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WRE 451: Hydrology, Irrigation and Flood Management

Course Content:
• Introduction
• Plant-soil-water relationship
• Consumptive use and estimation of irrigation water requirements
• Methods of irrigation
• Quality of irrigation water
• Problems of irrigated land
• Flood and its management

Lecture 2: Introduction and Water Law of Bangladesh

Multipurpose Irrigation Projects

Schematic Diagram of Irrigation System

Types of Surface Irrigation Systems

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Components of Drip Irrigation System

Components of a Typical Sprinkler Irrigation System

Schematic Diagram of a Typical Skimming Tubewell

SoilPlant Water Relationship

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SoilPlantWater Relationship

Water Policy (1999)

 Support of private development of groundwater irrigation for promoting agricultural growth will
continue, alongside surface water development where feasible.
 But there will be a renewed focus towards increasing efficiency of water use in irrigation through
various measures including drainagewater recycling, rotational irrigation, adoption of water
conserving crop technology where feasible, and conjunctive use of groundwater and surface water.
 Water allocations in irrigation systems have to be done with equity and social justice. At the same
time, serious consideration should be given to nonpoint pollution of water systems by fertilizer
and pesticides that are either leached to the groundwater or washed off the fields to rivers and
lakes.
 Encourage and promote continued development of minor irrigation, where feasible, without affecting
drinking water supplies
 Encourage future groundwater development for irrigation by both the public and the private sectors,
subject to regulations that may be prescribed by Government from time to time.
 Improve efficiency of resource utilization through conjunctive use of all forms of surface water and
groundwater for irrigation and urban water supply. Strengthen crop diversification programs for
efficient water utilization .
 Strengthen the regulatory system for agricultural chemicals that pollute ground and surface water,
and develop control mechanism for reducing nonpoint pollution from agrochemicals.
 Strengthen appropriate monitoring organizations for tracking groundwater recharge, surface and
groundwater use, and changes in surface and groundwater quality.

Objectives of Irrigation

 Ensure enough moisture essential for plant growth.

 Provide crop insurance against short duration drought.
 Cool the soil & atmosphere to provide a suitable surrounding.
 Wash out/dilute harmful salts, chemical in the soil.
 Reduce hazards of soil piping.
 Soften the tillage pan.

Irrigation: An engineering Perspective

 Planning and designing on efficient and low cost irrigation system.

 Controlling the various natural sources of water by the construction of dams and reservoirs, canals
and headworks and finally distributing the water to the agricultural land.
 Drainage of waterlogged areas and generation of hydroelectric power.

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Irrigation: Concept

 crop Plants need water for proper growth and development.

 The demand for water must be met by the water in the soil, via the root system .
 If the crop water demand is met by other ways (such as rainfall, capillary rise from groundwater
table, etc.), there is no need of irrigation.
 Irrigation requirement for cereals and noncereals are not the same.
 Among cereals, irrigation requirement of rice is the highest, whereas for wheat it is less.
 Proper irrigation scheduling also affects the irrigation requirement of different crops.

Irrigation: Challenges
Challenges in irrigation and water management for world food security and agricultural trade arise from
several aspects:

 Domestic resources condition

 Restriction of economic development level
 Climate change
 Pollution of water resources due to industrial and agricultural effluents
 Competition of water among different sectors of users
 International markets and price
 Agriculture is acknowledged as the principal source of income about 84% population involved.
 The existing land to man ratio for Bangladesh is 0.058 ha/person, cultivable land.
 Arable land is declining per year at the rate of about 1%.
 Land use intensity has already reached about 200%, perhaps the highest in the world.
 The excess water during the monsoon causes wide spread flooding, which damages crops.
 The inadequate flow in the river system during the dry months hampers irrigation

Irrigation: Impacts
Water maintains a host of natural ecosystems. Withdrawal of water from upstream can reduce the flow at
downstream needed to sustain natural ecosystem. The off take and diversion structures often deprive
downstream users of their water.
Some noted impacts of irrigation may

 Water quality degradation

 Groundwater abstraction
 Waterlogging and salinity
 Health risk be
 Simplification and homogenization of the world’s ecosystem

Cropping Pattern of Bangladesh

The crops in this country are grown throughout the year in three distinct cropping seasons:

 The first is the “KharifI” season lasting from the end of March to May, which is a moderately
humid period.
 The second is KharifII season or the hot monsoon season, covering the period from May to
September, is characterized by high humidity and low solar radiation. More than 80% of the total
annual rainfall occurs in this period.
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 Finally, we have the “Rabi” season from mid-October to early March, which is a cool, dry winter
season.
 The Kharif crops include rice, jute, sugarcane, sesame,
 Rabi crops include boro mugbean , etc. rice, wheat, potato, mustard, pulses, vegetables, spices, etc.
 60 variations in cropping patterns in Bangladesh (Hossain, 1990 ). Most of the cropping patterns are
based on rice or have rice in common with other crops.

Water dependency of the crop seasons are as follows:

 KharifI: Partially irrigation dependent

 Kharif II: Natural rainfall dependent
 Rabi: Irrigation dependent

Major Source of Irrigation Water in Bangladesh

 The major source of irrigation in Bangladesh is groundwater (covers about 76.5% of the total
irrigated area). Groundwater is abstracted through deep tube wells (DTW) and shallow tube wells
(STW).
 Groundwater in Bangladesh is available at comparatively shallow depths during the period of
August October (at the end of the rainy season) and is lowest in the period of April In May (at the
end of the dry season).
 Rajshahi , Bogra , Pabna, Mymensingh, and Dhaka, groundwater abstraction is causing a large
decline in groundwater levels during dry season.
 For maximizing the economic returns from the limited water resources available, it is more
advantageous to encourage the low water consuming crops.
 However, the cultural behavior could hamper the policy significantly.

Irrigation Expansion in Bangladesh

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Factors Affecting Irrigation Planning

 Soil  Commodity/product market

 Climate  National policy and priority
 Topography  Institutional infrastructure
 Water source  Economic factor
 Crop(s) to be cultivated  Environmental aspect
 Energy  Sociocultural aspect
 Labor
 Capital

Lecture 3: Introduction and Importance of Irrigation

Application of Weather Forecasting in Agricultural Decision Making
 Forecast of relevant components of climate variability in relevant periods, at an appropriate
scale, with sufficient accuracy and lead time for relevant decisions.
 Effective communication of relevant information.
 There should be existence of decision options that are sensitive to the incremental information
that forecasts, provide, and compatible with decision maker’s goals and constraints.
 There should be institutional commitment and favorable policies.

Application of Weather Forecasting

The major fields of applications of weather forecast are:
 Determination of appropriate time for sowing or harvesting of crop.
 Based on weather forecasts, producers may adjust their inputs.
 Estimation of irrigation or drainage need (or other cultural operation) and preparedness for
that.
 Preparedness for any hazardous weather phenomena.
 Analysis in exploring decision options and their risks.
 Use of seasonal forecasts in commodity forecasting for government policy support and for
decision making in industry.
 Development of trade policies based on commodity forecast.
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Agro-Climatic Indices
The most common agro-climatic indices are:
 Degree-Days or Growing-Degree-Days (GDD),
 Crop Heat Units (CHU),
 Heliothermal Units (HTU), and
 Photo-Thermal Units (PTU).

Degree-Days or Growing-Degree-Days (GDD)

 GDD is widely used for describing the temperature responses to growth and development of
crops.
 GDDs required to reach maturity (or to reach a particular phase) are calculated following
Nuttonson (1995):

Crop Heat Units (CHU)

Crop heat unit (CHU) for cereals may be calculated by the formula given by Cutforth and Shaykewich
(1990):

Heliothermal Units (HTU)

Heliothermal unit (HTU) is calculated by multiplying degree-days with daily actual sunshine hours:
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where SH is the daily actual sunshine hour.

Photo-Thermal Units (PTU)

Photo-thermal units (PTU) is the product of GDD and corresponding day length for that day. On daily
basis,

Problem
Find out the GDD, CHU, HTU, and PTU for wheat crop for the following days using the data given

Climatic Potential Yield

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 Crop yield in a particular environment is an interaction of genotype environment.

 The upper limit of crop production is set by the climatic conditions and the genetic potential of
the crop. Environmental potential can not be exploited unless the genetic potential reaches to
the environmental potential.
 Chang (1981) demonstrated that crop response to fertilizer application is reduced in areas of
low climatic yield potential.
 Crop production in an area can be better described as (Ghuman and Singh 1993):
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Lecture 4: Plant-Soil-Water Relationship

Plant-Soil Water Relationship
 Movement of water occurs in response to differences in the potential energy of water (from an
area of relatively high-water potential to an area of relatively low water potential).
 Although water uptake by plants is under physiological control, it is often described as a purely
physical process, as a consequence of gradients in water potential in the soil - plant -
atmosphere continuum (SPAC).
 The SPAC is the pathway for water moving from soil through plants to the atmosphere.
 The SPAC constitutes a physically integrated, dynamic system in which various flow processes
(e.g., solar energy interception, plant transpiration, water movement through plant system,
and water movement from soil to plant root hair) occur simultaneously and independently,
like links in a chain.
 About 99% of all the water that enters the roots leaves the plant’s leaves via the stomata.
 On a dry, warm, sunny day, a leaf can evaporate 100% of its water weight in just an hour.
 Water movement is due to differences in potential between soil, root, stem, leaf, and
atmosphere.
 Under normal conditions, the water potential in soil is higher than that in root saps or fluids.
 Typical moist soil might have a water potential of about -0.3 to -1.0 bar,
 root tissue about -4.0 bar, • stem about -7.0 bar,
 leaf about -10.0 to -12.0 bar, and
 typical dry atmosphere about -400 to -600 bar.


 Irrigation water management at any level requires a thorough understanding of the relations
between soil and water.
 These relations are governed by intermolecular forces and tensions, which give rise to the
“capillary phenomenon.”
 These forces in unsaturated soils are influenced by soil texture and structure.
 It is on the basis of the soil texture and structure that water retention capacity or water holding
capacity of the soil varies.
 Soil zone or root zone: depth at which
the root can be penetrated.
 This zone is important since the plants
uptakes water from here.
 Part of water from precipitation or irrigation
is being absorbed in this zone and the rest moves
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downward due to gravity, which is known as

gravity water.
Soil-Water Content
Water content of a soil specimen is defined as the amount of water that is lost from the soil upon
drying at 105°C, expressed either as the weight of water per unit of dry soil, or as the volume of water
per unit volume of soil in bulk.
Factors Affecting Soil Water
The amount of water that is retained in unit volume of soil depends on various factors:
 Size and distribution of soil particles (texture)
 Pore space and their orientation
 Organic matter content
 Soil structure
 Soil temperature
 Compaction/over-burden pressure

Soil physical properties influencing irrigation

The most important soil properties influencing irrigation:
 Soil texture
 Soil structure
 Infiltration capacity of soil
 Water holding capacity of soil
 Depth to water table
 Capillary conductivity
 Soil profile conditions
Soil Properties
The primary components of soil:
(a) Minerals (b) Water (c) Air
(d) Organic matter

Sand particle: 2 – 0.05 mm

Silt particle: 0.05 – 0.002 mm
Clay particle: < 0.002 mm
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Soil Texture
• Soils are generally called gravel, sand, silt or clay,
depending on their predominant size of particles
within the soil.
• Natural Soil is mixture of two or more of these
constituents.
• Organic materials are partly or fully decomposed
in natural soil.
• Texture can be determined from grain-size
distribution using textural classification chart.
• The geometry of voids created in the soil matrix is
dependent on the textural classification of soil. The
soil texture, therefore, influences considerably the
other phases (water and air) contained in the
spaces of soil matrix.
 Sandy soils are loose and non-cohesive and have a low water holding capacity. Such soils form
relatively simple capillary systems, which ensure good drainage and aeration.
 The clay particles are usually aggregated together into complex granules. Because of their
platelike shape, clay particles have a much greater surface area than cubes or spheres of
similar volume. Their extensive surface enables clay particles to hold more water and minerals
than sandy soils.

Soil Structure
The arrangement of individual soil particles is called soil structure.
 Sand-sized aggregates are more favorable for plant growth than very small and very large ones.
 Large pores induce aeration and infiltration, medium-sized pores facilitate capillary
conductivity, and small pores induce greater water holding capacity.
 Rounded edges of the aggregates result in better pore distribution than angular ones. Compact
soil restricts aeration and root spread.
 For optimum crop growth, soil structure should be such that the infiltration capacity is large,
the percolation capacity is medium and aeration is sufficient (not excessive).
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Problem
A cylinder was carefully pushed into the soil without disturbing the soil. The cross-sectional area of
the cylinder was 40 cm2. The length of the column of soil within the cylinder was 30 cm. The weight of
the soil within the cylinder was 1.65 kg when it was dried. The weight of the soil before drying was 2.0
kg. Determine the following:
1. Bulk density of soil
2. Percent moisture in weight basis 3. Percent moisture in volume basis
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Soil Water Measurement

 Good on-farm irrigation water management is the routine monitoring and measurement of soil
water.
 It helps to avoid economic losses due to:
 effect of under-irrigation and over-irrigation on crop yield and quality,
 waste of water and energy, and
 leaching of nutrients and agro-chemicals into surface and groundwater.
 Techniques for measuring the soil moisture can be grouped into the following two categories:
(a) Direct measurement
(b) Indirect measurement

Direct measurement
 Gravimetric or thermo-gravimetric procedure is the only one technique.
 Gravimetric technique requires the removal of soil from the field and conveyed to a laboratory
for weighing, and removal of soil moisture by heating in an oven.
Advantages of Gravimetric Method
 The direct and most reliable method
 Requires simple equipment, and the equipment are not so expensive, thus affordable by
various categories of users
 Requires less expertise, and thus usable by farmers
 With the use of core sampler (known internal diameter and height), both bulk density and
moisture percentage (in volume basis) can be determined with one set of sample
 No radiation hazard

Disadvantages of Gravimetric Method

 Compaction during core sampling may cause error in bulk density and volumetric moisture
content, if the edge of the core is not sharpened.
 For loose, upper-layer soil (especially after plowing and at the initial stage of the crop), core
sampling is problematic and estimation of bulk density may be erroneous.
 Destructive sampling is required, thus not possible of second time sampling at the same place.
 When the crop covers the field and root system develops, sampling hampers the crop.
 Time consuming and labor intensive
 Difficult in stony or rocky soil
Indirect measurement
The indirect measurement category includes:
1. Radiological method – Neutron scattering, gamma attenuation technique (e.g., using neutron
moisture meter)
2. Electromagnetic method – Time domain reflectometry (TDR), TDR FM, Diviner
3. Tensiometry method – using tensiometer
4. Psychrometer method
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Radiological method
In this method two approaches are used: (a) Neutron scattering, and (b) Gamma attenuation
Principle
 The neutron source emits (yields) a huge neutron per second. The hydrogen in soilwater
thermalizes (slow down) the fast neutrons and the slow neutron comes back near the source,
which is detected (counted) by a detector placed above the source.
 There is a direct relationship between the number of slow neutron coming back and the
amount of hydrogen present in soil.
 An electronic processor converts this information into amount of water.

Electromagnetic Method
Electromagnetic methods are based on measurements of electrical properties of soil, which are closely
related to soil-water.
 Dielectric constant of the soil water medium and then estimates soil-water content.
 This category includes time domain reflectometry (TDR), frequency domain reflectometry,
soil capacitance measurement, TDR FM, etc.
 TDR is accurate and automatable method for the determination of water content and
electrical conductivity of porous media.
 Another approach is resistivity method. Gypsum blocks are within this category.

• A TDR sensor/probe is inserted into the soil

and a step-voltage pulse is launched from a
TDR instrument through a coaxial cable to
the sensor.
• The sensor serves as a wave guide and the soil
as the dielectric medium.
• A part of the voltage pulse is reflected back when
it enters the sensor from the coaxial cable.
• Another part reaches the end of the sensor and is
then reflected back to the source.
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Tensiometry Method
 This method measures the potential or energy of soil water.
 Soil-water is brought into equilibrium with water in a porous sensing element, usually ceramic,
which is connected to a manometer or suction gauge, and the actual free energy or potential is
read directly in appropriate energy units.
When the water-filled (atmospheric pressure) porous ceramic cup of the tensiometer comes into
contact with the dry soil (negative pressure), the water from the cup comes out until equilibrium is
reached. The resulting rise in mercury in the tube (in case of mercury tensiometer) or the deflection
of the gauge (in case of gauge tensiometer) indicates the soil-water tension.

Limitations of Tensiometer
(a) It covers only a limited range of soil moisture scale, (0 – 0.8 bar).
(b) If the pores in the tensiometer are very small, the range can be extended slightly, but the
response time for fluctuations in soil potential becomes excessive. If the sensor is large with
relatively coarse pores, it can temporarily alter the soil environment by excess flow of water
from the device into the soil.
(c) Tensiometer readings reflect only the soil moisture tension (which is surrounding the porous
cup), but not the amount of water held in the soil.

Psychrometer Method
 This approach measures the vapor pressure of the water in equilibrium with the soil and hence
measures the total soil-water potential.
 The technique has been used mainly in the laboratory under rigorous controlled conditions,
but is now showing up in the field.
 In its simplest form, the extension of a strip of Cellophane in a gauge container embedded in
the soil is read remotely by the change of resistance in a frictionless potentiometer linked to
the system. More precise thermocouple psychrometers are now being used widely.
 This technique is one of the main tools of the plant physiologist in measuring water stress in
plants.
Soil moisture tension
 In unsaturated soils, water is held in the soil matrix under negative pressure due to attraction
of the soil matrix for water
 Instead of referring to this negative pressure the water is said to be subjected to a tension
exerted by the soil matrix
 The tension with which the water is held in unsaturated soil is termed as soil moisture tension
or soil-moisture suction. It is usually expressed in atm, bars, or kPa.
Other pressure units like cm of water or cm or mm of mercury are also often used.
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Soil moisture stress

 The osmotic pressure developed by the soil solution (soil-water-salt content) retards the
uptake of water by plants.
 Plant growth is a function of the soil moisture stress which is the sum of the soil moisture
tension and osmotic pressure of soil solution.
 For successful crop production in soils having appreciable salts, the osmotic pressure of the
soil solution must be maintained as low as possible by controlled leaching and the soil
moisture tension is the root zone is maintained in a range that will provide adequate moisture
to the crop.

Soil moisture characteristics

 Moisture extraction curves, also called moisture characteristics curves, which are plots of
moisture content versus moisture tension, show the amount of moisture a given soil holds at
various tensions.
 Knowledge of the amount of water held by the soil at various tensions is required in order to
understand the amount of water that is available to plants, the water that can be taken up by
the soil.
 Normally “Field capacity” ranges from 1/10 to 1/3 atm and “Permanent Wilting Point” ranges
from 10 to 20 atm.
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Plant-Soil-Water Relationship: Terminologies

Saturation Capacity
• Saturation capacity is the percentage water content of a soil fully saturated with water
and all its pores completely filled with water under restricted drainage.
• It is also termed as maximum water holding capacity.
• Complete saturation occurs in surface soil immediately after heavy irrigation and rain.
• The soil water is in free state and the tension at this stage is zero.

Field Capacity
 The soil is at field capacity when all the gravitational water has been drained and a vertical
movement of water due to gravity is negligible.
 Water removal for most of the soils will require at least 7 kPa (7 cbars) tension.
 Field capacity consists of two parts: i) Capillary water: water attached to soil particle due to
surface tension, and ii) Hygroscopic water: water can’t be removed through capillary and not
available to plants.
 Soil water tension at field capacity ranges from 0.1 atm to 0.33 atm.

Permanent Wilting Point (PWP)

 It refers to the soil moisture content at which plants do not get enough water to meet the
transpiration demand and remain wilted unless water is added to the soil.
 It is the moisture content of the soil when plants growing on that soil starts to show signs of
wilting due to moisture stress.
 Permanent wilting point is considered as the lowest limit of available water range.
 Soil water tension at PWP ranges from 7 to 32 atm depending on several factors.
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Oven Dry Soil

 Oven dry soil is used to describe the soil water status when a soil sample is dried at 105oC in a
hot air oven until the sample loses no more water i.e. for 24 hours.
 The equilibrium tension of soil water at this stage is 10,000 atm.
 Al estimations of soil water content are based on the oven dry weight of the soil and the soil at
this stage is considered to contain zero amount of water.

Lecture 5: Plant-Soil-Water Relationship (contd..)

Schematics of Soil Water Constants and Soil Water Ranges
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Soil Moisture Monitoring

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Terminologies

• Crop Period: Duration between the sowing and harvesting time.

• Base Period: Duration between first irrigation of a crop at the time of its sowing to its last irrigation
before harvesting. In general, crop period is higher than base period. However, those terms have been
used interchangeably. The unit is days and denoted as B.
• Delta: Total depth of water required for a crop to get mature including the percolation and
evaporation losses.
• Duty: It is the relationship between the volume of water and the area of the crop it matures. Duty is
represented by D. Duty of water for a crop is the number of hectares of land which the unit flow of
water for B days can be irrigated.

Volume 86400 B 8.64 B

•Total depth of water applied on field,delta (∆)= = (hector )= ( meter)
Area D D
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Factors Affecting Duty:

 Type of Crop
 Climate and Season
 Useful Rainfall
 Type of Soil
 Efficiency of Cultivation Method

It helps in planning and improving the efficiency of overall irrigation system. It also helps in:
 It helps in designing efficient canal irrigation system
 Helps in taking precautions in field preparation and sowing
 Helps in taking precautions in handling irrigation supplies

Terminologies
Irrigation Efficiency:
Water Conveyance Efficiency: Ratio of water delivered into field to water entering into the channel at
starting point. Considers convenience and transit loss.
Water Application Efficiency: Ratio of quantity of water stored in the root zone to the quantity actually
delivered in to the field. Also known as farm efficiency, considers water losses in the farm
Water Storage Efficiency: Ratio of the water store in the root zone during irrigation to water needed in
the root zone prior irrigation.
Water Use Efficiency: Ratio of the water beneficially used (including leaching water) to the quantity of
water delivered.
Water Distribution Efficiency:
d
(
ηd = 1−
D )
where D = mean depth of water stored during irrigation
d = average of the absolute values of deviations from the mean.

 If the depth of water penetration at five points in a field is 2.0, 1.9, 1.8, 1.6, and 1.5 m, how can
we calculate the distribution efficiency?
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Consumptive Use:
 For a crop, the total amount of water used in transpiration and evaporation in any specified
time. Denoted as Cu
 Varies for different crops.
 It can vary throughout a day, month or crop period.
 It is being used to determine the irrigation requirement.

Effective Rainfall:
Amount of precipitation available to meet the evapotranspiration of a crop. Denoted by R e.

Consumptive Irrigation Requirement (CIR):

Amount required to meet the evapotranspiration during full growth period.

CIR = Cu – Re.

Net Irrigation Requirement (NIR):

Amount of irrigation water required to meet not only the evapotranspiration but other (e.g., leaching)
NIR = Cu – Re + water lost as percolation.

Problem
1. After how many days, will you supply water to soil in order to ensure efficient irrigation of the
given crop if:
i) Field capacity of soil = 28%
ii) Permanent wilting point = 13%
iii) Dry density of soil = 13 gm/cc
iv) Effective root zone depth = 70 cm
v) Daily consumptive use = 12 mm
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2. From a stream, 130 l/s was diverted to an irrigation canal and 100 l/s was delivered to an
irrigation field (1.6 ha), which was being irrigated for 8 hours. The effective root zone depth
was 1.7m. The runoff loss in the field was 420 m3. The depth of water penetration varied
linearly from 1.7 m at the head of the field to 1.1 m at the tail end. Available moisture holding
capacity of the soil is 20 cm/m depth of soil. Calculate:
i) conveyance efficiency,
ii) application efficiency,
iii) storage efficiency,
iv) distribution efficiency.
Assume, irrigation was started at a moisture extraction level of 50% of the available
moisture.
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Consumptive Use
Estimation of Consumptive Use:
 Blaney-Criddle Equation
 Hargreaves Class A Pan Evaporation Method
 Penman’s Equation
These are indirect measurement methods.

Blaney-Criddle Equation
The monthly consumptive use is given by:
k.p
𝐶𝑢 = 40 ¿.8𝑡 + 32)
where, Cu = Monthly consumptive use in cm
k = Crop factor, under the environmental conditions of the particular area
t = Mean monthly temperature in °C
p = Monthly percent of annual day light hours that occur during the period
p
If ¿.8𝑡 + 32)is represented by f, we get
40
pt
𝐶𝑢 = 𝑘𝑓 If t is in ° F, 𝑓 = 40

It has been widely used for estimating seasonal water requirements all over the world.
• However, the k based estimation is found too low for short periods between irrigations.
• Therefore, the modified equation is: 𝐶𝑢 = 𝑘 Σ𝑓
• The value of crop factor should be determine for each crop at different place.
• Crop factor information is not available for our country (possible research area)
• Some factors i.e. humidity, wind velocity, elevation etc. are not considered.
• In subcontinent, this method is not widely used.
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Problem
3. Determine the volume of water required to be diverted from the head works to the irrigation
field (500 ha) using the following data. Assume 80% as the effective precipitation to take care
of the consumptive use of the crop. Also assume 50% efficiency of water application in the field
and 75% as the conveyance efficiency.
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Lecture 7: Consumptive Use and Irrigation Water Requirements

Hargreaves Class A Pan Evaporation Method
 Evapotranspiration is related to pan evaporation and a coefficient K, which is also termed as
consumptive use coefficient.
𝐶𝑢 𝑜𝑟 𝐸𝑡 = 𝐾. 𝐸𝑝
 K varies among crops, also location dependent.
 Dependent on crop growth stage (not constant all over the base period).
 Crops can be grouped in eight categories and coefficients have been suggested by researchers
in different parts of the world (Please consult with book).
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Class A Pan Evaporation

• Class A pan evaporation can be experimentally determined by directly measuring the
quantity of water evaporated from the standard class A pan.
• The pan is 1.2 m diameter, 25 cm deep and bottom is raised 15 cm above the ground.
• The depth of water is to be kept in a fixed range, where water surface is at least 5 cm
and never below 7.5 below the top of pan.
Pan evaporation (Ep) can be determined using Christiansen Formula:

𝐸𝑝 = 0.459. 𝑅. 𝐶𝑡. 𝐶𝑤 . 𝐶ℎ . 𝐶𝑠 . 𝐶𝑒
R = Extra terrestrial radiation (unit same as Ep)
Ct = Coefficient for temperature (°C), given by 𝐶𝑡 = 0.393 + 0.02796 𝑇𝑐 + 0.0001189𝑇𝑐2
Cw= Coefficient for wind velocity, given by: 𝐶𝑤 = 0.708 + 0.0034𝑊 − 0.0000038𝑊2
W is the wind velocity 0.5 meter above the ground (km/day).
Ch = Coefficient for relative humidity, 𝐶ℎ = 1.250 − 0.0087𝐻 + 0.75 × 104𝐻2 − 0.85 × 10−8𝐻4
H is the mean percentage of relative humidity at noon or average relative humidity for 11 and 18
hours.
Cs = Coefficient for sunshine 𝐶𝑠 = 0.542 + 0.008𝑆 − 0.78 × 10−4𝑆2 + 0.62 × 10−6𝑆3
S is mean sunshine percentage hours
Ce = Coefficient of elevation 𝐶𝑒 = 0.97 + 0.00984 𝐸
E is the elevation in 100 meters.
 29

Problem
4. Determine the pan evaporation from the following data for April.
Location: 15°19’,
Elevation +449 m
Mean Temperature = 31.8°C
Mean wind velocity = 183 km/day
Mean relative humidity = 40%
Mean sunshine percent = 89%
What is the consumptive use for the month for a crop having a consumptive use coefficient = 0.80.
 30

5. Compute the consumptive use and other irrigation requirement if application and conveyance
efficiency are 85% for the given crop data.
 31

Penman’s Equation
This equation has been derived by combining energy
balance and mass transfer approach for transpiration
and evaporation. Considering some modifications,
the final form is:
A . Hn+ Ea γ
𝐸𝑡= A +γ

A = Slope of the saturation vapor pressure

vs temperature.
 32

Factors Influencing Evapotranspiration

1. Crop factor
2. Weather factor
3. Soil factor
4. Management factors
Crop factors influencing ET include crop type, variety or species, stage of crop development (depth of
root zone, leaf area), etc. The major plant or crop factors as follows:
(a) Crop characteristics
(b) Leaf number and leaf area
(c) Leaf architecture
(d) Leaf rolling or folding
(e) Number and size of stomata
(f) Stomatal closure
(g) Root – its depth, density, pattern

Measurement of Evapotranspiration
1. Direct Measurement (Lysimeter Test)
2. Indirect Measurement
Crop Coefficient (Kc)
Crop coefficient (Kc) is defined as the ratio of the actual evapotranspiration of a disease free crop
grown in a large field adequately supplied evapotranspiration.

Reference Evapotranspiration
The reference evapotranspiration (ET0), as defined by FAO1992 (Smith et al. 1992), is the rate of
evapotranspiration from a hypothetic crop with an assumed crop height (12 cm) and a fixed canopy
resistance (70 s m-1) and albedo (0.23), which would closely resemble evapotranspiration from an
extensive surface of green grass cover of uniform height, the green grass actively growing, completely
shading the ground and having adequate water.

Crop Coefficient (Kc)

Kc depends on the following factors:
 Crop and cultivar type
 Growth stage or phase of the crop
 Percent ground coverage by the crop
 Management system (low or high management)
 Local climatic condition
 Length of growing season or individual growth stage
 Frequency of rainfall or irrigation
 Irrigation history in the early phase of development
 33

 Method of computing ET0

Evaporation Process
 Evaporation is an energy-dependent process. In addition, energy is also needed to vaporize
(heat of vaporization).
 The molecules of water at the surface of a water-body receive energy from solar radiation and
adjacent air. When the kinetic energy as developed from the solar radiation is sufficient enough
to overcome the intermolecular force, the water molecule escapes from the liquid surface (i.e.,
evaporates).
 As the evaporation process continues, the air over the water surface becomes humid, close to
saturation (decreases the vapor pressure difference of air and water), and the evaporation rate
decreases.
 Evaporation rate is controlled by vapor pressure deficit of air.

Factors Influencing Evaporation

Evaporation from free water surface is influenced by the following:
 Available energy
 Humidity of the adjacent air
 Temperature of the air
 Air movement/wind flow
 Purities/impurities of water
 Pressure over the water surface

Evaporation from Soil Surface

Evaporation of water from a thoroughly wetted soil may be characterized by three stages:
 A first or constant stage: when the soil surface is wet with water moving primarily in the liquid
phase, and evaporation is controlled by external condition.
 A second stage of short duration and of rapidly decreasing evaporation rate: this begins when
dry surface soil first appears.
 A third stage of slow evaporation rate: where water movement through the dry surface is
mainly by vapor diffusion, and evaporation is controlled by internal conditions.

Factors Influencing Evaporation from Soil

Evaporation from soil surface is influenced by the following:
 Degree of soil saturation/amount of soil water
 34

 Type of soil
 Percent shading
 Rate of drying
 Tillage or mulching
 Depth of tillage
Factors Influencing Deep Percolation
 Soil type (texture)
 Structure
 Sizes of pore
 Topography
 Soil management
 Surface cover
 Cropping pattern or rotation
 Crop root zone depth
 Rainfall pattern or irrigation type (sprinkler, drip irrigation emitters, etc.) and amount
 Stream size • Frequency of irrigation or intensity and temporal distribution of rainfall
 Opportunity time for infiltration and/or percolation
 Atmospheric evaporative demand
 Subsoil hydraulic conductivity
 Presence of plow pan or hard layer below the surface layer
 Depth to ground-water level
Deep percolation can be extremely variable within one soil type and within one field, and irrigation.
With saline water and gypsum application, can increase deep drainage. In cracking soil, the initial rate
of percolation is high unless the cracks (which are formed due to dryness) are filled up by swelling the
soil.

Lecture 8: Methods of Irrigation

Irrigation Methods
 The application of water to soils for crop use is referred to as irrigation.
 Irrigation types range from the simple hand watering method to the huge flood and furrow
irrigation systems.
 Surface (gravity-driven surface irrigation), sprinkler, drip/micro, and subsurface are types of
irrigation methods.
 Each system has its advantages and disadvantages.
 Water losses from irrigation vary with the type of irrigation method.
 The water management decisions strongly influence how uniform water can be applied
through different irrigation methods to provide optimal soil water conditions for crop growth
and marketable yields.
 The most appropriate irrigation method for an area depends upon:
 physical site conditions,
 the crops being grown,
 amount of water available, and
 management skill.
 35

Water application/irrigation methods can be classified based on different themes:

A. Based on energy/pressure required
B. Based on placement of irrigation water
C. Based on wetted area by irrigation

Classification system – A
Based on energy/pressure requirement, irrigation methods can be grouped as
1. Gravity irrigation and
2. Pressurized irrigation
Gravity irrigation may be subdivided based into:
1. Border irrigation
2. Basin irrigation (either check basin or contour basin)
3. Furrow irrigation

Pressure irrigation system may be subdivided into:

1. Drip irrigation
2. Sprinkler irrigation

Irrigation Methods Definition

Gravity irrigation: The water is not pumped but flows and is distributed to the crop field by gravity.
Pressurized irrigation: Irrigation system in which water is pumped and flows to the crop field by
pressure.
Surface irrigation: A form of irrigation where the soil surface is used as a conduit/conveyor.
Subsurface irrigation or subirrigation: Applying irrigation water below the ground surface either by
raising the water table within or near the root zone.
Border irrigation: Border irrigation is defined as the application of water to an area typically
downslope and surrounded by two border ridges or dikes to the ends of the strip.
Basin irrigation: Is the application of water to an area typically leveled to zero slope and surrounded
by dikes or check banks to prevent runoff.
Furrow irrigation: A partial surface flooding method of irrigation in which water is applied in furrows
(narrow channels dug between the rows of crops).
Sprinkler irrigation: Water is applied by means of nozzle or perforated pipe that operates under
pressure in the form of a spray pattern.
Drip irrigation: An irrigation system in which water is applied directly to the root zone of plants.
Flood irrigation: The entire soil surface of the field is covered by ponded water.
 36

Factors Affecting Irrigation Method Selection

 Soil type
 Field shape/geometry and topography
 Climate – evaporation rates, wind, and rainfall
 Water availability and its price
 Water quality
 The particular crop to be grown – physical requirements, crop layout, and water use
characteristics
 Required depth and frequency of irrigation application
 Labor requirements and its availability
 Energy requirement
 Economic factor – cost–benefit ratio, initial investment
 Compatibility with existing farm equipment
 Attainable irrigation efficiency of the proposed system
 Relative advantages and disadvantages of the available systems
 Type/level of technology at the locality
 Cultural factor/previous experience with irrigation
 Automation capacity
 Environmental conditions – impact and regulations
 Farm machinery and equipment requirements

Border Irrigation Method

 Border irrigation is a modern method of surface irrigation.
 Uses land to be formed into strips, bounded by ridges or borders.
 Prepared with zero side slope and a small but uniform longitudinal slope (~1%).
 Water is applied at the upper end of the border strip, and advances down the strip.
 Irrigation takes place by allowing the flow to advance and infiltrate along the border.
Advantages
I. Easy to construct and maintain
II. Operational system is simple and easy
III. High irrigation efficiencies are possible if properly designed, but rarely obtained in practice
due to difficulty of balancing the advance and recession phases of water application
IV. Natural drainage is facilitated through downward slope
V. Comparatively less labor is required
Limitations
I. Requires flat and smooth topography
II. Not suitable for sandy soils
III. Not suitable for crops which requires ponding water
IV. Higher amount of water is required compared to sprinkler or drip irrigation.
 37

Surface Irrigation System

Important variables in surface irrigation system includes:
 infiltration rate,
 surface roughness,
 size of stream,
 slope of land surface,
 erosion hazard,
 rate of advance,
 length of run,
 depth of flow,
 depth of water to be applied,
 infiltration depth.

Border Irrigation: Design Considerations

Design Parameters
The main design parameters for border irrigation system include the following:
• unit flow rate, Q or q
• length of border, L
• width of border, W
• slope, S
• cutoff time, tco

Two approaches:
(a) Design discharge (Q) for a predetermined border size and slope (L, W, S)
(b) Design L, W, S for a given Q

To ensure adequate spread of water over the entire border, a minimum allowable inflow rate qmin
must be used. The following equation was proposed by SCS (USDA, 1974) to estimate q min:
 38

When the soil erodibility causes restrictions on q, the maximum allowable inflow rate qmax can be
obtained using the empirical method proposed by SCS (USDA, 1974), where qmax is expressed as a
function of field slope S0 and type of crop, sod, and nonsod, by:

𝑞𝑚𝑎𝑥 = 𝐶 𝑆0 −0.75

When the dike height causes the restrictions on q, the maximum allowable inflow rate can be obtained
using Manning’s equation:
1
𝑞𝑚𝑎𝑥 = n 𝑦𝑚𝑎𝑥 5/3 𝑆0 ½
Where,
ymax=maximum allowable depth of flow assumed to equal 0.10m
n=roughness coefficient ( 0.4−0.25 ¿
So=field slope(m/m)[0.1-0.5%]
 39

Alazba (1998) derived empirical equation for border inflow rate and application time as follows:

L1.0562 × n0.1094 × k 1.225 ×a3.832

q apl =CUq
S 0.09 × D 0.823
req

The application time corresponding to design application are:

L1.1 × n0.0093 × S 00.0203 ×k 0.387 × D0.952

req
𝑇𝑎𝑝𝑙 = 𝐶𝑈𝑇 1.0885
q apl ×a 0.75

Problem
6. Design a border strip with the following data:
Field length, L = 180 m
Field slope, S0= 0.003
Infiltration family, IF = 0.5
k = 0.033 m/ha
 40

a = 0.63
Roughness coefficient, n = 0.15
Required depth of infiltration, Dreq= 0.08 m

Lecture 9: Methods of Irrigation

Basin Irrigation Method
 Water is applied to a completely level area
enclosed by dikes or borders (called basins)
to prevent runoff and to allow infiltration
after cutoff.
 Is the simplest of the surface irrigation
methods.
 The best performance is obtained when
advance time is minimized by using large
non-erosive discharges, and the basin surface
is precision leveled.
 Most commonly practiced worldwide.

 Generally the most expensive surface irrigation configuration to develop and maintain but
often the least expensive to operate and manage.
 Costs can vary greatly, depending on crop and soil (related to land gradation).
 Typical operation and maintenance costs for basin irrigation systems vary greatly, depending
on local circumstances and irrigation efficiencies achieved.
 Mainly two types of basin layouts are practiced worldwide: i) closed single basins (with or
without outflow or runoff), and ii) multiple basin layouts which are sequentially connected
through inter-basin flow.
 41

Advantages
 One of the major advantages of the basin method is its utility in irrigating fields with irregular
shapes and small fields
 Best suited for lands/crops where leaching is required to wash out salts from the root zone
 Water application and distribution efficiencies are generally high

Limitations
 It requires accurate land leveling to achieve high application efficiency
 Comparatively high labor intensive
 Impedes surface drainage
 Not suitable for crops which are sensitive to waterlogging
 Border ridges interfere with the free movement of farm machineries
 Higher amount of water is required compared to basin, sprinkler or drip irrigation.

Hydraulics in Basin Irrigation System

 Overland flow in surface irrigation systems is commonly described using a one dimensional
analysis.
 This assumption produces good results in cases when the flow can be considered linear such as
in furrow and border irrigation.
 It is more appropriate to simulate the hydraulic processes in contour layouts using a two-
dimensional flow simulation approach.
 There are two main processes involved in flow over porous media. One is the surface flow and
the other is the vertical movement (infiltration) of water into the soil.

Basin Irrigation Design Guidelines

The shape and size of basins are mainly determined by:
 the land slope,
 soil type,
 available stream size (the water flow to the basin),
 required depth of irrigation application, and
 farming practices

Basin Irrigation Design

 A two-dimensional simulation model, titled “contour basin simulation model” (COBASIM) was
developed by Khanna et al. (2003a, b).
 It was developed by Boonstra and Jurriens (1988) for level basin design. BASCAD simulates
advance with a zero-inertia model in real time, then uses a volume balance to determine a
single recession time and the final distribution of infiltrated water.
 SIRMOD, WinSRFR, SADREG
 42

Furrow Irrigation Method

 Furrow irrigation is one of the oldest
controlled irrigation methods.
 A furrow is a small, evenly spaced,
shallow channel installed down or
across the slope of the field to be
irrigated parallel to row direction.
 Water is applied to furrows using
small discharges to favor water
infiltration while advancing down the field.
 It is also known as partial surface flooding
method of irrigation (normally used with
clean-tilled crops).

Advantages
 Developed gradually as labor or economics allows
 Developed at a relatively low cost after necessary land forming activities are accomplished
 Erosion is minimal
 Adaptable to a wide range of land slopes
Limitations
• Not suitable for high permeable soil where vertical infiltration is much higher than the lateral entry
• Higher amount of water is required, compared to sprinkler or drip irrigation
• Furrows should be closely arranged

Furrow Irrigation Design

• Furrow irrigation involves the application of irrigation water at the top end of a field into furrows.
• The water then flows along these furrows to the bottom of the field, infiltrating into the soil along
the length of the furrow.
• Infiltration occurs laterally and vertically through the wetted perimeter of the furrow.
• To achieve the ultimate in furrow irrigation performance, the infiltration opportunity time should
equal the amount of time necessary to apply the required depth of water.

The time interval during which infiltration of water into the soil can occur is bounded by the advance
and recession functions and is often referred to as the infiltration opportunity time as described by
Holzapfel et al. (1984) and Foroud et al. (1996).
 43

Gross Water Needed for Furrows

Factors the farmer can readily vary or manage:

• furrow shape
• roughness
• length of furrow
 44

• irrigation set time

• flow rate (stream size for furrow)
• cutoff time
A furrow irrigation system has several design variables:
• the inflow rate
• the length of the run in the direction of the flow
• the time of irrigation cutoff
• Optimal furrow irrigation performance requires understanding of application efficiency and
distribution.
• Select a stream size appropriate for the slope, intake rate, and length of run. Or alternatively, optimal
furrow length and irrigation cutoff can be determined.
• With the proper cutoff ratio and gross application, you can achieve uniform water application and
minimize deep percolation and runoff.
• The best combination is the one which moves water to the end of the furrow within the
requirements of the cutoff ratio, is less than the maximum erosive stream size, and results in gross
applications that are not excessive.
•Time ratio (Rt) is defined as the ratio of the time required for the infiltration of total net amount of
water required for the root zone to the time when the water front reaches the end of the run.

• It plays a key role in determining optimum furrow length to achieve maximum irrigation efficiency.

• Advance ratio or cutoff ratio (Rcut) is defined as the ratio of time to reach the waterfront at the end
of furrow, to the time set for irrigation.
• A cutoff ratio of 0.5 is desired.
• The easiest way to change the advance time is by altering the furrow stream size.
 45

• For both level and sloping furrow systems, high uniformities (greater than 85%) require a
reasonably small advance ratio.

Stream Size
• Normally stream sizes up to 0.5 l/s is suggested.
• The maximum stream size that will not cause erosion will obviously depend on the furrow slope.
• It is advisable not to use stream sizes larger than 3.0 l/s.
Michael (1978) suggested the maximum non-erosive flow rate based on furrow:

Average Depth/volume
Average depth of applied (or infiltrated) water can be estimated from volume balance approach as
follows:
 46

Problem
7.Water is applied in a furrow using non-erosive maximum stream size. The length, width, and slope
of the furrow are 150, 0.75 m, and 0.4%, respectively. The stream is continued for 2 h. Estimate the
average depth of irrigation.

Sprinkler Irrigation Method

 47

Water is delivered through a pressurized pipe network to sprinklers, nozzles, or jets which spray the
water into the air, to fall to the soil as an artificial “rain”.
It is a pressurized system, where water is distributed through a network of pipe lines to and in the
field and applied through selected sprinkler heads or water applicators.
The basic components of any sprinkler system are:
• a water source
• a pump (to pressurize the water)
• a pipe network (to distribute the water throughout the field)
• sprinklers (to spray the water over the ground) and
• valves (to control the flow of water)
Advantages
 readily automatable,
 facilitates to chemigation and fertigation,
 reduced labor requirements needed for irrigation.
 can deliver precise quantities of water in a highly efficient manner and
 adaptable to a wide range of soil and topographic conditions.

Limitations
 many crops (citrus, for example) are sensitive to foliar damage when sprinkled with saline
waters.
 initially high installation cost and high maintenance cost thereafter (when needed).
When choosing a sprinkler type for irrigation, there are several considerations:
 Adaptability to crop, terrain, and field shape
 Labor availability and requirements
 Economics
 Automation facility
 Ability of the system to meet crop needs

Sprinkler Irrigation Method Design

Design aspects of sprinkler irrigation system are as follows:
 System layout
 Operating pressure, nozzle diameter, sprinklers discharge, and wetted diameter
 Spacings between sprinklers and laterals
 Design of main line and sublines
 Sprinkler line azimuth
 Pivot or ranger length
 48

 System capacity for water supply

 Pump design
The most common design criteria for sprinkler laterals is that sprinkler discharge should not vary by
more than 10% between the points of highest and lowest pressure in the system.
49
50
51
 52

Problem
A farm of 25 ha is planned to be brought under sprinkler irrigation. The textural class of the soil is
loam-to-silt loam, having moisture content at field capacity (FC) and permanent wilting point (WP) of
about 42% (by volume) and 26% (by volume), respectively. An infiltration test data showed that
constant (basic) infiltration rate is 2 mm/h. A hardpan (relatively impervious layer) exists at a depth
of 2.0 m below the soil surface. Long-term average reference evapotranspiration (ET0) rate in that
area is 4.5 mm/d. Vegetable crops are planned to grow in the farm, and the crop coefficient (Kc) at
maximum vegetative period is 1.1. The climate is moderately windy in a part of the season. Design the
sprinkler irrigation system (various components) for the farm. Assume standard value of any missing
data.
Assume
Irrigation hour = 4.0 hrs
Check the I, if I < the infiltration rate, it should be taken as the given infiltration rate.
Dml = 12.0 m
Dmm = 10.0 m
 53

Drip Irrigation
 Application of a constant steady flow of water to soil at low pressure.
 Water is applied directly to the root zone of plants by means of applicators (orifices, porous
tubing, perforated pipe, etc.).
 Applicators being placed either on or below the surface of the ground.
 Water loss is minimized as little splash.
 Most ideally suited to high-value crops.
 Properly managed systems enable the production of maximum yields with a minimum quantity
of water.

Advantages
 Highly efficient system
 Saves water
 Limited water sources can be used
 Correct volume of water can be applied in the root zone
 The system can be automated and well adapted to chemigation and fertigation
 Reduces nutrient leaching, labor requirement, and operating cost
 Other field operations such as harvesting and spraying can be done while irrigating
 Each plant of the field receives nearly the same amount of water
 Lower pressures are required to operate systems resulting in a reduction in energy for
pumping
Limitations
 High initial cost
 Technical skill is required to maintain and operate the system
 The closer the spacing, the higher the system cost per hectare
 Damage to drip tape may occur
 Cannot wet the soil volume quickly (to recover from moisture deficit) as other systems
 Facilitates shallow root zone
 Needs clean water
 54

Lecture 11: Irrigation Methods

Hydroponic Irrigation
 Hydroponic irrigation systems generally utilize drip irrigation with soilless growing media
such as sand, rock wool or coco-coir.
 The drip systems have barbed drip emitters and deliver water through distribution tubing to
the media.
 The duration of the watering cycle is few minutes.
 The irrigation cycle not only adds water, but also adds oxygen to the growing media.
 All nutrients, including micronutrients and oxygen, must be supplied by irrigation.
 Chemical is either mixed into the irrigation water in large tanks, or proportional injectors add
fertilizer solution to the irrigation water.
 In hydroponic (soil-less) system, the media allows growers to rapidly change nutrient
concentrations in order to “steer” the plant toward a desired endpoint.
 The media can range from sand to rock wool.
 55

 The most common growing media for hydroponic irrigation is rock wool or ground coconut
husks (coco-coir) in plastic bags.
 Nutrient and water recycle reduces the environmental impact of hydroponic irrigation.
 The fertilizer concentration in the return water must be monitored and adjusted to the
required nutrient recipe.
 Other required procedures for recycle include sand filtration and disinfection.
 System maintenance is especially critical in hydroponics because the water-holding capacity of
the growing media is low.
 In general, plants die after 2 days without irrigation.
 As with other drip irrigation systems, the following maintenance procedures are generally
necessary:
1. Injection of biocides
2. Injection of acids to prevent precipitation of salts
3. Flush drip laterals regularly
4. Filtration
5. Daily check of pump station pressure and operation
6. Weekly or monthly check of uniformity and flow rates of drip emitters
 Hydroponic systems are normally scheduled based on solar radiation intensity.
 Pyranometers are mounted outside the greenhouse and measure solar radiation.
 The normal schedule is one irrigation application per 80 J/cm2.
 The plants can be steered toward the desired growth characteristics by decreasing or
increasing the frequency.
 Decreasing the frequency can increase salinity in the growing media, which increases plant
stress, thus increasing fruit sugar and reproductive growth (more fruit rather than leaves).

Hydroponic Irrigation Scheduling

Develop an irrigation schedule based

on the solar radiation data shown in
Figure.
 56

 Volume application rate can be converted to depth application rate. Typical tomato plant
spacing is 0.2 m along the row and 1.5 m between rows. Thus, area per plant is approximately
(0.2 m) x (1.5 m) = 0.3 m2/plant.
Calculate depth applied per day in the greenhouse. The volume applied per plant is 2
Liters/day. The plant spacing is 0.3 m2

Irrigation Water Source

 57

Considerations for using surface water as a source

1. Lowest available water
2. Crop water requirement
 Water needed (CU) = ETs – Re
where, ETs = Crop water requirement; Re = Effective rainfall
3. Water quality (salinity & toxicity)
4. Water right – Other users
• Domestic water supply
• Navigation
• Fish culture
• Industry
• River morphology
5. Control structure
•Initial cost
• Operating & maintenance cost

Considerations for groundwater

1. Crop water requirement
2. Availability of surface water source
3. Position of ground water table
4. Water quality
5. Ground water recharge
6. Environmental impact

Conjunctive use of Surface Water & Groundwater

Use of both Surface Water (SW) & Groundwater (GW) Such a way that Groundwater recharges & draft
balances with each other. Factors governing the percentage of sharing SW & GW:
1. Natural recharge
2. Artificial recharge
3. Aquifer characteristics
4. Availability of surface water
5. Availability of fuel/energy
6. Operation & maintenance cost for pumps
7. Economic consideration

Storage of Surface Water or rainfall

 Availability of space
 Series of droughts can be easily overcome by GW storage
 Water table control easier
 Groundwater is recharged
Distribution System for Canal Irrigation
 58

 Main Canal
 Branch Canals
 Distributaries or major distributaries
 Minors or Minor Distributaries
 Watercourses
 59

Problem
The gross commanded area for a distributary is 6000 ha, 80% of which is irrigable. The intensity of
irrigation for Rabi season is 50% and that for Kharif season is 25%. If the average duty at the head of
the distributary is 2000 ha/cumec for Rabi season and 900 ha/cumec for Kharif season, find out the
discharge required at the head of the distributary from average demand considerations.
 60

Losses in Irrigation Canals

Evaporation
 Evaporation loss is small compared to seepage loss.
 Evaporation loss is 2 to 3 percent of total losses.
 Depends on temperature, wind velocity, humidity, etc.
 In summer, the loss is more about 7%.

Seepage
• Percolation

 Type of seepage
 Soil permeability
 Condition of the canal (new or old canal)
 Silt carried by the canal
 Velocity of water in the canal
 Cross-section of canal and the wetted perimeter
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Lecture 12: Irrigation Water Source and Quality

Sediment Load in Channel
• Sediment transport causes scouring and siltation of irrigation canals. Causing the
increase of maintenance.
• Peak flow is also dependent on the sediment movement. As be bed forms created due to
sediment movement can influence the flood elevation
• Due to siltation, the storage capacity can be reduced. Sedimentation also be responsible
for possible dredging

Threshold Sediment Movement

Design of Stable Channel

Regime channel, no silting or scouring channel. Natural channel can’t be such that, in artificial channel
the hypothesis is valid.
Regime Channel: Kennedy stated that non silting, non scoring channel is in regime. Lacey
differentiated between three regime conditions:
• True Regime
• Final Regime
• Initial Regime
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Regime Concept
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Problem
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Design a stable channel using Lacey’s regime concept if the flow is 50 m3/s and silt factor is 1.1

Irrigation Water Quality

Various impurities in irrigation water
Every water may not be suitable for plant life. The quality of suitable irrigation water is very much
influenced by the constituents of the soil which is to be irrigated. The various types of impurities:
• Sediment concentration in water.
• Total concentration of soluble salts in water.
• Concentration of sodium ions to other cations.
• Concentration of potentially toxic elements present in water.
• Bicarbonate concentration as related to concentration of Caplus Mg
• Bacterial concentration

Good quality water is essential for high production

 Physical : Color, odor, silt
 Chemical : Salt, alkaline
 Biological : Coliform

Salinity:
 Salts in soil or water reduce water availability to the crop to such an extent that yield is
affected.
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 Salts accumulate in the root zone and the crop cannot extract sufficient water from the salty
soil solution.
 The plants symptoms are Wilting or darker, bluish-green color & sometimes thicker leaves.
 Leaching can remove the accumulated salts from the root zone.

Water infiltration rate:

 Relatively high sodium or low calcium content of soil or water reduce the rate of infiltration to
such an extent that sufficient water cannot be infiltrated to the crop adequately from one
irrigation to the neat.
 This occurs within few centimeters of the soil surface and linked to the structural stability.
 When a high sodium surface is developed it weakens the soil structure. The soil particles
become finer and clog the soil pores.

Toxicity:
 Certain ions (sodium, chloride or boron) from soil or water accumulate in a sensitive crop to
concentration high enough to cause crop damage and reduce yield.
 Toxic ions absorbed with water in significant amounts, and transported to the leaves and they
accumulate during transportation.

Miscellaneous:
 Excessive nutrients reduce yield or quality.
 Unsightly deposits fruit or foliage reduces marketability.
 Excessive corrosion of equipment increases maintenance and repairs.
 Sediments tend to fill the cannels, lands etc.
 These include high nitrogen, prudence of silt, bicarbonate, iron.

Effect of Impure Irrigation Water

Sediment concentration of soluble salts
• The effect of sediment present in the irrigation water depends upon the type of irrigated
land.
• When fine sediment from water deposited on sand soils, the fertility is improved.
• The sediment has been derived from the eroded areas; it may reduce the fertility or
decrease the soil permeability.
• Sediment water increases the siltation and maintenance costs

Total concentration of soluble salts

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 Salts of calcium, magnesium, sodium and potassium, present in the irrigation water may
prove injurious to plants.
 Excessive quantities can reduce the osmotic activities of the plants, and may present
adequate aeration, causing injuries to plant growth.
 The injurious effects of salts on the plant growth depend upon the concentration of salts
left in the soil.

Electrical Conductivity (EC)

• It is the reciprocal of the Electrical resistivity. Quantitively the electrical resistivity is the
resistance, in ohms, of a conductor, metallic or electrolytis, which is 1 cm long and has a
crosssectional area of 1 cm2 at 25oC.
• The terms “Electrical Conductivity” and specific electrical conductance have identical
meanings.
• Standard method of evaluating total salts present in irrigation water. Electrical
conductivity is expressed as the reciprocal of ohm/cm or mhos/cm, milimhos per cm
(mmhos/cm) or decisiemens per meter (ds/m)
• For convenience in units, millimhos/cm (10-3 mhos/cm) or micromhos/cm (10-6
mhos/cm) are used.

Electrical Conductivity (EC) and suitability for water

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