1.

Irrigation–Principles
and Practices
Introduction 205
Lecture 1: Irrigation Concepts and Terminology 207
Lecture 2: Irrigation Scheduling and Delivery Systems 209
Demonstration 1: Field-Scale Irrigation 217
Demonstration 2: Garden-Scale Irrigation 219
Hands-On Exercises 1–6 221
Assessment Questions and Key 227
Resources 229
Supplements
1. Evapotranspiration (ET) and the Factors 233
that Aect ET Rates
2. Overview of “Water Budget Approach” 235
for Ecient Irrigation Management
3. Soil Moisture Sensing Instruments 237
Commonly Used for Irrigation Schedules
4. Overview of Dry Farming on the Central 239
California Coast
5. Nitrate Contamination of Groundwater 243
Appendices
1. Water Cycling Terms 245
2. Units of Water Measurement 247
3. Calculating Distribution Uniformity 248
 4. Estimating Soil Moisture By Feel 250
5. Critical Periods for Soil Water Stress by Crop 252
6. General Irrigation Rules 253
7. Irrigation for Various Vegetable Crops 254
8. Soil Probe and Soil Auger 256
9. Irrigation System Components 257
10. Field Irrigation Schedule 259
11. Garden Irrigation Schedule 260
12. Amount of Water Needed to Pre-Irrigate 261
a Dry Soil to Dierent Depths
13. Sample Sprinkler and Drip Tape Application 262
Rate Calculations

Part 1 – 204 | Unit 1.5

Irrigation—Principles & Practices
Introduction: Irrigation
UNIT OVERVIEW MODES OF INSTRUCTION

Effective irrigation practices can > LECTURES 2 LECTURES, 1 1.5 HOURS

improve yields and quality, mini- Lecture 1 covers the role of irrigation water along with
irrigation concepts and terminology. It nishes with a brief
mize water use, and protect natu-
overview of differences and similarities between garden-
ral resources. This unit introduces and eld-scale irrigation.
students to the basic concepts, tools, Lecture 2 focuses on techniques used to determining
and skills used to deliver water ef- when to irrigate and how much water to apply. Note: If
possible, have soil at different moisture levels available
ciently and effectively on both a eld
to demonstrate the “feel” approach to judging soil
and garden scale. Students will learn moisture.
about the role of irrigation water > DEMONSTRATION 1: FIELDSCALE IRRIGATION 2 HOURS
in agriculture, the movement and This eld-scale demonstration illustrates how to gauge soil
cycling of water in agricultural sys- moisture by feel and how to establish, use, and maintain
tems, and the environmental factors eld-scale irrigation equipment.

that inuence the type, frequency, > DEMONSTRATION 2: GARDENSCALE IRRIGATION 2 HOURS
and duration of irrigation. This garden-scale demonstration illustrates how to gauge
soil moisture by feel and how to establish, use, and main-
tain garden-scale irrigation equipment.
Two lectures and two demonstrations
introduce the resources and essential skills > EXERCISES 13: FIELD AND GARDENSCALE IRRIGATION
needed to determine the proper timing and SAMPLE CALCULATIONS 0.5 HOUR EACH
volume of irrigation. The rst lecture covers Given evapotranspiration information and output data for
basic irrigation concepts and terminology. drip and sprinkler irrigation systems, students will review
The second lecture addresses the use of both how to calculate the needed frequency and duration of irri-
quantitative (water budget and soil moisture gation for a 1-acre eld and a 100-square-foot garden bed.
sensors) and qualitative (feel) approaches to
determine irrigation timing, outlines envi- > EXERCISE 4: CALCULATING A WATER BUDGET FOR A ONE
ronmental factors that inuence irrigation ACRE BLOCK OF VEGETABLES 0.5 HOUR
decisions, and describes irrigation delivery Students will use their region’s evapotranspiration infor-
systems. Through exercises and problem mation to calculate the needed frequency and duration of
solving, students will practice calculating irrigation for a 1-acre eld.
water budgets used to develop irrigation
> EXERCISES 56: HOW MUCH WATER DO I NEED? HOW MANY
schedules and determine total water volume
ACRES CAN I IRRIGATE? SAMPLE CALCULATIONS 0.5 HOUR
needs per unit of time. The latter calcula-
EACH
tions will help the student determine needed
irrigation delivery systems. Supplements to Students will practice calculating total water volume needs
the lectures offer additional information on per unit of time to determine the need for irrigation infra-
using the water budget approach to man- structure.
age irrigation efciently, along with details > ASSESSMENT QUESTIONS 0.5 HOUR
on water sensor technologies, dry farming
Assessment questions reinforce key unit concepts and skills.
techniques, and health and environmental
impacts of nitrates contamination. > POWERPOINT
See casfs.ucsc.edu/about/publications and click on Teaching
Organic Farming & Gardening.

Unit 1.5 | Part 1 – 205

Introduction Irrigation—Principles & Practices
LEARNING OBJECTIVES

CONCEPTS SKILLS
• The role of irrigation water in agricultural • How to determine the timing and volume of
systems irrigation using qualitative approaches: Gauging
relative measures of eld capacity using the feel
• The movement and cycling of water in
method
agricultural systems: E.g., transpiration,
capillary action, evaporation, • How to determine the timing and volume of
evapotranspiration, evapotranspiration rate, irrigation using quantitative approaches: Water
percolation budgeting calculations using evapotranspiration
rates and calibrated water delivery systems
• Water quantity measurements: E.g., acre/
feet, acre/inch, one hundred cubic feet (CCF), • How to calculate total water volume needs per
gallons/minute (GPM) unit of time to determine the need for irrigation
infrastructure
• Relevant measurements of soil moisture: Soil
saturation, gravitational water, eld capacity, • How to access web-based irrigation information
permanent wilting point
• How to determine the appropriate irrigation
• Environmental factors that inuence the type, delivery system to use for specic crops and
frequency, and duration of irrigation settings
• Different way to determine the need for
irrigation: qualitative (feel method) and
quantitative (water budgeting, soil moisture
sensors)

Part 1 – 206 | Unit 1.5

Irrigation—Principles & Practices Introduction
Lecture 1: Irrigation—Concepts & Terminology
Pre-Assessment Questions
1. Why is water important for growing crops?
2. How is water volume commonly measured in agricultural systems?
3. How does irrigation water cycle through an agricultural system?
4. How does water stress negatively aect crop development and yield?

A. The Role of Irrigation Water in Agriculture Systems

1. Sustains soil biological and chemical activity and mineralization during dry periods: In
seasonally dry areas, irrigation water articially extends the time period in which soil
biological activity and nutrient release are elevated, creating more optimal growing
conditions for cultivated crops
2. Promotes soil solution and nutrient uptake: Irrigation water becomes the medium into
which soil nutrients are dissolved (soil solution) and through which nutrients are made
available for plant uptake
3. Provides carbohydrate building block: 6 CO2+ 6 H2O —> C6H12O6 + 6 O2: Through the
process of photosynthesis, water molecules taken up by plants are broken down and their
constituent atoms rearranged to form new molecules: carbohydrates and oxygen
4. Provides plant structure/support: Water molecules contained within the water-conducting
vascular bundles and other tissues of plants serve to provide physical support for the plant
itself
5. Promotes the maintenance of optimal temperatures within the plant: The loss of water
through the process of evapotranspiration (dened below) liberates heat from the plant,
thereby regulating plant temperature
6. Protects crops from frost damage: Irrigation water is commonly used to lower the freezing
temperature in orchard systems during threats of damaging frost
7. Reduces plant stress: By reducing stress on the plant, proper irrigation improves plants’
resistance to pest and disease damage and improves crop quality (see E, below)

B. Water Cycling in Agricultural Systems

1. Denition of terms (see also Appendix 1, Water Cycling Terms)
a) Transpiration: Water transfer to the air through plant tissues.
b) Evaporation: The loss of water to the atmosphere from soil and plant surfaces
c) Evapotranspiration: The loss of water to the atmosphere by the combined processes of
evaporation and transpiration (see more at Supplement 1, Evapotranspiration (ET) and
the Factors that Aect ET Rates)
d) Capillary action: The movement of water through very small pores in the soil from
wetter areas to drier areas. Water may move vertically and horizontally.
e) Inltration: The process by which water on the ground surface enters the soil
f ) Percolation: The gravitational process of water moving downward and through the soil
horizons

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Lecture 1: Irrigation—Concepts & Terminology Irrigation—Principles & Practices
C. Units of Water Measurement
1. Denition of terms
a) Acre inch: The equivalent volume of water application that would cover one acre of
land one inch deep in water. Example: On average, approximately one inch of water is
lost through evaporation and plant transpiration each week from May 15th–October 15
along the central coast of California (see Appendix 2, Units of Water Measurement)
b) Acre foot: The equivalent volume of water application that would cover one acre of land
one foot deep in water
c) Gallons per minute (GPM): The number of gallons being delivered through an irrigation
system in one minute
d) One hundred Cubic Feet (CCF): Term commonly used by municipal water providers as a
means of water measurement based on volume. 1 CCF equals 748 gallons.
e) Pounds per square inch (PSI): Water pressure in irrigation systems is measured in PSI.
Determining your irrigation system’s specic PSI is important in irrigation planning.
f ) Distribution Uniformity (DU): A measure of how uniformly water is applied to the area
being irrigated, expressed as a percentage. The higher the DU percentage, the more
uniform the application. See Appendix 3, Calculating Distribution Uniformity, for
additional information.

D. Overview of Garden vs. Field-Scale Irrigation (to be further discussed in Lecture 2,

Irrigation Scheduling and Delivery Systems)
1. Features of garden- and eld-scale cropping systems that inuence irrigation
a) Gardens: Smaller, more diverse, hand cultivated
b) Garden irrigation water sources
i. Domestic wells with 2 to 5 horsepower submersible pumps (10 gal/minute average
output)
ii. Municipal water systems (for urban gardens)
c) Fields: Larger blocks of plantings, mechanically cultivated
d) Field irrigation water sources; note—minimum 10 gal/acre/minute recommended
i. Agricultural wells (10 horse power or larger electric or diesel pumps / 50 gallons per
minute minimum)
ii. Surface sources supplemental to well water (ponds, creeks)
iii. Water district deliveries from surface sources supplied through canals or pipe lines
2. Similarities in irrigation scheduling and delivery systems between the two settings
a) Whether operating on a garden scale or eld scale, irrigation managers need to make
decisions about water application rates and type of delivery based on crop needs, weed
management, disease potential, evapotranspiration (ET) rates, and harvest schedules
while using both labor and water wisely and eciently.
3. Dierences in irrigation scheduling between the two settings
a) Garden scale: Typically use “soil moisture by feel” (qualitative) approach to determine
need for irrigation, as well as scheduling and reference to local ET rates (see Lecture 2
and Appendix 4, Estimating Soil Moisture by Feel)
b) Field scale: Typically use water budgeting (quantitative) approach along with
tensiometers or other moisture monitoring devices to determine need for irrigation (see
more at Lecture 2, as well as Supplement 2, Overview of “Water Budget” Approach for
Ecient Irrigation Management, and Supplement 3, Soil Moisture Sensing Instruments
Commonly Used for Irrigation Schedules)

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Irrigation—Principles & Practices Lecture 1: Irrigation—Concepts & Terminology
Lecture 2: Irrigation Scheduling & Delivery
Systems
Pre-Assessment Questions
1. How do you determine when it is time to irrigate?
2. What is the fundamental dierence between a qualitative (by feel) and quantitative (water
budget, soil moisture meter) approach to determining when to irrigate?
3. How do you determine how much water to apply?
4. What are some of the environmental factors that may inuence the frequency or duration
of irrigation?
5. What are some of the environmental factors that may inuence the type of irrigation used?
6. What are some of the dierent irrigation delivery systems available?

A. Denitions of Terms Specic to Soil Moisture Assessment (see also illustrations in

Appendix 1, Water Cycling Terms)
1. A number of terms are used when discussing the amount of moisture in the soil and plant’s
ability to access that moisture
a) Soil saturation: When all the pores of a given soil are lled with water. Soil rarely remains
saturated once watering (rain or irrigation) stops because gravitational water percolates
(drains) down to deeper soil strata.
b) Gravitational water: The water that will drain from a saturated soil if no additional water
is added. This water is not available for plant growth.
c) 100% of eld capacity: The point reached when no additional gravitational water
drains from a previously saturated soil. At 100% eld capacity the largest pores of the
soil structure (macropores) have been drained of water and replaced with air, while
micropores still retain water. This water is available to plants, which have the ability to
move water against gravity due to the upward pulling force produced by transpiration.
At eld capacity, an improved soil retains the maximum amount of water available to
plants, as well as optimal air space for aerobic microbial activity and plant growth.
d) 50% of eld capacity: The amount of water remaining in the soil when 1/2 of the water
held in the soil at eld capacity has evaporated, drained, and/or has been transpired
by growing plants; 50%–60% of eld capacity in the root zone of the crop is the soil
moisture level at which most crops should be irrigated
e) Permanent wilting point (PWP): The point at which soil moisture has been reduced to
where the plant cannot absorb it fast enough to grow or stay alive
f ) Plant available water (PAW): The water content held in the soil between eld capacity
and permanent wilting point that is available for uptake by plants
g) Soil water potential: The amount of energy required to remove water from the soil. This
measurement increases as soils dry, which then increases the possibility of transpiration
rates exceeding the rate of uptake, leading to plant stress.
h) Management allowable depletion (MAD): Maximum amount of soil water the irrigation
manager allows the crop to extract from the active rooting zone between irrigations.
This amount can vary with crop, stage of growth, potential for rainfall, and the soil’s
water holding capacity.

Unit 1.5 | Part 1 – 209

Lecture 2: Irrigation Scheduling & Delivery Systems Irrigation—Principles & Practices
B. Soil Moisture, Plant Stress, and Crop Productivity
1. Yield may be reduced due to water stress
a) Water-stress-sensitive stages of crop development (prioritized); see also Appendix 5,
Critical Periods for Soil Water Stress by Crop
i. Flowering
ii. Yield formation/fruit set
iii. Early vegetative growth/seedling stage
iv. Fruit ripening
2. General signs of water stress
Plants can show some water stress and still recover—however, extreme lack of water will
cause permanent wilting (see below) and death. Signs of water stress include:
a) Graying leaves: A change in leaf color from a vibrant green to a dull gray-green or bluish
color
b) Loss of sheen: Plant leaves change from glossy to dull in appearance
c) Insect damage: The presence of cabbage aphids on Brassica family crops (broccoli,
cabbage, kohlrabi, etc.) often indicates dry conditions
d) Damage to the root system: Upon closer examination, plants that look dry even after
watering often have root damage, e.g., from symphylans, and can’t take up sucient
water
e) Red or purple leaf color: Can indicate dry conditions, saturated conditions (anaerobic),
or root damage
f ) Development of small spines on the leaf margins or increased spinyness on stems: This
condition is especially likely to occur in lettuce and related species such as endive that
experience water stress
g) Wilting: Pay attention to the time of the day. If plants wilt early in the cool of the day,
this can be a sign that they need water. Some wilting in the mid-day heat (e.g., zucchini,
winter squash) is a plant-protective strategy to reduce transpiration losses.
h) Slower than expected growth: This can be detected over time with a practiced eye
3. Water stress increases crops’ susceptibility to pests and pathogens
Crops repeatedly subjected to water stress will be less resistant to both pest and
pathogens
4. Permanent wilting point
Permanent wilting point is dened as the point at which soil moisture is too low for the
plant to take up water against the pull of gravity. Crop plants reaching permanent wilting
point often do not grow well thereafter, are non-productive, or die.

C. Factors Inuencing Frequency and Volume of Irrigation

A number of factors, from climate and soil type to stage of crop maturity, must be considered
in determining when and how much to irrigate. Factors include:
1. Climate
a) Air temperature: Increased air temperatures will increase the rate of evapotranspiration
(ET)
b) Precipitation: In areas of regular summer rainfall, where precipitation exceeds ET,
irrigation is seldom required. Irrigation demands are based on ET rates. Where ET
exceeds precipitation, irrigation is required.
c) Humidity: Increased humidity will decrease the rate of ET
d) Wind: High wind speeds increase ET

Part 1 – 210 | Unit 1.5

Irrigation—Principles & Practices Lecture 2: Irrigation Scheduling & Delivery Systems
 2. Soils
a) Sandy soils drain rapidly and do not hold water well
b) Silty soils drain slowly and hold water well
c) Clay soils drain very slowly and hold water tightly
d) Loam soils both drain well and hold water well
e) Agricultural soils improved with organic matter (cover crops, compost) maintain good
drainage and moisture retention properties (for more on this topic, see discussion in
Unit 1.6, Selecting and Using Cover Crops and Unit 1.7, Making and Using Compost)
3. Stage of development and crop natural history
a) “Water-loving” crops (e.g., celery) demand less uctuation in soil moisture levels (see
Appendix 6, General Irrigation Rules, and Appendix 7, Irrigation for Various Vegetable
Crops)
b) Drought-tolerant crops (e.g., tomato varieties, winter squash varieties, Amaranth, etc.)
may require little or no irrigation (see Supplement 4, Overview of Dry Farming on the
Central California Coast)
c) Maturation period: Prior to harvest, many crops (e.g., onions and garlic) require a
gradual reduction in irrigation to encourage maturation
d) The specic watering needs of tree fruits are highly variable, and depend on a
combination of the tree’s age and size, rootstock, and your soil and climate. In general,
deciduous fruit trees need readily available moisture in the root zone through harvest
to promote canopy development, extension growth, fruit sizing, and fruit maturation.
This normally means letting the soil dry down to no more than 6–8” deep between
irrigations and replacing water based on local ET rates to ensure high fruit quality.
e) Citrus and other evergreen fruit trees also need regular water delivery for the same
reasons noted above for deciduous fruit. In the case of citrus, which are often owering,
setting fruit, and maturing fruit simultaneously, consistent water delivery is important to
maintain citrus tree health, vegetative vigor, and fruit quality. Both irrigation and rainfall
should be monitored year round, and the soil should only be allowed to dry to a depth
of 3–4”, followed by an irrigation set to replace water lost to ET. Underwatering citrus as
fruit ripens can lead to small fruit and dry, aky interiors.
f ) Vase life of cut owers can be improved—in some cases dramatically—by developing
an irrigation schedule that delivers water to crops ready to harvest at least 12 hours but
not more than 24 hours prior to harvest. This will help ensure that stems have full turgor
and stress can be minimized, allowing stems to maintain turgor through post-harvest
uptake rather than trying to compensate for an already extant water decit. With
reduced stress, plants will consume stored nutrients more slowly, extending the time
that cut stems remain strong and vibrant.

D. Determining When to Irrigate and How Much Water to Apply

1. Measuring soil moisture by feel: A qualitative approach
a) Measuring soil moisture by feel includes learning how to judge soil moisture by forming
soil into a cast or ball, and by “ribboning” soil (see Appendix 4 and the NRCS publication
Estimating Soil Moisture by Feel and Appearance noted in the Resources). This takes
practice! Knowing the percent of soil moisture present can help determine whether
irrigation is needed.
b) Shovels, trowels, and soil augers can be used to obtain soil samples to a depth of up to
12 inches in the crop root zone for accurate moisture assessment (see illustrations in
Appendix 8, Soil Auger and Soil Probe)

Unit 1.5 | Part 1 – 211

Lecture 2: Irrigation Scheduling & Delivery Systems Irrigation—Principles & Practices
 2. Considerations for determining irrigation scheduling using the “feel” approach
a) The “feel” method is more commonly used by irrigation managers in garden and small
farm systems as a low-tech, low-cost way to assess irrigation needs in diverse cropping
systems
b) Irrigation managers must be familiar with soil type and appropriate methods of soil
moisture assessment to make accurate irrigation scheduling decisions
c) The “feel” approach to irrigation management requires a high level of intuition and
experience, and an extensive knowledge of the specic requirements of the various
crops being irrigated. Once understood, it can be a quick decision-making tool.
d) In deciding when and how much to irrigate, the irrigation manager must take into
account a variety of factors in addition to soil moisture, including crop needs, and
timing of harvest (see D. Factors Inuencing Frequency and Volume of Irrigation, and
below), as well as weed management operations to determine an optimum application
time and rate
3. Determining irrigation scheduling using the water budget approach
a) Water budgeting is often compared to managing a savings account: The starting point is
eld capacity (see denitions, above), and as water is removed and the “savings balance”
drops, it is replaced as needed by the crop. Water budgeting is a quantitative approach
using existing models that analyze temperature and crop water use to determine
evapotranspiration (ET) rates. Growers use these models to determine irrigation timing
and amounts.
b) When seasonal ET exceeds precipitation, irrigation is required to sustain planted crops
c) Resources for determining regional average ET (e.g., CIMIS; see Resources section); you
can use this regional average when determining a water budget
d) Replacing estimated water loss through ET with calibrated irrigation systems
i. Once the ET rate of your site is determined, this estimated volume of water may
be replaced through the use of calibrated irrigation systems that deliver water at
a known rate and volume. The Hands-on Exercises in this unit oer examples of
how to calculate the irrigation time and frequency required to replace water with a
calibrated irrigation system.
e) Irrigation scheduling in dierent systems based on water budgeting approach
i. Once the evapotranspiration rate for a crop in full canopy (in gallons/week) and
the water delivery rates (in gallons/hour) of the irrigation system are estimated, the
amount of time required to replace water lost may be calculated (see Hands-On
Exercises). This calculation will provide the total number of hours required to replace
the water lost through evapotranspiration. (An additional 10% should be calculated
in to compensate for delivery system ineciencies.)
ii. The frequency of irrigation should correspond to the time period required for the
soil in the root zone of the crop to dry to approximately 50% of eld capacity. Due to
shallow root systems and greater susceptibility to water stress, annual crop culture
often requires a higher frequency of irrigation (2–3 times/week for many crops).
iii. Established orchards, which have deep root systems and are less susceptible to water
stress, often require less frequent but larger volumes of water to be delivered in
each irrigation. In both situations the estimated amount of water lost through ET is
replaced as needed to maintain the health of the crop.

Part 1 – 212 | Unit 1.5

Irrigation—Principles & Practices Lecture 2: Irrigation Scheduling & Delivery Systems
 f ) Once a decision is made to irrigate, and a volume is determined, the timing of the water
application must take into account timing of future harvest and weed management
operations
g) Disadvantages: Water budget approach is not easy to apply to small, diverse systems
h) Advantages: Water budget approach can be an eective tool to increase water use
eciency
4. Determining irrigation scheduling using tensiometers and other soil moisture sensors (see
Supplement 3, Soil Moisture Sensing Instruments Commonly Used for Irrigation Schedules)
a) As the cost of simple soil moisture sensors drops, many growers are beginning to
incorporate these instruments in their systems to monitor soil moisture levels. Such
devices provide site-specic data points that may be more accurate than CIMIS data and
can be used in combination with other techniques to inform irrigation decisions.
i. Soil tensiometers and Electrical Resistance Sensing Devices (ERSDs) are the
instruments most commonly used to measure soil moisture on California’s Central
Coast farms. Both must be carefully installed directly in the wetted area of the crop’s
root zone at a number of sites throughout the eld for accurate monitoring (see
Supplement 3 for details).
ii. Soil moisture sensors are often used in pairs at dierent depths, e.g., at 6 and 12
inches deep, to provide the irrigator with information on below-ground moisture
dynamics
iii. Tensiometers and ERSDs provide soil/water tension readings that can be used to
establish irrigation schedules adequate to maintain soil moisture at levels conducive
to good crop growth and productivity
5. Other factors to consider when determining whether irrigation is needed
a) How do the plants look? See above for list of general signs of water stress.
b) Weather patterns: E.g., a crop may look stressed at midday, but knowing that the
weather will cool overnight and be foggy in the morning may mean that irrigation is not
immediately required. Therefore observing the crops throughout the day is important.
c) After a cool period, the rst hot day may trigger plants to look stressed, but in fact they
may not need irrigation
d) Soil type: Soil type and organic matter levels will determine in part how the soil holds
water (see the NRCS reference Estimating Soil Moisture by Feel and Appearance in
References)
e) Type of crop: Dierent crops, dierent needs (Appendix 7, Irrigation for Various
Vegetable Crops)
f ) Stage of development: Some crops benet from being slightly water stressed early in
their growth cycle (e.g., tomatoes, beans, cucumbers and other cucurbits), or do not
need irrigation once the plants begin to die back (e.g., potatoes). Others, particularly
small-seeded crops such as lettuce and carrots, require that soils be kept moist in order
to germinate eectively.
g) Optimal moisture for harvest: It is critical to maintain full turgor for leafy crops and cut
owers, particularly if they will not immediately go into a cooler or receive some form
of hydrocooling, as is done with brassicas and similar crops (see more at C. Factors
Inuencing Frequency and Volume of Irrigation, above)

E. Problems with Overapplying Water

1. In many areas, fresh water is a limited resource. Irrigation practices that optimize the
available supply are critical.
2. The energy and environmental costs involved in transferring water and “lifting” it to
irrigation systems via pumps, etc., can be signicant

Unit 1.5 | Part 1 – 213

Lecture 2: Irrigation Scheduling & Delivery Systems Irrigation—Principles & Practices
 3. Over application of irrigation water has the potential to germinate weed seeds that would
have otherwise remained dormant in the soil, leading to higher labor costs for weed
removal and/or signicant crop competition resulting in decreased yields
4. Overapplying water can lead to unnecessary nutrient leaching, soil compaction, decreased
water inltration rates, erosion, and nutrient leaching (see Supplement 5, Nitrate
Contamination of Groundwater)

F. Irrigation Delivery Systems (see also Appendix 9, Irrigation System Components)

1. Sprinklers
a) Micro sprinklers
i. Micro sprinklers are commonly used in small-scale orchards and vineyards
ii. Micro sprinklers are commonly used in garden-scale production systems that require
frequent, light irrigation to help germinate small seeds
iii. Micro sprinklers provide uniform application of water and the relatively small droplet
size minimizes soil surface crusting and aggregate dispersion
iv. Small droplet size is not optimal for distribution uniformity (DU) or water use
eciency in windy areas
b) Hand-moved aluminum pipe with impact or rotator type heads
i. Hand moved aluminum irrigation pipe is the most commonly used sprinkler
irrigation delivery system in both small- and large-scale farming operations due to
relatively low cost, long life, ease of use, and durability
ii. Hand moved aluminum pipes typically use 3-inch diameter lateral lines with a
3-gallon-per-minute (GPM) sprinkler head mounted on an 18” riser on each 30’ long
section of pipe. Lateral lines are typically spaced either 30’ or 40’ feet apart.
iii. Aluminum sprinkler pipes tted with impact heads typically require an operating
pressure of at least 45 psi for optimum uniformity. Rotator type heads require higher
pressure.
c) Hand-moved PVC “riser system” for garden-scale applications
i. A simple-to-build, portable riser system of PVC and micro sprinklers can serve the
same purpose as hand-moved aluminum pipe at a garden scale (see Appendix 9)
ii. Because it has xed spray patterns of 180º and 360º at a pre-determined distance,
the riser system can be sized to suit your garden’s scale and design, assuming basic
irrigation knowledge and access to a standard irrigation supply store
iii. The pattern and uniformity of distribution are relatively consistent and predictable,
and the system can be run with minimal attention to water pressure
iv. Spray distribution, while still gentle on surface soil structure, is delivered in relatively
large drops and risers are only 15”–18” tall, so loss of uniformity and evaporation due
to wind exposure is minimized
v. The riser system can be used to germinate seeds, to grow overhead irrigation-loving
or tolerant crops and low-growing crops to maturity, and can be used to pre-irrigate
and ush weeds in smaller areas. In the absence of timely rains, the riser system can
also be used to establish cover crops and until cover crop height exceeds the height
of the risers.
d) Oscillators
i. Oscillating sprinklers are often used in garden settings, and are a relatively low-cost,
easy-to-use water delivery system
ii. Advantages of oscillators
• Oer large degree of coverage size and pattern exibility in a single unit

Part 1 – 214 | Unit 1.5

Irrigation—Principles & Practices Lecture 2: Irrigation Scheduling & Delivery Systems
 • Relatively low cost, readily available, and can be operated with little technical
background
• Although vulnerable to the wind, oscillators may be useful when “rough irrigating”
large blocks of garden beds, e.g., prior to planting cover crops, and when irrigating
large garden spaces
iii. Drawbacks
• Relatively low uniformity of water distribution and high rates of evaporation,
especially in hot and/or windy situations (see below for more on distribution
uniformity, or DU)
• Can take time to adjust and “dial in” to provide full, even coverage of the desired
area without also delivering water beyond the boundary of the desired irrigation
set
• Inconsistent distribution pattern and vulnerability to wind redistribution;
neighboring crops may be subject to “drift” and thus increased disease incidence if
they are prone to fungal disease of the leaf canopy
• Can be dicult or time consuming to accurately determine output/distribution,
thus leading to over- or underwatering. For example, the adjustability of
oscillators means the same device could water a 4’ x 12’ section or a 30’ x 30’
section. So an irrigation set of the same duration will deliver very dierent
amounts to these two garden plots.
• Quality of oscillator brands—their useful lifespan, adjustability, and distribution
uniformity—varies widely, and it can be dicult to know quality until you’ve
invested considerable time and energy
2. Drip irrigation
a) Drip irrigation has many advantages over sprinkler or ood irrigation, including
application uniformity, the ability to apply water exactly where it is needed, and the
potential reduction of disease and weed incidence in irrigated systems
b) Drip irrigation refers to both rigid ½ inch poly tubing with inline emitters and the thin
wall tubing commonly referred to as “drip tape.” Drip tape is available in an assortment
of wall thicknesses and emitter spacings and is relatively low cost, but also much less
durable compared to the rigid poly tubing.
c) Drip tape is commonly used in small-scale vegetable production systems as a means of
conserving water and minimizing weed and disease pressure
d) Depending on the water source, drip tape and tubing often require ltration to limit
clogging of emitters
e) Drip tape and poly tubing with inline emitters require pressure regulation to optimize
application uniformity
f ) Drip tape and poly tubing with inline emitters require a grade of 2% or less and runs of
no more that 300 feet for optimum distribution uniformity
g) Careful consideration must be given to design when setting up a drip irrigation system
to optimize distribution uniformity and system function

G. Environmental Factors Inuencing the Type of Irrigation Used

1. Climate and incidence of plant pathogens (see also Unit 1.9, Managing Plant Pathogens)
a) Overhead irrigation may encourage the growth and spread of certain plant pathogens
on crops in certain climates (e.g., Phytopthora spp. on melons, cucumber, peppers, and
tomatoes along coastal California).

Unit 1.5 | Part 1 – 215

Lecture 2: Irrigation Scheduling & Delivery Systems Irrigation—Principles & Practices
H. Importance of Distribution Uniformity (DU)
1. Distribution Uniformity (DU) refers to how uniformly water is made available to plants over
an area via an overhead sprinkler or drip irrigation system
2. DU can be measured using a simple “catch bucket” test and the “low quarter DU”
calculation for both overhead and sprinkler irrigation systems (see Appendix 3 for details).
Note that measuring DU of oscillating sprinklers may require more catch buckets to
accurately measure uniformity throughout the coverage area (corners, edges, and middles
of beds).
3. It is important to maintain high DU in order to optimize water use and ensure that the
entire crop is receiving the intended amount of irrigation

Part 1 – 216 | Unit 1.5

Irrigation—Principles & Practices Lecture 2: Irrigation Scheduling & Delivery Systems
Demonstration 1: Field-Scale Irrigation
for the instructor
INTRODUCTION PREPARATION AND MATERIALS

This demonstration offers students • Map of farm irrigation system

an in-eld look at the tools and Irrigation equipment:

techniques used to deliver irrigation • Established set of aluminum pipe with sprinklers
water efciently from the mainline • Component pieces of sprinklers
irrigation infrastructure through the • Established set of drip irrigation
specic irrigation delivery system
• Component pieces of drip irrigation equipment
used on your farm. The instructor
• Tools for setting up and adjusting irrigation equipment
should begin with an explanation
• Irrigation schedules (see Appendix 10, Field Irrigation
of the irrigation infrastructure used
Schedule)
to deliver water to and through the
farm, then explain how to set up, PREPARATION TIME
adjust, and maintain the specic 1.5 hours
irrigation system(s) currently in use.
DEMONSTRATION TIME
2 hours

DEMONSTRATION OUTLINE

A. Irrigation Infrastructure
1. Explain the layout and identify major components of
the farm irrigation water delivery system from source to
crop

B. Measuring Flow Rate

1. Demonstrate how to determine ow rate using a garden
hose and a 5-gallon bucket

C. Sprinkler Irrigation Systems

1. Demonstrate a typical eld layout and a typical orchard
layout of a hand-moved aluminum sprinkler system.
Include the following demonstrations:
a) The proper technique for moving and laying out
sprinkler pipes
b) Flushing the system clean
c) Sprinkler head adjustment
d) Layout design and pipe hook-up
2. Demonstrate and explain the importance of proper
head adjustment and timing as it relates to application
uniformity

Unit 1.5 | Part 1 – 217

Instructor’s Demonstration 1 Outline Irrigation—Principles & Practices
 3. Demonstrate and explain how to determine optimum operating pressure
4. Students are given the opportunity to unhook, move, and hook up a sprinkler set.
The sprinkler set is then turned on and adjusted.

D. Drip Irrigation Systems

1. Demonstrate and explain several examples of drip irrigation header set-ups
2. Demonstrate and explain how to turn on a drip system and set pressure and check for
leaks
3. Demonstrate the following:
a) How a gate-valve and ball-valve work
b) How to set up a drip irrigation header
i. How to properly punch holes in the 2” oval tube
ii. How to install the barbed connectors into the oval tube
iii. How to connect the T-tape to the various types of connectors
iv. How to splice T-tape for repairs
v. How to cap ends of T-tape
vi. How to determine proper system pressure
vii. How to properly roll out and roll up T-tape for placement and storage
4. Have students cut and splice T-tape

E. Review and Discuss Irrigation Scheduling

1. Review the calculations in Hands-on Exercises 1–3 to determine the volume of water
and the frequency of irrigation necessary to replace the water lost through regional
evapotranspiration
2. Assign Exercise 4: Calculating irrigation requirements using regional
evapotranspiration data
3. Describe and demonstrate the use of an irrigation schedule for tracking and planning
irrigation (see Appendix 10)

F. Review and Discuss Exercises 5 and 6

1. Exercise 5: How much water is needed to irrigate a given area of land?
2. Exercise 6: How much area can one irrigate with a given ow rate?

G. Discuss water delivery systems needed to deliver the volumes of water given in
Exercises 5 and 6

Part 1 – 218 | Unit 1.5

Irrigation—Principles & Practices Instructor’s Demonstration 1 Outline
Demonstration 2: Garden-Scale Irrigation
for the instructor
OVERVIEW PREPARATION AND MATERIALS

Students must be able to accurately • Drip irrigation system

gauge soil moisture and use scale- • Oscillators

appropriate irrigation tools and • Fan
techniques in order to irrigate garden • Rose
crops efciently and effectively. • Micro sprinklers
The following demonstration
• Garden riser
provides an overview of the basic
• Rain gauge
skills, concepts, and tools used in
• Ross
garden-scale irrigation. During this
demonstration, the instructor should • Soil moisture chart (see Appendix 4, Estimating Soil
Moisture by Feel)
discuss the different approaches
• Blank irrigation schedule (see Appendix 11, Garden
to irrigation (qualitative and
Irrigation Schedule)
quantitative) as well as demonstrate
• Soil samples or pre-irrigated soils at varying percentages of
the tools and techniques used to eld capacity
monitor soil moisture and schedule
irrigation. PREPARATION TIME
1.5 hours

DEMONSTRATION TIME
2 hours

Unit 1.5 | Part 1 – 219

Instructor’s Demonstration 2 Outline Irrigation—Principles & Practices
DEMONSTRATION OUTLINE

A. Irrigation Management by Percent Field Capacity

1. Review terms
a) Soil saturation
b) Gravitational water
c) 100% of eld capacity
d) 50% of eld capacity
i. Review 50% of eld capacity as critical moisture level for most cultivated annual
crops
e) 25% of eld capacity
f) Permanent wilting point
2. Review exceptions to the to the 50% eld capacity general rule (see Appendix 6,
General Irrigation Rules)
3. Review the stages of crop development at which plants are most sensitive to drought/
water stress (listed from most to least sensitive; see also Appendix 5, Critical Periods
for Soil Water Stress by Crop)
a) Flowering
b) Yield formation/fruit set
c) Early vegetative growth/seedling stage
d) Fruit ripening
4. Have students gauge soil moisture (in percent eld capacity) by feel and appearance
using Appendix 4, and the USDA publication Estimatng Soil Moisture by Feel and
Appearance (see Resources)
5. Review how to develop an irrigation schedule based on an estimated frequency of dry
down to 50% of eld capacity (see Appendices 11 and 12, Amount of Water Needed to
Pre-Irrigate a Dry Soil to Different Depths)
6. Discuss and demonstrate how to properly maintain seedbed soil moisture for small-
and large-seeded direct-sown crops
7. Discuss and demonstrate how to assemble, use, and repair garden-scale irrigation
equipment (T-tape, oscillators, micro sprinklers, garden riser, etc.) in delivering water
effectively and efciently
8. Discuss and demonstrate how to assemble and repair the PVC portions of a garden-
scale irrigation system

B. Irrigation Management Using the Water Budgeting Approach

1. Estimating crop Evapotranspiration (ET) loss from a crop in full canopy
a) The use of California Irrigation Management Information Systems (CIMIS) data to
determine average weekly ET (see Resources section)
2. Review and discuss the calculations used in developing a weekly irrigation schedule to
replace water lost through estimated ET for drip-irrigated crops. Assign and review the
Garden Irrigation Exercise (see next section).
3. Discuss and demonstrate the use of rain gauges in monitoring the volumes of water
delivered to replace water losses through ET in overhead-irrigated crops. Note the
issue and challenge of achieving an adequate level of distribution uniformity in using
oscillators, and the importance of identifying areas (middle, ends, and corners of beds)
that may be receiving too much or too little irrigation.

Part 1 – 220 | Unit 1.5

Irrigation—Princples & Practices Instructor’s Demonstration 2 Outline
Hands-On Exercises 1–3: Sample Calculations—
Replacing Water Lost through Evapotranspira-
tion (ET) Using the Water Budgeting Approach
for the student
In Hands-on Exercises 1 through 3, you EXERCISE 1
will see sample calculations for the The following sample calculation will show you how to
calculate the amount of irrigation time and frequency of
amount of irrigation time and frequency irrigations required to replace the amount of water lost
of irrigations required to replace water through evapotranspiration from a 1-acre block of vegetables
lost through evapotranspiration (ET) in full canopy using drip irrigation.
from a 1-acre block of vegetables using A. NUMBER OF GALLONS LOST THROUGH
drip irrigation and sprinkler irrigation, EVAPOTRANSPIRATION ET IN A 1ACRE FIELD
as well as a 100-square-foot garden bed • Daily average summer evapotranspiration rate (ET) for an
actively growing crop in full canopy in Santa Cruz = 0.15 inch/
(respectively). day
• Multiply this by 7 days/week = 1.05 inches/week
• There are 27,158 gallons of water in an acre inch (the volume
of water needed to cover an acre of land to a 1-inch depth)
• An acre = 43,560 square feet (roughly 208 feet x 208 feet)
• Multiplying 1.05 inches/week (ET) x 27,158 gallons/acre
inch = 28,516 gallons/acre of water lost each week through
evapotranspiration in an actively growing crop in full canopy
in Santa Cruz, California

B. DRIP IRRIGATION OUTPUT CALCULATIONS

• Flow rate of high ow T-tape drip irrigation ribbon with 8-inch
emitter spacing at 10 pounds per square inch (psi) = .74
gallons/minute/100 feet
• There are 14,520 feet of row per acre when beds are spaced
36 inches center-to-center
• To determine gallons/hour/acre emitted from one acre of
drip irrigation ribbon, divide 14,520 (the number of row
feet/acre) by 100 = 145 (the number of 100-foot lengths of
drip irrigation ribbon in 1 acre). Multiply 145 by .74 gallons/
minute/100 feet (the amount of water delivered through each
100 feet of ribbon) = 107.4 gallons/minute/acre.
• 107.4 gallons/minute x 60 minutes = 6,446 gallons/hour/acre.
Two lines of drip tape would provide twice this volume, or
12,892 gallons/hour/acre.

Unit 1.5 | Part 1 – 221

Students’ Hands-On Exercises 1-3 Irrigation—Principles & Practices
 C. CALCULATING IRRIGATION REQUIREMENTS • There are roughly 109 sprinkler heads per acre
• 28,516 gallons/acre are lost through using 20-foot pipes set 20 feet apart (20 feet x 20
evapotranspiration each week from an actively feet = 400 square feet. 43,560 square feet/acre
growing crop in full canopy. The drip system divided by 400 = 109)
described above is capable of delivering 6,450 • 109 sprinkler heads x 3 gpm each = 330 gallons
gallons/hour/acre @ 10 psi. To calculate the per minute
amount of irrigation time required to replace the
amount of water lost through ET complete the • 330 gal/min x 60 minutes/hour = 19,800 gallons/
following: hour/acre

• Divide 28,516 gallons/acre (ET) by 6,450 gal/ C. CALCULATING IRRIGATION REQUIREMENTS:

hour/acre (irrigation system application rate) = • 28,516 gallons/acre are lost through
4.4 hours of irrigation time required each week. evapotranspiration each week from an actively
Running the one acre of single line drip irrigation growing crop in full canopy. The sprinkler system
with 8 inch emitter spacing for 4.4 hours each is capable of delivering 19,800 gallons/hour/acre
week will apply 28,516 gallons/acre (~1.05 inches/ @ 45psi. To calculate the amount of irrigation
acre), which is the amount of water needed to time required to replace the amount of water lost
replace what is lost through ET. This total of 4.4 through ET complete the following:
hours/week should be divided into 2–3 evenly
timed irrigation sets. • Divide 28,516 gallons/acre (ET) by 19,800 gallons/
hour/acre (irrigation system application rate) = 1.4
hours of irrigation time required each week.
EXERCISE 2
• Running the one acre sprinkler system for 1.4
The following sample calculation will show you
hours each week will apply 28,516 gallons/acre
how to calculate the amount of irrigation time and
(~1.05 inches/acre), which is the amount of water
frequency of irrigations required to replace the
needed to replace that lost through ET. This total
amount of water lost through evapotranspiration
of 1.4 hours/week should be divided in to 2–3
from a 1-acre block of vegetables using sprinkler
evenly timed irrigation sets/ week of 40 or 30
irrigation.
minutes respectively.
A. NUMBER OF GALLONS LOST THROUGH *Note: It is also important to factor in an additional
EVAPOTRANSPIRATION ET IN A 1ACRE FIELD 10–20% for evaporative loss due to extreme heat
• Daily average summer evapotranspiration rate and wind conditions. It is further advisable to use
(ET) for an actively growing crop in full canopy in several rain gauges to check the actual amount
Santa Cruz = .15 inch/day applied and to assess uniformity of applications.
See Appendix 2: Calculating Sprinkler and Drip
• Multiply this by 7 days/week = ~1.05 inches/week
Distribution Uniformity, for additional information.
• There are 27,158 gallons of water in an acre inch
(an acre inch is the amount of water needed to D. CALCULATING AN ADDITIONAL 1020% WOULD
cover an acre to a 1-inch depth) PROCEED AS FOLLOWS:
• 28,516 + 10% (.10 x 28,516) = 31,368 gallons/
• An acre = 43,560 square feet (roughly 208 feet x
acre; 28,516 + 20% (.20 x 28,516) = 34,239 gallons/
208 feet)
acre. Dividing each of the above by the irrigation
• Multiplying 1.05 inches/week (ET) x 27,158 system output results in the following: 31,368
gallons/acre inch = 28,516 gallons/acre of water gallons/acre divided by 19,800 gallons/hour/acre
lost each week through evapotranspiration in an = 1.6 hours of irrigation time each week. 34,239
actively growing crop in full canopy in Santa Cruz, gallons/acre divided by 19,800 gal/hour/acre = 1.7
California. hours of irrigation time each week. These totals
of 1.6 and 1.7 hours/week should also be divided
B. SPRINKLER IRRIGATION OUTPUT CALCULATIONS into 2–3 irrigation sets each week for annual
• Flow rate from a 1/8 inch nozzle running at an vegetables.
operating pressure of 45 psi is about 3 gallons per
minute (gpm)

Part 1 – 222 | Unit 1.5

Irrigation—Princples & Practices Students’ Hands-On Exercises 1-3
EXERCISE 3 C. CALCULATING IRRIGATION REQUIREMENTS
The following sample calculation will show you • 62.31 gallons of water are lost from a single
how to calculate the amount of irrigation time and 100-square-foot garden bed through
frequency of irrigations required to replace the evapotranspiration each week. Four lines of high
amount of water lost through evapotranspiration ow T-tape deliver 50.1 gallons/hour @ 10 psi. To
from a 100-square-foot garden bed. calculate the amount of irrigation time required
to replace the amount of water lost through ET,
A. CALCULATING THE NUMBER OF GALLONS LOST complete the following:
THROUGH EVAPOTRANSPIRATION ET IN A
100SQUAREFOOT GARDEN BED • 62. 31 gallons/week (ET) divided by 50.1 gallons/
hour (output ) = 1.25 hours (or 75 minutes) of
• Daily average summer evapotranspiration rate
irrigation time @ 10 psi. This application of water
(ET) in Santa Cruz = 0.15 inch/day
should be divided between two to three equally
• Multiply this by 7 days/week = 1.05 inches/week long irrigation sets each week, 40 or 25 minutes in
length respectively.
• 25-foot x 4-foot garden bed = 100 square feet
• 20% more time should be added to compensate
• 100 square feet x 144 (square inches/foot) =
for evaporative losses, leakage, etc. These
14,400 square inches
respective times should be increased to two
• 100 square feet to 1 inch in depth = 14,400 cubic 45-minute sets or three 30- minute sets/week.
inches
• 1,728 cubic inches/ cubic ft.
• 1 cubic foot = 7.48 gallons
• 14,400 cubic inches (100-square-foot garden bed)
divided by 1,728 cubic inches = 8.33 cubic feet
• 8.33 cubic feet x 7.48 gallons/cubic foot = 62. 31
gallons/week lost through ET

B. DRIP IRRIGATION OUTPUT CALCULATIONS

• Flow rate of high ow T-tape irrigation ribbon
with 8-inch emitter spacing @ 10 psi = .74 gallons/
minute/100 feet (assuming 100% eciency)
• There are 133 emitters/100 ft @ 8-inch spacing
• .74 divided by 133 = 0.00556 gallons/minute/
emitter
• .00556 X 60 (inches/hour) = .334 gallons/hour/
emitter
• A 25-foot row of T-tape = 300 inches
• 300 inches divided by 8-inches emitter spacing =
37.5 emitters/row
• 37.5 emitters/row x 4 rows t-tape/bed = 150
emitters/ bed
• 150 x .334 gallons/hour/emitter = 50.1 gallons/
hour

Unit 1.5 | Part 1 – 223

Students’ Hands-On Exercises 1-3 Irrigation—Principles & Practices
Hands-On Exercises 4: Calculating a Water
Budget for a One-Acre Block of Vegetables
Using Sprinkler Irrigation
for the student
In the following exercise you will B. SPRINKLER IRRIGATION OUTPUT CALCULATIONS
calculate the amount of irrigation • Step 4: Flow rate in gallons per minute (gpm) from an
individual sprinkler head _____
time and frequency of irrigations
required to replace the amount of • Step 5: Given: There are roughly 109 sprinkler heads per acre
using 20 foot pipes set 20 feet apart. (20 feet x 20 feet = 400
water lost through evapotranspiration square feet. 43,560 square feet/acre divided by 400 = 109)
in your area from a one-acre block of
• Step 6: 109 sprinkler heads x _____ gallons/minute each =
vegetables using sprinkler irrigation. _____ gallons per minute
• Step 7: ______ gallons/minute x 60 minutes/hour = _______
EXERCISE 4 gallons/hour/acre total
A. NUMBER OF GALLONS LOST THROUGH C. CALCULATING IRRIGATION REQUIREMENTS
EVAPOTRANSPIRATION ET IN A ONE
• To calculate the amount of irrigation time required (in
ACRE FIELD:
hours/week) to replace the amount of water lost through
• Step 1: Daily average summer evapotranspiration each week, complete the following
evapotranspiration rate (ET) for an calculations:
actively growing crop in full canopy in
your area = _____ inches/day • Divide the total in Step 3 _____ gallons/acre ET by the total in
Step 7 _____ gallons/hour/acre from the irrigation system =
• Step 2: Multiply this by 7 days/week = _____ hours of irrigation time required each week. This total
_____ inches/week time should be divided in to 2–3 irrigation sets for mixed
Given: There are 27,158 gallons of water vegetable operations.
in an acre inch (the amount of water * Note: It is also important to factor in an additional 10–20% for
needed to cover an acre to a 1-inch evaporative losses due to extreme heat and wind conditions.
depth) It is further advisable to use several rain gauges to check
Given: An acre = 43,560 square feet the actual amount applied and to assess uniformity of
(roughly 208 feet x 208 feet) application.

• Step 3: Multiplying _____ inches/week

(ET) x 27,158 gallons/acre inch = _____
gallons/acre of water lost each week
through evapotranspiration in an actively
growing crop in full canopy in your area.

Part 1 – 224 | Unit 1.5

Irrigation—Princples & Practices Students’ Hands-On Exercise 4
Hands-On Exercises 5 & 6: Sample Calculations—
How Much Water Do I Need? How Many Acres
Can I Irrigate?
for the student
OVERVIEW GIVEN:
In the following exercises you will • At any time during the summer the entire 10 acres may be in
production
calculate the total rate and volume of
• The daily average evapotranspiration rate (ET) during the
irrigation water that must be delivered
summer months is about 0.30 inch per day
to support two hypothetical farming
• There are 27,158 gallons of water in an acre inch
operations. This information will help
• You only plan to run the pump 12 hours per day
you determine the irrigation system
needed to support the delivery of this • There are 10,080 minutes per week (60 minutes/hour x 24
hours/day x 7 days/week)
volume of water.
• There are 5,040 minutes per week at 12 hours per day (10,080
divided by 2)
EXERCISE 5: HOW MUCH WATER DO I NEED?
SOLUTION
• I have 10 acres that I want to farm. The
climate is Mediterranean with a fairly dry 1. Multiply 0.30 inches (ET) by 7 (days per week) to get 2.1
summer season. There is no well or pump inches per week
on the property. The property is situated 2. Assume that your application will be 75% ecient and
over an aquifer that has an adequate multiply 2.1 (inches per week) by 1.25 to get 2.625 inches
water supply. I have adequate capital per week (application rate to supply actively growing crops
to invest in a well and pump to supply with adequate moisture for maximum yield during summer
irrigation water for my farm. I need to months)
decide how much water I need (ow
rate in gallons per minute) to irrigate the 3. Multiply 2.625 inches per week by 27,158 (gallons per acre
entire 10 acres, so that I can have the inch) to get 71,290 gallons per acre per week
proper-sized well and pump installed. 4. Multiply 71,290 (gallons per week) by 10 (acres) to get 712,900
gallons per week
5. Divide 712,900 (gallons per week) by 5,040 (minutes per week
at 12 hours per day) to get 141.44 gallons per minute
Your pump and well will have to deliver 141.44 gallons of
water per minute to keep your 10-acre farm productive during
the summer months. If you were willing to irrigate 24 hours
per day you would only need an output of 70 GPM (gallons
per minute).

Unit 1.5 | Part 1 – 225

Students’ Hands-On Exercises 5 & 6 Irrigation—Principles & Practices
EXERCISE 6: HOW MANY ACRES CAN I IRRIGATE? 4. Multiply 1.4 (inches per week ET) by 27,158
Someone has just oered you 10 acres of (gallons per acre inch) to get 38,021 gallons per
farmland in the Pajaro Valley on the central coast acre per week to keep your full canopy crops
of California. There is a pump and well on the supplied with adequate water during the summer
property capable of delivering 15 GPM. There are months
no other sources of water in the area. Your daily 5. Assuming your application eciency is 75%,
average ET in the summer is 0.20 inch. How many multiply 38,021 by 1.25 to get 47,526 gallons per
acres of irrigated vegetables can you plant during week
the summer months without running short of
water? 6. Divide 75,600 (maximum pump output per week)
by 47,526 (weekly crop need per acre) to get 1.6
GIVEN acres
• The daily average ET during the summer months Your 15 GPM well is capable of irrigating 1.6 acres
is about 0.20 inch per day of actively growing crop in full canopy during
• There are 27,158 gallons of water in an acre inch the summer months assuming 75% application
eciency and with application happening 12
• The pump ow rate is 15 gallons per minute hours per day. If you are will-ing to irrigate 24
• You are only able to run the irrigation 12 hours per hours per day then you can irrigate 3.2 acres.
day during peak use If you increase your eciency by only using
SOLUTION overhead during the night, and utilize drip tape,
you could in-crease your crop area slightly. If you
1. Multiply 15 gallons per minute (GPM) by 60 (min plant crops with a low moisture requirement and if
per hr) to get 900 gallons per hour your soil and climate are conducive to dry farming
2. Multiply 900 gallons per hour by 84 (hours per (deep clay soil, mild summer temperatures, and
week @ 12 hours per day) to get 75,600 gallons at least 30 inches of precipitation annually during
per week maximum pump output the winter) you might be able to farm the entire
10 acres.
3. If your average ET during the summer months is
.20 inches per day for an actively growing crop in
full canopy, then multiply .20 (daily ET) by 7 (days
per week) to get 1.4 inches per week

Part 1 – 226 | Unit 1.5

Irrigation—Principles & Practices Students’ Hands-On Exercises 5 & 6
Assessment Questions
1) Describe four functions of water in an agricultural system.

2) What is soil saturation?

3) What is eld capacity?

4) What is the level of soil moisture at which most crop plants require additional water?

5) Describe two ways that agriculturists determine the need for irrigation.

6) Number the following stages of crop development in terms of their sensitivity to drought/water
stress (1 being most sensitive and 4 being least sensitive):
____ Flowering
____ Yield formation/fruit set
____ Early vegetative growth
____ Fruit ripening

7) The soil water condition between eld capacity (FC) and permanent wilting point (PWP) is
referred to as:

Unit 1.5 | Part 1 – 227

Assessment Questions Irrigation—Principles & Practices
Assessment Questions Key
1) Describe four functions of water in an
agroecosystem.
• plant support/turgidity
• nutrient transport (soil solution)
• plant cooling through transpiration
• plant nutrient (photosynthesis)
• soil moisture for soil organisms
2) What is soil saturation?
When water is lling all the available pore
spaces in a given soil
3) What is eld capacity?
A soil is at eld capacity when the free water/
gravitational water drains from a saturated soil
4) What is the level of soil moisture at which
most crop plants require additional water?
50% of eld capacity
5) Describe two ways that agriculturists
determine the need for irrigation.
• Qualitative: Measuring for relative
percentages of eld capacity in the root zone
of the crop
• Quantitative: Determining the
evapotranspiration rate of a given site and
systematically replacing the amount of water
lost each week through calibrated water
delivery systems
6) Number the following stages of crop
developmental in terms of their sensitivity to
drought/water stress (1 being most sensitive
and 4 being least sensitive):
1. Flowering
2. Yield formation/fruit set
3. Early vegetative growth
4. Fruit ripening
7) The soil water condition between eld
capacity (FC) and permanent wilting point
(PWP) is referred to as:
Plant available water (PAW)

Part 1 – 228 | Unit 1.5

Irrigation—Principles & Practices Assessment Questions Key
Resources
PRINT RESOURCES for irrigating eld and row crops. Discusses
TECHNICAL RESOURCES energy and management considerations such
as when to irrigate, how much water to apply,
Cleveland, David A. and Daniela Soleri. 1991. Food and how to monitor soil moisture, offers design
from Dryland Gardens: An Ecological and Social considerations and troubleshooting ideas, and
Approach to Small-Scale Household Food Produc- provides an overview of system uniformity and
tion. Tucson, AZ: Center for People, Food and the efciency.
Environment.
An overview of small-scale and community- Hanson, Blaine, Larry Schwankl, and Terry Prich-
based food production techniques intended ard. 1999. Micro-irrigation of Trees and Vines. Pub-
for use by development educators and rural lication 94-01. UC Irrigation Program, UC Davis.
organizers in less developed nations. Encourages Oakland, CA: Division of Agriculture and Natural
the development of gardens that serve local Resources.
needs, that are based on local knowledge, Offers an overview of the rationale for micro-
and that conserve natural resources and the irrigation and how to assemble, operate, and
biodiversity of traditional crops. Includes an maintain such a system.
excellent section on the principles and practices
of low-technology garden-scale irrigation. Hanson Blaine, Steve Orloff, and Blake Sanden.
2007. Monitoring Soil Moisture for Irrigation Water
Hanson, Blaine. 2009. Measuring Irrigation Flow Management. Publication 21635. UC Irrigation Pro-
Rates. Publication 21644. UC Irrigation Program, gram, UC Davis. University of California, Agricul-
UC Davis. Oakland, CA: University of California ture and Natural Resources: Oakland, California.
Division of Agriculture and Natural Resources. Describes techniques for monitoring soil
Provides growers and irrigation professionals moisture as an alternate method to water-based
with information about devices typically used balance methods of managing irrigation water.
to measure ow rates on farms. Includes Using this method you can “see” what is going
descriptions of the various ow meters, their on in the soil and determine answers to some
installation and operation, and the calculations key irrigation management questions.
for determining ow rates and amounts of
applied water. Schwankl, Larry, Blaine Hanson, and Terry Prich-
ard. 2008. Maintaining Micro Irrigation Systems.
Hanson, Blaine, Larry Schwankl, and Allen Fulton. Publication 21637. UC Irrigation Program, UC Da-
2004. Scheduling Irrigations: When and How Much vis. Oakland, CA: University of California Division
Water to Apply. Publication 3396. UC Irrigation Pro- of Agriculture and Natural Resources.
gram, UC Davis. Oakland, CA: University of Califor- Discusses the maintenance issues of
nia Division of Agriculture and Natural Resources. microirrigation systems that can be used on tree
A technical reference for irrigation tools and crops, row crops, and trees and vines.
techniques used in production agriculture.
Includes many common calculations used to RESOURCES ON WATER ISSUES
determine when to irrigate and how much water California Roundtable on Water and Food Supply.
to apply. 2011. Agricultural Water Stewardship: Recommen-
dations to Optimize Outcomes for Specialty Crop
Hanson, Blaine, Larry Schwankl, Steve Orloff, and Growers and the Public in California, June 2011.
Blake Sanden. 2011. Sprinkle Irrigation of Row and Ag Innovations Network.
Field Crops. Publication 3527. Oakland, CA: Divi-
sion of Agriculture and Natural Resources.
Provides practical information on the design,
management, and maintenance of the sprinkle
irrigation methods commonly used in California

Unit 1.5 | Part 1 – 229

Resources Irrigation—Principles & Practices
California Roundtable on Water and Food Supply. WEBBASED RESOURCES
2014. From Crisis to Connectivity: Renewed Think-
Appropriate Technology Transfer for Rural Areas
ing about Managing California’s Water and Food
(ATTRA) – Drought Resource Guide
Supply, April 2014. Ag Innovations Network.
attra.ncat.org/downloads/drought_RL.html
Roundtable members identied agricultural
water stewardship as a key area of importance Provides a list of journals and websites with
for sound long-term water management. information on general farm management
The group held a series of meetings to build practices that can help mitigate the impacts
a common understanding of agricultural of drought conditions. Accompanies ATTRA
water use, develop a unied set of principles Powerpoint presentations on drought.
that underlie long-term solutions, and create California Agricultural Water Stewardship Initiative
recommendations for decision-makers and
agwaterstewards.org
the public on balanced solutions to tough
agricultural water issues. These reports are the The California Agricultural Water Stewardship
product of those efforts. Initiative (CAWSI) works to raise awareness
about approaches to agricultural water
Carle, David. 2009. Introduction to Water in Cali- management that support the viability of
fornia. California Natural History Guides No. 76. agriculture, conserve water, and protect
Berkeley, CA: University of California Press. ecological integrity in California. A project of
Describes the journey of California’s water, from the Community Alliance of Family Farmers,
snowpack to eld and faucet. Discusses the role CAWSI’s website includes a resource library,
of water in agriculture, the environment, and case studies, information on on-farm practices,
politics, and includes an update on recent water an events calendar, and other resources.
issues facing the state.
California Irrigation Management Information
Donahue, John M., and Barbara Rose Johnston. Systems (CIMIS)
1997. Water, Culture, and Power: Local Struggles in www.cimis.water.ca.gov
a Global Context. Washington, DC: Island Press. California weather information site designed
Presents a series of case studies from around the to help growers, turf managers, and others
world that examine the complex culture and properly time irrigation applications.
power dimensions of water resources and water
resource management. Drought Proong Your Farm Checklist
aginnovations.org/agwaterstewards.org/uploads/
Mount, Jeffrey F. 1995. California Rivers and docs/Cahn-drought_proong_checklist.pdf
Streams: The Conict between Fluvial Process and
Based on a presentation at the 2010 Ecological
Land Use. Berkeley, CA: University of California
Farming Conference in Asilomar, CA by
Press.
Michael Cahn, Cooperative Education irrigation
Provides an overview of processes shaping and water resources advisor for Monterey
California’s rivers and watersheds, and the County. Outlines general strategies and specic
impact on water-ways of different land use steps to take in drought proong your farm.
practices, including agriculture.
Effective Irrigation Practices To Improve Short Term
Pielou, E. C. 1998. Fresh Water. Chicago, IL: Uni- and Long Term Water Management
versity of Chicago Press.
ftp://ftp-fc.sc.egov.usda.gov/CA/news/
A natural history of fresh water that includes an Publications/factsheets/eective_irrigation_
explanation of the dynamics of the water cycle practices.pdf
and groundwater.
Provides step-by-step guidelines to maximize
distribution uniformity, minimize evaporation
losses, and optimize water application timing
and amount decisions for a variety of irrigation
systems, including furrow irrigation, hand-
moved and solid set sprinklers, microirrigation,
and drip irrigation.

Part 1 – 230 | Unit 1.5

Irrigation—Principles & Practices Resources
Estimating Soil Moisture by Feel and Appearance. UC Division of Agriculture and Natural Resources:
USDA NRCS Program Aid Number 1619 Institute for Water Resources, Water and Drought
www.nrcs.usda.gov/wps/portal/nrcs/detail/mt/ Online Seminar Series
newsroom/?cid=nrcs144p2_056492 ciwr.ucanr.edu/California_Drought_Expertise/
PDF available at: www.ext.colostate.edu/sam/ Insights__Water_and_Drought_Online_Seminar_
moisture.pdf Series/
This online seminar series from the University of
This user-friendly guide describes how to use
California, Agriculture and Natural Resources,
the “feel and appearance” method to estimate
developed with support from the California
soil moisture. Includes photos of a range of
Department of Water Resources, brings timely,
soils at various moisture levels and provides
relevant expertise on water and drought from
useful guidelines for estimating soil moisture
around the UC system and beyond directly
conditions, e.g., by using the “squeeze test.”
to interested communities. Topics include
Irrigation Scheduling: The Water Balance Approach. using agroecological practices to enhance the
resilience of organic farms to drought, vineyard
www.ext.colostate.edu/pubs/crops/04707.html
irrigation with limited water, saving water in the
Describes the water balance approach to landscape, and much more.
irrigation scheduling.
UC Division of Agriculture and Natural Resources:
Measuring and Conserving Irrigation Water Irrigation
attra.ncat.org/attra-pub/summaries/summary. www.anrcatalog.ucdavis.edu
php?pub=332
Publications and instructional materials on
Describes how to nd the net water application irrigation.
rate for any irrigation system. Explains how to
calculate the number of hours the system should The WATER Institute
be operated, describes several ways to measure www.oaecwater.org
owing water in an open channel or pipeline,
The WATER Institute (Watershed Advocacy,
and offers suggestions for irrigating with limited
Training, Education, & Research), based at
water supplies.
the Occidental Arts and Ecology Center in
Methods of Determining When To Irrigate. Coop- Occidental, California, promotes understanding
erative Extension, College of Agriculture and Life of the importance of healthy watersheds
Sciences, The University of Arizona. to healthy communities. The Institute’s
website offers numerous resources and links
cals.arizona.edu/pubs/water/az1220/
to important readings about water politics,
Details a variety of techniques used to determine conservation, traditional practices, and water
irrigation scheduling, including the “feel history.
method,” tensiometers and other soil moisture
measuring devices, infrared thermometers that
measure the temperature of the plant canopy,
and computerized irrigation models.

UC Davis Small Farm Center, Family Farm Series

Publications: Vegetable Crop Production—Tips on
Irrigating Vegetables
www.sfc.ucdavis.edu/Pubs/Family_Farm_Series/
Veg/vegcrop.html
Information on pre-irrigation, timing, irrigation
system options, and other useful tips for
irrigating vegetable row crops.

Unit 1.5 | Part 1 – 231

Resources Irrigation—Principles & Practices
Part 1 – 232 | Unit 1.5
Irrigation—Principles & Practices
SUPPLEMENT 1

Evapotranspiration (ET) & Factors that Aect

ET Rates
Many factors affect ET, including weather parameters such as solar radiation, air tempera-
ture, relative humidity, and wind speed; soil factors such as soil texture, structure, density,
and chemistry; and plant factors such as plant type, root depth, foliar density, height, and
stage of growth.

Evapotranspiration (ET) = Evaporation + Transpiration

Evaporation is the transformation of water from a etc.) will transpire more. It is equivalent to breathing
liquid into a gas. Water volatilizes into the air easily, for us – adults use more air than children do, you
especially when it is hot and windy. Evaporation use more when you are exerting yourself, etc.
happens only at the surface of a liquid, so the great- Evapotranspiration or ET is the combined use of
er the surface area-to-volume ratio of the water, the water by plants and loss of water into the air (see
greater the evaporation rate. This means that you Appendix 1: Soil Moisture Terms, for an illustra-
lose more water to evaporation from water sprayed tion). The evapotranspiration rate is the amount of
in drops into the air than you do from water in a water that needs to be replaced over a given amount
drip tape line or in an irrigation canal or ditch. of time to make up for the water that has been
The evaporation rate (i.e., the time it takes for used or volatilized. The evapotranspiration rate is
a certain amount of water to volatilize) for a given measured for mature plants in a given region on a
day can be measured. One way is by placing a given day.
known quantity of water in a container of a known Precipitation and irrigation are the two primary
surface area and timing how long it takes to disap- sources of water that plants use. Plant leaves and
pear. In California, we can also look up this value soil surfaces temporarily retain some part of the wa-
on a state-run website indexed by geographical area, ter applied to the eld, but this part readily evapo-
the California Irrigation Management Information rates. What remains percolates into the soil. Plants
System website, www.cimis.water.ca.gov. extract the inltrated water through their roots and
Transpiration is the transformation and use of transport it up to their leaves for photosynthesis. In
water by a plant. The plant uses water to transport addition to water, plants need carbon dioxide (CO2)
nutrients and air, to maintain its structure, and to and light for photosynthesis. In order to take in CO2
thermoregulate (maintain optimal temperature). The from the atmosphere, plants open their stomata,
transpiration rate of a plant is the amount of water the microscopic pores on the undersides of leaves.
a plant uses up over a given amount of time. This It is during this process that they lose water to the
value is harder to measure, as it is difcult to assess atmosphere.
the minimum amount of water that a plant needs
to be healthy. The plant could be using less water Some Environmental Factors Affecting the
than you are giving it. You could measure this in a Rate of Transpiration
very controlled environment by giving similar plants
different amounts of water and seeing the effects. LIGHT
Fortunately, this can also be looked up. Transpira- Plants transpire more rapidly in the light than in
tion rates found in reference tables are generally for the dark. This is largely because light stimulates the
mature plants; any plants that are working more opening of the stomata. Light also speeds up tran-
(owering, setting fruit, at a critical stage of growth, spiration by warming the leaf.

Unit 1.5 | Part 1 – 233

Supplement 1: Evapotranspiration (ET) Irrigation—Principles & Practices
TEMPERATURE
Plants transpire more rapidly at higher tempera-
tures because water evaporates more rapidly as the
temperature rises. At 86°F, a leaf may transpire three
times as fast as it does at 68°F.
HUMIDITY
When the surrounding air is dry, diffusion of
water out of the leaf happens more rapidly.
WIND
When there is no breeze, the air surrounding a
leaf becomes increasingly humid thus reducing the
rate of transpiration. When a breeze is present, the
humid air is carried away and replaced by drier air.
SOIL WATER
A plant can continue to transpire rapidly if its
water loss is made up by replacement water from
the soil. When absorption of water by the roots fails
to keep up with the rate of transpiration, loss of
turgor (rigidity caused by pressure of water against
cell walls) occurs and the stomata close. This im-
mediately reduces the rate of transpiration (as well
as of photosynthesis). If the loss of turgor extends to
the rest of the leaf and stem, the plant wilts.
The volume of water lost in transpiration can be
very high. It has been estimated that over the grow-
ing season, one acre of corn plants may transpire
400,000 gallons of water. As liquid water, this
would cover the eld with a lake 15 inches deep.

Part 1 – 234 | Unit 1.5

Irrigation—Principles & Practices Supplement 1: Evapotranspiration (ET)
SUPPLEMENT 2

Overview of the “Water Budget Approach” to

Irrigation Management
Water budgets are analogous to maintaining a balanced checkbook. Additions of irrigation
water or rainwater are “deposits” and water use by plants as well as evaporation from the
soil surface are “withdrawals.” The starting point for a water budget is a soil saturated
from either irrigation or rainfall. From that initial point of saturation, water depletion is
monitored and water is applied as needed to maintain a “balanced” system to optimize
plant growth.

This “quantitative” water budget approach to ir- available for a limited number of economically
rigation scheduling has been used successfully by important crops typically produced in large-scale
large-scale farming operations in arid regions of the systems. For this reason this system of irrigation
western United States since the early 1980s. Through management is typically not used in small-scale
a network of regional weather stations, daily weather diverse systems. In its simplest terms the crop evapo-
data including reference ETo is made available to transpiration rate (ETc) equals the crop coefcient
growers in many agricultural regions throughout (Kc) multiplied by the reference crop evapotranspi-
the west. Weather information from these stations is ration rate (ETo).
commonly used by large-scale irrigation managers
ETc = Kc x ETo
and research plot managers to assist in accurately de-
Using corn as an example:
termining how much water to apply to crops in order
• A corn crop at 10 days from emergence would
to avoid over application of irrigation water, while at
have an estimated Kc value of .25
the same time maximizing crop yields of agronomic,
• A corn crop at 45 days from emergence would
orchard and vegetable crops. Though this system of
have an estimated Kc value of .50
irrigation scheduling is simply not practical or ap-
• A corn crop at 100 days from emergence would
propriate for diverse small-scale agricultural systems,
have an estimated Kc value of 1.00
many of the principles are applicable and can be ef-
If the corn crop had been irrigated at time of
fectively used by irrigation managers of smaller scale
planting and the daily ETo averaged .15 inches per
systems as a means of increasing overall irrigation
day for the rst ten days since emergence, then your
efciency on their farms and in their gardens.
irrigation calculation for day ten would be as fol-
From an irrigation standpoint the most impor-
lows:
tant data from this network of weather stations is
what is referred to as “reference crop evapotrans- ETc = .25 (Kc) x 1.5 (.15” ETo per day X 10 days)
piration” (ETo). The ETo is the estimated daily ETc = .375 inches
rate of evapotranspiration from a reference crop,
Based on this equation would you irrigate the
which is either grass or alfalfa in full canopy. In
corn with .375 inches of water on day ten?
most locations these data are given in “inches per
You might be better off accessing soil moisture
day.” With these data a grower can calculate “crop
using a shovel and the “feel” method at this growth
evapotranspiration”(ETc) and determine how much
stage. The Kc is not an absolute number but only
water to apply to an actively growing crop.
an estimate since it would change on a daily basis
The other critical piece of information needed
from emergence of the crop through to matura-
for the irrigation rate calculation is the “crop
tion. What is most important to understand from
coefcient”(Kc). The crop coefcient reects the
this example is that most vegetable crops, when in
stage of growth of the crop from seedling through
full canopy, have a Kc value of 1. If we can get an
full canopy. Crop Coefcient (Kc) information is
accurate estimation, from a local weather station, of

Unit 1.5 | Part 1 – 235

Supplement 2: Overview of the “Water Budget Approach” Irrigation—Principles & Practices
the average ETo rate then we can easily determine
an approximate weekly rate of irrigation for most
crops —when in full canopy—typically grown in
small-scale diverse systems.
For example if our average ETo is about .15
inches per day then we would need to apply roughly
1 inch per week of irrigation water to a crop in full
canopy.
ETo .15” per day times 7 days per week = 1.05”
There are many other considerations to take into
account when making irrigation decisions, includ-
ing soil type, crop type, time of harvest, and irriga-
tion system application uniformity, but the “water
budget” method of irrigation rate calculation does,
when used properly, provide a basis for sound deci-
sion making in small-scale farms and garden—es-
pecially when used in conjunction with the “feel”
method.

Part 1 – 236 | Unit 1.5

Irrigation—Principles & Practices Supplement 2: Overview of the “Water Budget Approach”
SUPPLEMENT 3

Soil Moisture Sensing Instruments Commonly

Used for Irrigation Scheduling
Information from soil moisture sensing instruments can help inform decisions about when
and how much to irrigate vegetable, vine, and tree crops. Although these instruments
can’t replace the knowledge and experience gained from both qualitative (“by feel”) and
quantitative approaches to measuring soil moisture discussed elsewhere in this unit, they can
be used in tandem with these methods to help determine crops’ needs.

There are a number of soil moisture sensors available different depths at the same location. The deeper
to growers, but two general categories have come to location tends to maintain a higher percentage of
be industry standards because of their relative low moisture compared to the more shallow placement,
cost, accuracy, reliability, and ease of use. Currently, and this difference provides the irrigator with a
tensiometers and electrical resistance sensing devices good representation of below-ground moisture dy-
(ERSDs) are the instruments most commonly used in namics that can be a great help in determining both
California’s Central Coast region. timing and amounts of water needed to meet the
crop’s needs over time.
Tensiometers Tensiometers should be placed at a number of
In simple terms, a tensiometer is a tightly sealed plas- locations across the eld to reect different soil and
tic water-lled tube with a semi-porous ceramic tip irrigation conditions. They should be left in place for
at the bottom, which is buried in the soil. A vacuum the duration of the crop cycle and read as often as
gauge near the top of the tube (above grade) provides once a day to inform irrigation scheduling decisions.
constant readings that reect soil moisture conditions Placement location and method of installation
at the depth of the ceramic tip. are critical for accuracy. Tensiometers should be
Starting from a point of eld capacity, as plant placed within the root zone directly in the “wetted”
roots extract available water from the soil, water is area that receives either drip or sprinkler irrigation.
pulled from the sealed tensiometer tube into the sur- In sprinkler-irrigated systems, place the tensiometers
rounding soil. This “pull” or “tension” is measured between sprinkler risers where maximum uniformity
in centibars on the vacuum gauge attached to the is often observed. In drip-irrigated systems, place the
tensiometer. The dryer the soil becomes, due to plant tensiometers off to the side of the drip line but still
extraction of irrigation water, the higher the centibar within the wetting pattern of the drip.
readings; thus a reading of 0 reects saturation and Prior to placing the tensiometer in the soil the
a reading of 100 reects very dry soil. Irrigation is semi-porous ceramic tip must be soaked in water
often required at readings between 30 and 50 centi- overnight to insure that it is adequately moist so
bars, although this can vary considerably depending that water can easily move from the sealed tube into
on crop, soil type, and climate. the surrounding soil.
To install the tensiometer the irrigator makes a
Placement hole in the ground to the desired depth and the same
diameter as the tensiometer. There are dedicated
Tensiometers are placed directly into the most active tools for this purpose, but a soil probe can be used
part of the crop’s root zone, at depths ranging from as long as it is the same diameter as the tensiometer.
6 inches to as deep as 48 inches. The most common A slurry of soil and water is poured into the bottom
placement depths are 6 and 12 inches for shallow- of the hole to ensure good tensiometer-soil contact
rooted crops (e.g., strawberries). (critical for accurate readings), and the tensiometer
Two tensiometers are often placed next to each is then pushed into place in the hole. The tensi-
other so that soil moisture can be monitored at ometer location should be marked with a ag to

Unit 1.5 | Part 1 – 237

Supplement 3: Soil Moisture Sensing Instruments Irrigation—Principles & Practices
facilitate locating the instruments for monitoring. depths and locations, similar to tensiometers, and
Once installed it usually takes several readings over like the tensiometer, a soil/water slurry is used when
a period of several days to start getting accurate the ERSD is installed to establish good soil contact
readings. with the instrument.
Tensiometers have a water reservoir above the To get a reading from the ERSD the irrigator uses
sealed column of water that resupplies the plastic a small, inexpensive, hand-held electrical resistance
column, since the plant roots constantly extract very meter that is temporarily connected to the wire
small quantities of water from the sealed tube. To leads from the buried ERSD. The meter allows a
rell the sealed tube the irrigator simply unscrews very low electrical current to ow between the two
the cap on the reservoir and this opens the seal electrodes in the ERSD and displays an electrical
below the reservoir, allowing the excess water in the resistance reading. This reading reects the amount
reservoir to ow into the lower tube. Once the tube of moisture within the porous material, since the
is lled, a small hand-held suction pump is used to buried ERSD takes on the moisture properties of the
remove air bubbles from the tube. The lid of the surrounding soil. Due to the electrical conductiv-
reservoir is then retightened, sealing the lower tube. ity potential of water, the higher the concentration
It is important to follow all of the manufacturer’s of moisture within the porous block the lower the
recommendations for installation and maintenance, resistance and, conversely, the lower the concentra-
including the use of an additive to minimize algal tion of moisture within the block the higher the
contamination of the water in the tensiometer. resistance.
When used properly, tensiometers will provide At eld capacity the block is wet; as the grow-
accurate “soil/water tension” readings on a range of ing plants start to extract moisture from the soil,
crops. These readings provide the irrigation manager the moisture is also pulled from the ERSD and the
with critical information that can be used to estab- conductivity reading will reect this change in soil
lish irrigation schedules adequate to maintain soil moisture. Note that high salt concentrations in the
moisture at levels conducive to good crop growth soil solution will affect the accuracy of the read-
and productivity. ing, since salts increase electrical conductivity. This
potential salt impact needs to be taken into account
Electrical Resistance Sensing Devices when deciding which monitoring tool is best suited
In many ways electrical resistance sensing devices to your farm.
(ERSDs) are similar to tensiometers—the main dif- Electrical resistance sensing devices are rela-
ference is the method used to measure soil moisture. tively inexpensive and easy to install and monitor.
ERSDs utilize two “electrodes” cast into a porous Like tensiometers, they are left in the eld for the
material (often gypsum based). The two electrodes in duration of the cropping cycle and provide criti-
the “block” are attached to wires that run from the cal irrigation scheduling information that enables
ERSD to the surface. These wires are often protected the irrigation manager to make informed decisions
within a ½-inch PVC tube that is attached to the about irrigation frequency and quantity based on
ERSD. The ESRDs are buried in the soil at various site-specic data.

Part 1 – 238 | Unit 1.5

Irrigation—Principles & Practices Supplement 3: Soil Moisture Sensing Instruments
SUPPLEMENT 4

Overview of Dry Farming on the Central

California Coast
“Dry farming” is a term that growers and consumers on California’s Central Coast use to
describe summer- and fall-harvested orchard, vineyard, and vegetable crops grown without
supplemental irrigation following planting. Rather than rely on irrigation, dry-farmed
crops draw on a reserve of soil moisture “captured” by the grower following winter and
early spring rains.

A limited number of geographic regions are suited February and into March. High pressure then
to dry farming, which requires adequate winter dominates the region from April through September
rainfall and, in the case of annual crops, a summer- and often into October, pushing rainfall to the north
time marine inuence that generates cool mornings during the Central Coast’s long “summer drought.”
and warm afternoons. These conditions, combined Thus the region rarely receives signicant rainfall
with careful soil preparation, appropriate variety from May through September.
selection, adequate plant spacing, and vigilant weed Rainfall amounts vary considerably across the
control are all required for successful dry farming. Central Coast, inuenced in large part by the loca-
tion, height, and orientation of the area’s numerous
A Note About Dry Land Farming mountain ranges. Steeper ranges parallel to the coast
can cause signicant orographic (mountain-induced)
“Dry land farming” is another term commonly used
lifting of moisture-laden air, resulting in high rainfall
in agricultural production. The term typically refers
amounts on the west side of these slopes. These
to winter grain production on non-irrigated crop-
ranges also create rain shadows on the east (inland)
land. Dry land grain is planted in fall and harvested
sides, reducing rainfall in these areas. From San Luis
in spring/early summer, relying on winter rainfall
Obispo County in the south to San Mateo County in
for growth and development. A dry land grain crop
the north, rainfall amounts vary from approximately
usually requires between 10 and 15 inches of annual 8 inches up to approximately 35 inches per year
precipitation for economic yields. In areas where depending on the effects of the mountain ranges and
rainfall is less than 10 inches, with careful soil man- specic storm dynamics.
agement, grain can be produced every other year.
The important distinction between dry farming ADEQUATE WINTER RAINFALL
and dry land grain production is that the grain crop A minimum of 20 inches of rainfall during the
is “rain irrigated” during most of its growth cycle. rainy season is required to create an adequate re-
In contrast, dry-farmed crops experience little or serve of soil moisture for growing most dry-farmed
no rainfall during the growth cycle of the crop. In crops. The challenge for the dry-farm grower is
this supplement we are specically referring to “dry to capture and hold as much of this precipitation
farming.” in the soil as possible so that the spring-planted
dry-farmed crops can access this “stored” moisture
Criteria for Successful Dry Farming during the dry summer months.
MEDITERRANEAN CLIMATE MARITIME INFLUENCE
Central California’s Mediterranean climate cre- The valleys along the coast in Central California
ates the conditions that make dry farming possible. that receive signicant summer time marine inu-
In normal years Central Coast rainfall is generated ence in the form of early morning fog and mild
by storms that develop in the Gulf of Alaska and afternoon high temperatures (highs in the mid 80ºs)
sweep south and then east, moving from the Pacic and evapotranspiration (ET) rates in the range of
Ocean across the region from November through .15 inches per day are ideal for dry farm production.

Unit 1.5 | Part 1 – 239

Supplement 4: Overview of Dry Farming Irrigation—Principles & Practices
 Higher afternoon temperatures and ET rates in soil particles; these channels can be thought of as
the range of .33 inches per day, typically encoun- capillaries within the soil horizon. Polar bonds
tered in the more inland valleys with less marine between water molecules and the forces of cohesion
inuence, are much less suited to dry farming, facilitate water’s upward movement through the
especially of tomatoes, since it can be difcult for soil: as water near the soil surface evaporates, water
the plants to access deeper moisture quickly enough lower in the soil is pulled nearer the surface, much
to maintain turgidity during periods of high evapo- like liquid being drawn through a straw. Thus in
transpiration. However, some crops can be suc- elds destined for dry farming it is critical to break
cessfully dry farmed in inland valleys: although not up the capillaries near the surface to minimize the
within the scope of this article, wine grapes, olives, evaporative loss of residual rain moisture during late
and apricots are successfully dry farmed in Cali- spring and summer.
fornia on small acreages in areas with little or no This breaking of capillaries is typically accom-
maritime inuence. plished with relatively shallow (8”–10”) mechanical
soil tillage. Commonly used tillage tools include ro-
SOIL TYPE
totillers and disc harrows, often followed by second-
The best soils for dry farming have relatively ary tillage implements such as spring tooth harrows.
high clay content. Sandy loam soils or loam soils The resultant tilled zone is called a “dust mulch.”
that overlay deeper clay soils also work well for dry This dust mulch provides an effective barrier to the
farming. Soils higher in sand content do not hold potential evaporative loss of residual rain moisture
soil moisture as well as clay and clay loam soils and held within the root zone of the soon-to-be-planted
therefore are typically not used for dry farming. And dry-farmed crop.
because organic matter increases the soil’s porosity, When creating the initial dust mulch, timing is
it does not improve conditions for dry farming. critical: the grower must trap as much rain moisture
A grower considering dry farming should bore in the soil as possible, yet avoid working the soil
numerous holes up to 4 feet deep throughout the when it is too wet. Wet soils, especially “heavier”
production area using a 2-inch slide hammer and soils high in clay content, are subject to clod forma-
soil probe to obtain soil “plugs”: soils suitable for tion and compaction caused by tractor operations.
dry farming will exhibit continuity within the dif- It is also important to minimize tillage depth
ferent horizons and a loam or sandy loam upper when preparing soil for planting annual dry-farmed
horizon going directly to clay. Horizons with a crops, since deeper tillage could disrupt the lower
larger particle size, e.g., containing sand or gravel, soil capillaries that are critical for soil water move-
will impede water’s ability to be drawn upward to ment below the tilled zone. The dust mulch needs to
the plant’s root zone, thus making dry farming less be maintained with fairly frequent and light tillage
feasible. Preparing and planting a small area of the operations (every two or three weeks) from the time
eld is the best way to determine whether the site of initial tilling until the crops are too large to culti-
and conditions are suited to dry farming. vate effectively.
Although dry farming relies on winter rainfall,
Soil Preparation several scenarios can necessitate irrigation prior to
Soil preparation that conserves or “traps” winter planting. During dry springs it is sometimes neces-
rainfall is critical for successful dry farming. In the sary to pre-irrigate the beds before planting using
spring, prior to planting, residual rain moisture is either overhead irrigation or drip lines in order to
typically lost from the root zone as water percolates establish an optimal stand. When a mechanical
down through the soil horizon with the help of spader is used to incorporate a high residue cover
gravity. High clay content in the soil, and to a lesser crop prior to dry farming it is often necessary, in the
extent soil organic matter (humus), greatly facili- absence of post-tillage rain events, to pre-irrigate
tates the soil’s ability to hold water in the root zone with overhead sprinklers to facilitate the cover
against the pull of gravity. crop’s breakdown. On a garden scale, you may need
As the weather warms, soil moisture is also lost to hand water the newly planted plants to assist in
through surface evaporation. Evaporation occurs as rooting and uniform establishment.
water is drawn upward via small channels between

Part 1 – 240 | Unit 1.5

Irrigation—Principles & Practices Supplement 4: Overview of Dry Farming
 Plant Spacing and Weed Control
The typical springtime dry farm tillage and crop culture Dry-farmed crops with extensive root systems
sequence at the UCSC Farm is as follows: can effectively extract deep residual rain mois-
1 Flail mow cover crop ture from a fairly large area within their roots’
2 Incorporate cover crop residue with mechanical spader grasp. Competition from other nearby crop
3 Form beds with rolling cultivator plants or weeds can result in water-stressed
4 In the absence of rain, pre-irrigate beds with over head plants that produce very little fruit and remain
irrigation at a rate of 1.5 inches per acre (when spring stunted. For this reason it is critical to plant
rains are adequate this step is unnecessary) out dry-farmed crops in a much wider spac-
ing than is typically used for irrigated crops
45 Wait for weed ush and create dust mulch with rolling
of the same type. Good weed management in
cultivator
a dry farm system is also critical, since most
6 Maintain dust mulch with rolling cultivator as needed weeds have aggressive root systems capable of
until planting time outcompeting most crop plants for both water
7 At time of planting break open bed middles with and nutrients.
Alabama shovels and plant tomato transplants deeply As an example of plant spacing, irrigated
into moisture using hand trowels tomatoes are commonly spaced 2 feet apart
8 Cultivate with sweeps and side knives when rst weeds within the row with rows spaced 4 feet apart,
appear in furrow bottoms or as necessary to maintain a density of roughly 5,400 plants per acre.
dust mulch A typical spacing for dry-farmed tomatoes
9 Once plants reach adequate height, reform beds by (depending on soil type and rainfall amounts)
throwing dirt into bed middles with rolling cultivator would be 6 feet between rows and 6 feet
—when timed well this last cultivation pass will also between plants, for a total plant population of
eectively smother weeds starting to establish within 1210 plants per acre. As you can see from this
the plant line example a signicant yield reduction can be
expected from most dry-farmed crops simply
based on per acre plant populations. A higher
price premium for dry-farmed tomatoes will
Variety Selection
often make up for the yield loss related to
In any dry farming system, variety selection is absolutely wider spacing.
critical. Varieties that do well as dry-farmed crops typi-
cally have an aggressive root system capable of reaching Crops Suitable for Dry Farming
deep into the soil horizon to tap the stored rain moisture.
Tomatoes are the most notable dry-farmed
It is interesting to note that growers in the Central
crop produced in the Central Coast region.
Coast region have trialed literally hundreds of varieties of
Dry-farmed tomatoes are typically transplant-
heirloom, open pollinated and hybrid tomatoes and, to
ed into the eld from May through June. It is
date, none have compared to ‘Early Girl’ in their ability
advantageous to plant the tomatoes as deep as
to set roots deep and consistently produce a high yield
possible into the residual rain moisture after
of high quality, avorful, and marketable fruits with no
the dust mulch has been created and when soil
irrigation. ‘New Girl’, a recently introduced variety, is
temperatures are adequate for strong growth
closely related to ‘Early Girl’ and appears to have many of
(>55 ºF). Growers often plant several succes-
the same favorable characteristics.
sions spaced 2 to 3 weeks apart to provide an
extended fall harvest period. Some growers
stake and tie the tomatoes for ease of harvest
and to enhance fruit quality, while others let
the plants vine out on the ground without
support.

Unit 1.5 | Part 1 – 241

Supplement 4: Overview of Dry Farming Irrigation—Principles & Practices
 ‘Early Girl’ and/or ‘New Girl’ are currently the paction from cultivation operations. Problem weeds
tomato varieties of choice. The fruits are easy to are much easier to deal with when irrigation is
handle, they don’t crack, and the avor is remark- eliminated for a season and weed seed development
able. However, when grown without irrigation, is easily minimized in a dry-farmed block. If water is
these varieties are prone to a physiological condi- a limited resource on a farm then dry farming makes
tion known as blossom end rot. Blossom end rot is perfect sense as a means of maintaining production
related to the plant’s inability to move calcium to the while eliminating the need for irrigation. Forcing
blossom end of the fruit, which is exacerbated when deep rooting of dry-farmed crops can also facilitate
water is limited. The symptom is a black sunken the extraction of nutrients that have leached below
spot on the blossom end of the fruit that—depend- the root zone of most irrigated crops through exces-
ing on the severity of the symptom—is prone to rot. sive rainfall or irrigation.
Although the condition often becomes less prevalent Dry farming also heightens the intensity of crop
as the season progresses, it may affect 10–20% of avors. This is particularly true of tomatoes, which
the crop. Fruit showing symptoms of blossom end are highly sought after by savvy consumers and the
rot are not marketable. Central Coast region’s chefs. As a result, the produc-
Other annual vegetable crops that have been tion and sale of dry-farmed tomatoes has become
successfully dry farmed in the Central Coast region an important and economically viable niche market
include dry corn, dry beans, and winter squash, all for small-scale organic specialty crop growers on the
of which are direct seeded into residual rain mois- Central Coast.
ture after the creation of the dust mulch. In a trial Finally, although dry farming may not be appro-
conducted at the UCSC Farm in the mid 1990s we priate for every cropping system and region, under-
showed no signicant difference in yield between standing the basic principles of dry farming can lead
irrigated and dry-farmed Red Curry, Butternut, and to a greater knowledge of the complexities of water
Spaghetti winter squashes. and soil dynamics, tillage, weed management, and
fertility management. This knowledge can in turn
Advantages of Dry Farming lead to a greater understanding of your particular
As a rotation within a diverse irrigated cropping sys- production system. In regions where conserving
tem, dry farming has many advantages. The lack of water is critical, applying dry farming principles to
irrigation in a dry-farmed production block can lead irrigated systems can result in improved water use
to improved soil tilth, since dry surface soil is not efciencies, better weed management, and improved
prone to compaction or clod formation from both soil tilth and productivity.
foot trafc associated with harvest and tractor com-

Part 1 – 242 | Unit 1.5

Irrigation—Principles & Practices Supplement 4: Overview of Dry Farming
SUPPLEMENT 5

Nitrate Contamination of Groundwater

Irrigation accounts for nearly one-third of all water use in the United States, or 128 billion
gallons/day.1 In arid western states, and California in particular, irrigation accounts for
more than half of all water used. California uses about 24.4 billion gallons/day to irrigate
some 9 million acres. This is about 6 times the amount of domestic water used by the entire
U.S. population.2

While these statistics clearly illustrate the enormous in 1960 to 21 million tons in 2010.5 In 2007, Cali-
quantity of water used in agriculture, they also sug- fornia farmers applied 740,00 tons of nitrogen in
gest that irrigation has far-reaching consequences fertilizers to 6.7 million acres of irrigated farmland.6
on water quality. With cheap sources of nitrogen and water available,
In an effort to maximize crop yields, many farm- our current agricultural system is based on the lib-
ers apply nitrogen-based synthetic fertilizers. More eral application of synthetic fertilizers and irrigation
than half of the nitrogen applied may go unused by water to ensure high yields, often at the expense of
crops, ending up in surface water runoff or leaching environmental and public health.
into groundwater and causing severe water quality California’s Central Valley is home to some of
and other public health concerns for rural com- the most heavily fertilized cropland and some of the
munities, many populated by poor, immigrant farm most polluted water in the United States. Communi-
workers.3 4 As this supplement illustrates, how farm- ties there are particularly vulnerable to public health
ers use irrigation and apply fertilizers affects not effects of nitrate contamination because groundwa-
only their crops, but also their neighbors. ter provides drinking water for the majority of resi-
Synthetic nitrogen-based fertilizers were made dents. Additionally, rural communities in the valley
possible because of the Haber-Bosch process, which are generally poor and populated by immigrants and
converts stable, inert nitrogen gas (N2) unavailable minorities least able to afford treatment costs and
to plants into the reactive ammonia molecule (NH3) most vulnerable to discriminatory decision-making.
readily available for plant uptake. Once the process Tulare County, the second most productive
was commercialized, synthetic fertilizer use skyrock- agricultural county in California, includes many of
eted, as farmers were no longer dependent only on these communities. Though it generates nearly $5
their soil organic matter, compost, cover crops, and billion in revenue from agriculture each year, it has
livestock manure for nitrogen. Fertilizer use in the the highest poverty rate in California and is popu-
United States increased from about 7.5 million tons lated mainly by minorities (66%), most of whom

1,2 Kenny, Joan F. et al. Estimated use of water in the United States
in 2005. Circular 1344, pp. 23–24. U.S. Department of the
Interior: U.S. Geological Survey.
pubs.usgs.gov/circ/1344/pdf/c1344.pdf
3 J.L. Hateld, J. L., and J. H. Prueger. 2004. Nitrogen over-use,
under-use, and eciency. New directions for a diverse planet,
proceedings of the 4th International Crop Science Congress, 5 USDA, Economic Research Service. Fertilizer use and price.
26 Sep – 1 Oct 2004, Brisbane, Australia. Published on CDROM. Table 1: U.S. consumption of nitrogen, phosphate, and potash,
Website: www.cropscience.org.au 1960-2011. www.ers.usda.gov/data-products/fertilizer-use-
4 Moore, Eli, and Eyal Matalon. 2011. The human costs of and-price.aspx#26720
nitrate-contaminated drinking water in the San Joaquin 6 Harter, Thomas. 2009. Agricultural impacts on groundwater
Valley. Pacic Institute. www.pacinst.org/wp-content/uploads/ nitrate. Southwest Hydrology, July/August 2009.
sites/21/2013/02/nitrate_contamination3.pdf www.swhydro.arizona.edu/archive/V8_N4/feature2.pdf

Unit 1.5 | Part 1 – 243

Supplement 5: Nitrate Contamination of Groundwater Irrigation—Principles & Practices
are Latino.7 8 9 The average per capita income in the High nitrate levels in water can cause a num-
county is $18,021.10 Here, one in ve small public ber of health problems, including skin rashes, eye
water systems and two in ve private domestic wells irritation, and hair loss. More severe is “Blue Baby
surpass the maximum contaminant level (MCL) Syndrome” (methemoglobinemia), a potentially fatal
for nitrates.11 12 As a result, residents of towns like blood disorder in infants caused by consumption of
Seville, East Orosi, and Tooleville are paying $60 nitrate-contaminated water. Direct ingestion, intake
per month for nitrate-contaminated water they can’t through juices from concentrate, and bottle-fed
safely use, and must spend an additional $60 to infant formula are all potential threats to children.
purchase bottled water for drinking and bathing. Nitrate contamination has also been linked to thy-
In contrast, San Francisco water customers pay $26 roid cancer in women. Widespread contamination of
per month for pristine water from the Hetch Hetchy groundwater through leached fertilizer has rendered
water system in Yosemite. drinking water in rural communities across the
The economic cost of nitrate contamination in country not only unusable, but dangerously so.
drinking water is not the only cost to these commu- While nitrate contamination is an acute problem
nities. Farm workers make up a signicant segment in California, it exists across the country. The EPA
of the population of small towns throughout the estimates that over half of all community and do-
Central Valley and are both directly exposed to the mestic water wells have detectable levels of nitrates.14
hazards of heavy fertilizer use in the elds and in Rural communities that rely on private wells (which
the air, and through excess nitrogen leached into are unregulated), or lack access to adequate water
groundwater drinking supplies. Scientists estimate treatment facilities, have the most insecure water
that 50–80% of nitrogen applied in fertilizer is un- supplies.
used by plants. Of that, about 25% volatilizes into In the short term, municipalities must devise a
the atmosphere (some as nitrous oxide, the most plan to reduce the disproportionately high cost of
potent greenhouse gas). As a result, approximately water to these communities. One potential solution
30–50% of nitrogen applied in fertilizer—about 80 is a fee attached to the purchase of fertilizer used to
pounds per acre in California—leaches into ground- subsidize water costs for communities with contami-
water beneath irrigated lands and into public and nated water. Communities with contaminated water
private water supplies.13 could also be added to a nearby water district with
access to clean water.
In the longer term, the obvious solution is to sub-
stantially reduce synthetic fertilizer and water use
in agriculture. Treatment, while effective on a small
scale, cannot keep up with the vast quantities of
7 USDA, National Agricultural Statistics Service. 2012. 2012
Census of agriculture, county data. www.agcensus.usda.gov/
nitrates continually entering groundwater supplies
Publications/2012/Full_Report/Volume_1,_Chapter_2_County_ through fertilizer application. Similarly, reduced
Level/California/st06_2_002_002.pdf irrigation on farms, drawn mostly from uncon-
8 USDA Economic Research Service. 2012. County-level data sets. taminated sources, frees up new sources of drink-
www.ers.usda.gov/data-products/county-level-data-sets/poverty.
aspx#Pa8c98972d6c14aaea0543afd59db4088_3_382iT4
ing water for nearby communities. Lastly, to reach
9,10 United States Census Bureau. State and county QuickFacts, a truly sustainable and equitable system of water
Tulare County. distribution, residents of rural communities must
quickfacts.census.gov/qfd/states/06/06107.html be included in the planning and decision-making
11 California State Water Resources Control Board. Groundwater process as members of local water boards, irrigation
ambient monitoring and assessment (GAMA). Domestic well
project, groundwater quality data report, Tulare County focus districts, and planning commissions to establish and
area. Table 2: Summary of detections above drinking water safeguard their right to uncontaminated water.
standards.
www.swrcb.ca.gov/gama/docs/tularesummaryreport.pdf
12 Brown, Patricia Leigh. 2012. The problem is clear: The water is
lthy. New York Times, November 13, 2012.
www.nytimes.com/2012/11/14/us/tainted-water-in-california-
farmworker-communities.html?pagewanted=all&_r=0
13 Harter, Thomas. 2009. Agricultural impacts on groundwater 14 California State Water Resources Control Board. 2010.
nitrate. Southwest Hydrology, July/August 2009. Groundwater information sheet.
www.swhydro.arizona.edu/archive/V8_N4/feature2.pdf www.waterboards.ca.gov/gama/docs/coc_nitrate.pdf

Part 1 – 244 | Unit 1.5

Irrigation—Principles & Practices Supplement 5: Nitrate Contamination of Groundwater
Appendix 1: Water Cycling Terms

Transpiration: water loss from the plant leaves Over application of irrigation water can cause
Evaporation: water loss from the soil surface leaching of nitrogen and phosphorus from
the root zone and can cause contamination of
Transpiration + Evaporation = Evapotranspiration (ET)
aquifers, streams, ponds, and lakes.
Initial state Initial state
SATURATION

Illustrations by José Miguel Mayo

Initial state
Unit 1.5 | Part 1 – 245
Appendix 1: Water Cycling Terms Irrigation—Principles & Practices
Appendix 1 (cont.): Water Cycling Terms

Amount of water the soil can hold against Point at which the plant can no longer
the pull of gravity. uptake water held tightly to the surface of
the soil particles.

Initial state Initial state

Illustrations by José Miguel Mayo

Part 1 – 246 | Unit 1.5

Irrigation—Principles & Practices Appendix 1: Water Cycling Terms
Appendix 2: Units of Water Measurement

Illustration by José Miguel Mayo

Unit 1.5 | Part 1 – 247

Appendix 2: Units of Water Measurement Irrigation—Principles & Practices
Appendix 3: Calculating Distribution Uniformity
(DU)
Distribution uniformity (DU) is dened as how uniformly water is distributed
over an area being irrigated, and is expressed as a percentage: the higher the
percentage value, the more evenly water is distributed.

You can calculate distribution uniformity by collecting water in sample “catch

buckets” or rain gauges laid out in an even grid between irrigation pipes or
along the length of a drip line (see diagram). DU can then be determined using
the “low quarter DU” method, in which the average of the lowest quarter of the
samples collected is divided by the average of all samples to get a percent of
distribution: 80% or above is considered an acceptable distribution uniformity.

SPRINKLER UNIFORMITY OF DISTRIBUTION TEST

Low quarter DU calculation: Average volume of lowest 25% of catch buckets divided by Average
volume of all samples collected = Distribution Uniformity (DU)

Illustration by José Miguel Mayo

Part 1 – 248 | Unit 1.5

Irrigation—Principles & Practices Appendix 3: Calculating Distribution Uniformity (DU)
Appendix 3 (cont.): Calculating Distribution
Uniformity (DU)

To calculate DU:
1 Lay out an evenly spaced grid of catch buckets or rain gauges between irrigation pipes or along
a drip line (see diagram on page I-248); note that if you use the Taylor style rain gauges that
measure precipitation/irrigation in inches, you can simultaneously test for application rate as well
as uniformity
2 Run irrigation for 5 minutes
3 Measure and record the volume of water in each catch bucket or rain gauge
4 Rank the volume of water collected in each bucket or rain gauge, from lowest to highest
5 Calculate the average volume collected in the lowest 25% of catch buckets or rain
gauges, and divide that number by the average volume of all the samples collected to
get DU (measured as a percentage)

Example:
Average volume of lowest 25% of catch buckets = 4 inches
Average volume of all samples collected = 5 inches
DU = 4 divided by 5 = 80%

A low DU percentage (less than 80%) indicates poor distribution uniformity, i.e., one area of the
eld or bed is receiving signicantly more irrigation water than other areas. Sources of poor
uniformity can include malfunctioning or clogged sprinkler heads, dierences in nozzle orice
sizes across a eld, improper pipe spacing, improper operating pressure (too high or too low),
windy conditions, and dierences in pressure due to slope.

A similar DU test can be done for drip irrigation systems:

1 Once the system is brought up to pressure, collect water for a set amount of time (e.g., 5 minutes)
in shallow containers placed beneath emitters, evenly spaced along drip lines at a number of
locations in the eld. Bury the trays to grade level so that they do not create undulations that
might impact distribution uniformity
2 Make sure that all the containers are under the emitters for the same length of time
3 Measure and record the volume of water in each container
4 Use the “low quarter DU” method to calculate distribution uniformity. Note that using a
number of containers/data points divisible by four will make the calculations easier.

Drip system uniformity can also be tested by taking pressure measurements using Shrader valves
throughout the eld. See a presentation of this method at:
www.agwaterquality.org/toms%20presentation%20DU%20in%20Drip%20and%20Sprinkler.pdf

Unit 1.5 | Part 1 – 249

Appendix 3: Calculating Distribution Uniformity (DU) Irrigation—Principles & Practices
Appendix 4: Estimating Soil Moisture by Feel
SOIL MOISTURE LEVEL COARSE LIGHT MEDIUM HEAVY
% OF FIELD CAPACITY SAND LOAMY SAND, FINE, SANDY LOAM, CLAY LOAM, CLAY
SANDY LOAM SILT LOAM

0–25% Dry, loose, single Dry, loose, clods Crumbly, dry, Hard, rm baked,
grained, ows easily crushed and powdery, will barely cracked. Usually too
No available soil through ngers. will ow through maintain shape. stiff or tough to work
moisture. Plants wilt. No stain or smear ngers. No stain or Clods, breaks down or ribbon1 by squeezing
on ngers. smear on ngers. easily. May leave between thumb or
slight smear or stain forenger. May leave
when worked with slight smear or stain.
hands or ngers.

25–50% Appears dry; will not Appears dry; may May form a weak Pliable, forms a ball;
retain shape when tend to make a cast2 ball2 under pressure will ribbon but usually
Moisture is available, squeezed in hand. when squeezed in but will still be breaks or is crumbly.
but level is low. hand, but seldom crumbly. Color is May leave slight stain
will hold together. pale with no obvious or smear.
moisture.

50–75% Color is darkened Color is darkened Color is darkened Color is darkened

with obvious with obvious from obvious with obvious moisture.
Moisture is available. moisture. Soil may moisture. Soil moisture. Forms a Forms good ball.
Level is moderate to stick together in very forms weak ball or ball. Works easily, Ribbons easily, has
high. weak cast or ball. cast under pressure. clods are soft with slick feel. Leaves stain
Slight nger stain, mellow feel. Will on ngers.
but no ribbon when stain nger and
squeezed between have slick feel when
thumb and forenger. squeezed.

75% to eld Appears and feels Appears and feels Appears and feels Color is darkened.
moist. Color is moist. Color is moist. Color is Appears moist; may
capacity darkened. May darkened. Forms darkened. Has a feel sticky. Ribbons out
(100%) form weak cast or cast or ball. Will not smooth, mellow easily, smears and stains
Soil moisture ball. Will leave wet ribbon, but will feel. Forms ball and hand, leaves wet outline.
level following an outline or slight show smear or stain will ribbon when Forms good ball.
irrigation. smear on hand. and leave wet outline squeezed. Stains and
on hand. smears. Leaves wet
outline on hand.

1
Ribbon is formed by squeezing and working soil between thumb and forenger
2
Cast or ball is formed by squeezing soil in hand

See also:
USDA, Natural Resources Conservation Service. 1998. Estimating Soil Moisture by Feel and Appearance. Program Aid Number 1619. www.nrcs.
usda.gov/wps/portal/nrcs/detail/mt/newsroom/?cid=nrcs144p2_056492

Part 1 – 250 | Unit 1.5

Irrigation—Principles & Practices Appendix 4: Estimating Soil Moisture by Feel
Appendix 4 (cont.): Estimating Soil Moisture
By Feel

Using the “squeeze test” to

estimate soil moisture

Illustrations by José Miguel Mayo

Unit 1.5 | Part 1 – 251

Appendix 4: Estimating Soil Moisture by Feel Irrigation—Principles & Practices
Appendix 5: Critical Periods for Soil Water Stress
by Crop

Apples: During spring growth, owering, Lettuce, leaf: All stages, pre-harvest
fruit set and development Melons: Flowering and fruit set
Arugula: During vegetative growth Onions, garlic, shallots: During bulb
Basil: Maturity, to prevent stress-induced enlargement
owering Parsley: All stages
Beans: Flowering, seed set, pod Parsnips: Early root development
development
Peas: Flowering, pollination, pod
Beets: Regular water as roots develop enlargement
Broccoli: Head development Pears: During spring growth, owering and
Brussels Sprouts: Vegetative and sprout fruit set
development Peppers: All stages, but allow dry-down
Cabbage: Head development between waterings
Carrots: Early root development, regular Plums: During spring growth, owering,
water to prevent cracking fruit set and development
Cauliower: Head development Potatoes: Tuber enlargement, from ower
Cilantro: During vegetative growth to die-back

Collards: During vegetative growth Pumpkins: Flowering, fruit set and

development
Corn: During crown root development, at
pollination and kernel development Radishes: All stages

Cucumbers: Flowering and fruit Small grains: During crown root

development development, heading, owering

Eggplant: All stages Squashes (summer and winter): Flowering,

fruit development
Fennel: Bulb development
Tomatoes: All stages, but especially
Kiwifruit: During spring growth, owering, owering and fruiting
and fruit set
Flowers: Bud development through
Leeks: All stages owering, and pre-harvest
Lettuce, head: Head development,
pre-harvest

Part 1 – 252 | Unit 1.5

Irrigation—Principles & Practices Appendix 5: Critical Periods for Soil Water Stress by Crop
Appendix 6: General Irrigation Rules

During the owering and fruit set stages of crop development, plants are most sensitive to
drought/water stress.
Most crops require irrigation when the soil moisture in the root zone of the plant has
decreased to ~50% of eld capacity. Use Appendix 4, Estimating Soil Moisture By Feel, to
help you determine the moisture content of the soil.
Seed beds containing small-seeded, directly sown crops require light and frequent water
applications. Apply water each time 50% of the surface soil has dried down, showing
discoloration (see Appendix 6, Garden-Scale Seed Bed Irrigation in Unit 1.4, Transplanting
and Direct Seeding).
Seed beds containing large-seeded, directly sown crops require less frequent water applications.
Apply water each time the soil at the depth of the seed has dried to 50% of
eld capacity. Use Appendix 4 to help you determine the moisture content of the soil.

ADDENDA TO THE GENERAL RULES

1 Potatoes: Phase 1 and phase 4 (the planting and maturation stages) require the full soil
moisture uctuation between 50% and 100% of eld capacity. Phase 2 and phase 3 (tuber
initiation and enlargement) demand less of a uctuation, responding favorably to a moisture
swing between 75% and 100% of eld capacity.
2 Other Solanaceae family crops (e.g., tomatoes, peppers, eggplant) respond favorably to a full
swing between 50% and 100% of eld capacity
3 Cut owers: Irrigation 24 hours prior to harvest will help assure full turgor pressure at harvest
time and increase the vase life of the stems or bouquets
4 Leafy greens: 50% of eld capacity minimum
5 Alliums: 50% of eld capacity minimum
6 Established fresh beans and peas: 50% of eld capacity minimum
7 Celery responds favorably to a moisture swing between 75%–100% of eld capacity
8 It is important to calculate irrigation system uniformity. This information is critical for accurate
determination of irrigation application rates; see Appendix 3: Calculating Distribution
Uniformity (DU)
9 For best yield, turgidity and post harvest handling of brassicas, lettuce, leafy greens and carrots
it is advisable to irrigate as close to harvest as possible, especially during warm weather
10 Over application of irrigation water will increase cost of production, limit deeper rooting of
some crops, potentially leach water-soluble nutrients from the root zone, enhance weed
pressure, and enhance soilborne and foliar disease pressure
11 Delivery system design is critical when utilizing well water when the pump delivers water
directly to the delivery system

Unit 1.5 | Part 1 – 253

Appendix 6: General Irrigation Rules Irrigation—Principles & Practices
Appendix 7: Irrigation for Various Vegetable Crops
SHALLOW ROOTS – 6 to 24 inches

MEDIUM ROOTS – 24 to 40 inches

DEEP ROOTS – more than 40 inches
Arugula: Frequent shallow water to maintain avor and succulence and support SHALLOW
rapid growth.
Asparagus: Water deeply and infrequently. Allow to dry down between watering. DEEP
Basil: Somewhat thirsty. Important to water prior to harvest. MEDIUM
Beans, fresh: Can drink lots of water because they are fast growing. Once MEDIUM
fruit is set, can often “nish” the crop with less or no water to enhance avor.
Vulnerable to disease with overhead water.
Beans, dry: Treat as fresh beans until seeds begin to mature, then gradually cease SHALLOW
application of water.
Beets: Give adequate supply of water as lack thereof during warm weather causes plants MEDIUM
to bolt or beet roots to crack and become tough and woody.
Broccoli: Commercial growers use 1-1-1/2” per week. Extra water during crown SHALLOW
development will add bulk to the harvest.
Brussels Sprouts: Not very ecient at water uptake so require evenly moist soil to SHALLOW
function at best. 70-80% of the roots are concentrated at top 8-12” of soil.
Cabbage: Needs even moisture or heads will crack. Not very ecient at SHALLOW
water uptake.
Cabbage, Napa: Keep ground moist. SHALLOW

Carrots: Need deep watering until later stages of root development, at which time MEDIUM
excess water can cause roots to crack. Cracking is also caused by too great a
uctuation between wet and dry.
Cauliower: Keep soil evenly moist. SHALLOW

Celeriac: Thirsty like celery, but more tolerant of wet/dry swings. SHALLOW

Celery: Thirsty; needs frequent irrigation to get well established. Do not overhead water SHALLOW
because susceptible to fungal disease. Heavy feeder.
Chard: Likes moist roots, bolts from water stress. MEDIUM
Cilantro: Keep moist to forestall bolting. SHALLOW

Corn: Adequate moisture is critical from tasseling through kernel formation and harvest. SHALLOW
Do not over water dry corn (e.g., popcorn and ornamental) at maturity; let it dry out
on stalk.
Cucumber: Sensitive to disturbance. Needs consistently moist soil, watered at base. SHALLOW TO
Susceptible to fungal disease spread through wet leaves. Lack of water when fruits MEDIUM
are developing will cut down on production.
Eggplant: Need sucient moisture. Will always benet from supplemental fertility. MEDIUM
Fennel: Likes adequate moisture but not demanding. MEDIUM

Part 1 – 254 | Unit 1.5

Irrigation—Principles & Practices Appendix 7: Irrigation for Various Vegetable Crops
Appendix 7 (cont): Irrigation for
Various Vegetable Crops SHALLOW ROOTS – 6 to 24 inches

MEDIUM ROOTS – 24 to 40 inches

DEEP ROOTS – more than 40 inches

Flowers: Root depth and water needs vary by species. Generally important to supply
regular water during bud formation and owering.
Garlic: Likes steady supply of water. Stop watering several weeks before harvest to SHALLOW
reduce succulence and therefore reduce rot during drying.
Kale: Average water needs, except during warm weather when more water is MEDIUM
required to prevent wilting.
Kohlrabi: Must have even moisture to be tender. SHALLOW

Leeks: Never let the soil dry out. SHALLOW

Lettuce: Water consistently to avoid bitter taste. SHALLOW

Musk melons: Like a constant supply of moisture. Susceptible to foliar disease, so MEDIUM
avoid overhead watering.
Onions: Steady supply of moisture; if too dry, onions get a strong unpleasant avor. SHALLOW
Avoid water on leaves to minimize downy mildew.
Parsley: Somewhat thirsty. SHALLOW

Parsnips: Water lovers. DEEP

Peas: need adequate moisture at owering and pod enlargement. Avoid water on MEDIUM
leaves to minimize mildew.
Peppers: Constant and even moisture from ower through fruit. Peppers like to dry MEDIUM
down before being watered again. Will always benet from supplemental fertility.
Potatoes: Even moisture. This is especially critical during period of tuber enlargement SHALLOW
which begins at blossom. Cut back on water as vines die back, to cure the skins.
Pumpkins: Water deep and infrequent. DEEP
Radishes: Need adequate moisture – dry soil results in tough, woody radishes, and SHALLOW
vulnerability to ea beetles. Moisture swings cause cracking.
Rutabaga: Provide even moisture. Roots will become tough as a result of the DEEP
development of extra xylem cells if always forced to bring water up from a deep
soil level.
Salad mix: Water consistently for succulent growth and to avoid bitter taste. SHALLOW

Spinach: Keep evenly moist to forestall bolting. SHALLOW

Squash, summer: Rapid growth and ongoing fruit production requires frequent MEDIUM
deep water.
Squash, winter: Do well with deep and infrequent waterings. Avoid overhead water DEEP
to prevent foliar disease.
Tomatoes: Like to dry down before being watered again. When blossoming begins, DEEP
keep soil moisture a little bit drier. Imbalances of moisture may lead to blossom
end rot and fruit cracking.
Turnips: Roots will become tough as a result of the development of extra xylem cells MEDIUM
if always forced to bring water up from a deep soil level.

Unit 1.5 | Part 1 – 255

Appendix 7: Irrigation for Various Vegetable Crops Irrigation—Principles & Practices
Appendix 8: Soil Probe & Soil Auger

Soil Probe Soil Auger

Illustrations by José Miguel Mayo

Part 1 – 256 | Unit 1.5

Irrigation—Principles & Practices Appendix 8: Soil Auger and Soil Probe
Appendix 9: Irrigation System Components

An easy-to-build, portable riser system of PVC and micro sprinklers can be used to
irrigate garden beds.

Illustration by José Miguel Mayo

Unit 1.5 | Part 1 – 257

Appendix 9: Irrigation System Components Irrigation—Principles & Practices
Appendix 9 (cont.): Irrigation System
Components

1 Overhead aluminum irrigation 2 Impact heads (also known as 3 Latches connect 4 End caps seal 5 and 6 : T’s and
pipes deliver water to elds. Pipes sprinkler heads) come up from the pipes to each other o the open end elbows are used to
(also called joints) commonly come main pipe and release water over and to end caps, T’s of a line of pipe. connect pieces of
in three sizes (2”, 3” and 4”) the eld. Risers connect impact and elbows. pipe together.
head to pipes. If necessary, valves
can be added to risers to shut o
individual sprinklers.

Illustrations by José Miguel Mayo

Part 1 – 258 | Unit 1.5

Irrigation—Principles & Practices Appendix 9: Irrigation System Components
Appendix 10: Field Irrigation Schedule

CROPS/FIELD DATE TIME AMOUNT INCHES IRRIGATION METHOD COMMENTS

Unit 1.5 | Part 1 – 259

Appendix 10: Field Irrigation Schedule Irrigation—Principles & Practices
Appendix 11: Garden Irrigation Schedule
CROP/BED DATE TIME AMOUNT IRRIGATION METHOD COMMENTS
INCHES OR TIME

Part 1 – 260 | Unit 1.5

Irrigation—Principles & Practices Appendix 11: Garden Irrigation Schedule
Appendix 12: Amount of Water Needed to
Pre-Irrigate a Dry Soil to Dierent Depths
(Approximate)*

SOIL TYPE INCHES WATER PER FOOT INCHES WATER TO REACH

SOIL DEPTH 6 FEET DEEP

CLAY 1.41.8 8.610.8

SILTY CLAY 1.61.9 9.611.4
SANDY CLAY 1.61.9 9.611.4
SILTY CLAY LOAM 2.22.3 13.013.7
CLAY LOAM 2.02.2 12.213.0
SANDY CLAY LOAM 2.02.2 12.213.0
SILTLOAM 1.82.0 10.812.2
LOAM 1.71.9 10.111.4
VERY FINE SANDY LOAM 1.71.9 10.111.4
SANDY LOAM 1.11.3 6.57.9
LOAMY VERY FINE SAND 1.11.3 6.57.9
LOAMY FINE SAND 1.01.2 5.87.2
LOAMY SAND 0.71.0 4.35.8
VERY FINE SAND 0.71.0 4.35.8
FINE SAND 0.71.0 4.35.8
SAND 0.71.0 4.35.8
COARSE SAND AND GRAVEL 0.40.7 2.24.3

* Based on available water holding capacity; plants have dried soil to permanent wilting point, 15 ATM.
Assumes the soil is uniform throughout irrigation depth.

Appendix 12: Amount of Water Needed to Pre-Irrigate Unit 1.5 | Part 1 – 261
Irrigation—Principles & Practices
Appendix 13: Sample Sprinkler & Drip Tape
Application Rate Calculations

Sample sprinkler application rate Sample drip application rate

calculation calculation
You determine your sprinkler risers put out Flow rate: .75 gpm per 100 feet @ 8 psi
3.5 gpm (gallons per minute) at 60 psi (from label)
30 x 30 spacing = 64 risers per acre Bed spacing 2 feet
64 risers x 3.5 gpm = 227.5 gpm Bed length 100 feet
227.5 gpm x 60 = 13,650 gph (gallons per 218 beds per acre
hour)
218 beds x 100 feet = 21,800 bed feet
There are 27,154 gallons of water per acre
per acre inch
Total bed feet divided by 100 = 218
27,154 divided by 2 = 13,577 gallons 100-foot sections per acre
So, 13,650 gallons per hour = 218 sections x .75 gpm (per 100 feet) =
approximately .5 inch application 163.5 gpm
rate per hour
163.5 gpm x 60 minutes per hour =
If your application uniformity is 80% then 9,810 gph
your application rate is closer to .4 inches
9,810 gph = 2.76 hours to apply
per hour (.5 x 80%)
one acre inch
27,154 gallons per acre inch divided by
9,810 gph = 2.76 hours

Part 1 – 262 | Unit 1.5

Irrigation—Principles & Practices Appendix 13: Sample Sprinkler & Drip Tape Application Rate