Department of Civil Engineering

University of Engineering and Technology Peshawar

CE-402: Irrigation Engineering

Lecture 4
Crop water requirements and its
measurement, Quality of irrigation
water

8th Semester (4th Year)

Civil Engineering
Spring 2021

Lecturer: Alamgir Khalil

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Soil-Moisture-Irrigation Relationship

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Soil-Moisture-Irrigation Relationship (cont.)

➢ The water below the water table is known as groundwater and above the
water table as soil moisture.
➢ Below the ground surface is the soil zone or root zone that is penetrated by
the roots of vegetation.
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Soil-Moisture-Irrigation Relationship (cont.)

➢ Root zone is most important from irrigation point of view, because the plants
take their water supply from this zone.
➢ When water falls over the ground, a part of it get absorbed in the root zone
and the rest flows downward under the action of gravity.
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Field Capacity
➢ After the rain or irrigation is stopped, part of water held in the larger
pores of soil moves downward called percolation or drainage. As the
drainage stops the large soil pores are filled with both air and water
whereas the small pores are still full of water. The soil, at this stage, is
said to be at field capacity.

➢ The field capacity is the amount of maximum moisture that can be held
by the soil against gravity. It is expressed as percentage. (or)
The field capacity is the moisture content of the soil after free drainage
has removed most of the gravity water.

➢ The concept of field capacity is useful in arriving at the amount of water

available in the soil for plant use.
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Field Capacity (cont.)

➢ The field capacity is thus the water content of a soil after free drainage
has taken place for a sufficient period. This period of free gravity drainage
is usually taken as 2 to 5 days.

𝑊𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑤𝑎𝑡𝑒𝑟 𝑟𝑒𝑡𝑎𝑖𝑛𝑒𝑑 𝑖𝑛 𝑎 𝑐𝑒𝑟𝑡𝑎𝑖𝑛 𝑣𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑠𝑜𝑖𝑙

𝐹𝑖𝑒𝑙𝑑 𝐶𝑎𝑝𝑎𝑐𝑖𝑡𝑦 = × 100
𝑊𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑡ℎ𝑒 𝑠𝑎𝑚𝑒 𝑣𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑑𝑟𝑦 𝑠𝑜𝑖𝑙

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Soil Moisture Conditions

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Soil Water Availability and Classes

➢ Gravitational water:
• It is the water in the large pores that moves downward freely under the influence
of gravity
• It drains out so fast that it is not available to the crops.

➢ Capillary Water:
• It is the amount of water retained by the soil after gravitational water has drained
out.
• It is the water in the small pores which moves because of capillary forces and is
called capillary water.
• Capillary water is the major source of water available for the plant

➢ Hygroscopic Water
• Soil moisture further reduced by ET until no longer moves because of capillary
forces. The remaining water which is held on particle surfaces so tightly is called
hygroscopic water.
• the water is held by adhesive force. And therefore, it is unavailable to the plant.
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Soil Water Availability and Classes (cont.)

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Soil water parameters and classes of water

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Typical water extraction pattern in uniform soil profile.

Source: Chapter 3 “Crops”, Irrigation Guide, Natural Resources Conservation Service- National Engineering Handbook. 11
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Readily Available Moisture

➢ It is that portion of the available moisture that is most easily
extracted by plants and is approximately 75 to 80% of the
available moisture.

Permanent Wilting Point

➢ It is that water content at which plants can no longer extract
sufficient water from the soil for its growth and wilts up. Thus,
water available to plants is the difference of field capacity water
and permanent wilting point water. This is known as available
moisture or maximum storage capacity of soil.

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Illustration of soil saturation, field capacity and permanent wilting point.

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General Relationship Between Soil Water Characteristics and Texture

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Available Moisture

➢ The difference in water content of the soil between field

capacity and permanent wilting is known as available water or
available moisture.

Soil Moisture Deficiency

➢ Soil moisture deficiency or field moisture deficiency is the water
required to bring the soil moisture content of the soil to its field
capacity.

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Depth of water stored in root zone

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Depth of water stored in root zone (cont.)

➢ In order to estimate the depth of water stored in the root zone of soil
containing water up to field capacity, let,

d = depth of root zone (in meters)

FC = Field Capacity (expressed as ratio);
𝛾𝑑 = unit weight of dry soil; and
𝛾𝑤 = unit weight of water.

Considering unit area (1 sq. meter) of soil area;

𝑊𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑤𝑎𝑡𝑒𝑟 𝑟𝑒𝑡𝑎𝑖𝑛𝑒𝑑 𝑖𝑛 𝑢𝑛𝑖𝑡 𝑎𝑟𝑒𝑎 𝑜𝑓 𝑠𝑜𝑖𝑙
𝐹𝐶 =
𝑊𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑠𝑜𝑖𝑙 𝑜𝑓 𝑢𝑛𝑖𝑡 𝑎𝑟𝑒𝑎

𝑊𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑤𝑎𝑡𝑒𝑟 𝑟𝑒𝑡𝑎𝑖𝑛𝑒𝑑 𝑖𝑛 𝑢𝑛𝑖𝑡 𝑎𝑟𝑒𝑎 𝑜𝑓 𝑠𝑜𝑖𝑙

=
𝛾𝑑 × 1 × 𝑑
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Depth of water stored in root zone (cont.)

Weight of water retained in unit area = 𝐹𝐶 ∙ 𝛾𝑑 ∙ 𝑑

𝐹𝐶 ∙ 𝛾𝑑 ∙ 𝑑
Depth of water stored (in depth d) =
𝛾𝑤

This depth of water will be available for evapotranspiration.

Available 𝛾𝑑 ∙ 𝑑
= [Field Capacity – Permanent Wilting Point]
moisture depth 𝛾𝑤

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Estimation of Depth and Frequency of Irrigation based on

Soil Moisture Concept
➢ Water is consumed be plants through roots. Significant moisture
should be available in the root zone.
➢ Soil moisture in the root zone varies from field capacity moisture
content to wilting point moisture content.
➢ Soil moisture content is not allowed to deplete up to wilting point.
➢ The optimum level up to which the soil moisture is allowed to deplete
is called optimum moisture content.
➢ The irrigation water should be supplied as soon as the moisture falls
up to optimum level (i.e., fixing irrigation frequency)
➢ The quantity should be sufficient to bring the moisture to the field
capacity making allowance for application losses thus fixing the water
depth.
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Depth and Frequency of Irrigation

Depth of water 𝛾𝑑 ∙ 𝑑
= [Field Capacity – Optimum Moisture Content]
in root zone, 𝑑𝑤 𝛾𝑤
If 𝐶𝑢 is the daily consumptive use rate, frequency of watering is given by;
𝑑𝑤
𝑓𝑤 = (days)
𝐶𝑢
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Example 3.7 (Punmia)

After how many days will you supply water to soil (clay loam) in order to
ensure sufficient irrigation of a given crop if;
1) Field capacity of soil = 27%
2) Permanent wilting point = 14%
3) Density of soil = 15 kN/m3
4) Effective depth of root zone = 75 cm
5) Daily consumptive use of water for a given crop = 11 mm

Assume readily available moisture = 80% of available moisture.

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Irrigation Efficiencies

➢ Efficiency is the ratio of water output to the water input and is

expressed as percentage.

➢ The objective of efficiency concepts is to show when improvements

can be made which will result in more efficient irrigation. The
following are the various types of irrigation efficiencies;

✓ Water Conveyance Efficiency

✓ Water Application Efficiency
✓ Water Use Efficiency
✓ Water Storage Efficiency
✓ Water Distribution Efficiency

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Water Conveyance Efficiency

➢ It is the ratio of water delivered to the farm or irrigation plot to the
water supplied or diverted from the river or reservoir.

𝑊𝑓
𝜂𝑐 = × 100
𝑊𝑟

Where
𝜂𝑐 = water conveyance efficiency
𝑊𝑓 = water delivered to the farm or irrigation plot
𝑊𝑟 = water supplied or diverted from the river or reservoir

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Water Application Efficiency

➢ It is the ratio of quantity of water stored into the root zone of the crops to
the quantity of water delivered to the field. It focuses the attention of the
suitability of the method of application of water to the crops.

𝑊𝑠
𝜂𝑎 = × 100
𝑊𝑓

Where
𝜂𝑎 = water application efficiency
𝑊𝑠 = water stored in the root zone during irrigation
𝑊𝑓 = water delivered to the farm or irrigation plot

✓ The common sources of loss of irrigation water during water application are (i) surface
runoff 𝑅𝑓 from the farm and (ii) deep percolation 𝐷𝑓 below the farm root-zone soil.
Hence,
𝑊𝑓 = 𝑊𝑠 + 𝑅𝑓 + 𝐷𝑓
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Water Use Efficiency

➢ It is the ratio of water beneficially used, including leaching water to the

quantity of water delivered.

𝑊𝑢
𝜂𝑢 = × 100
𝑊𝑑

Where
𝜂𝑢 = water use efficiency
𝑊𝑢 = water used beneficially or consumptively
𝑊𝑑 = water delivered

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Water Storage Efficiency

➢ It is the ratio of the quantity of water stored in the root zone during
irrigation to the water needed in the root zone prior to irrigation.

𝑊𝑠
𝜂𝑠 = × 100
𝑊𝑛

Where
𝜂𝑠 = water storage efficiency
𝑊𝑠 = water stored in the root zone during irrigation
𝑊𝑛 = water needed in the root zone prior to irrigation

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Water Distribution Efficiency

➢ Water distribution efficiency evaluates the degree to which water is
uniformly distributed throughout the root zone. The more uniformly the
water is distributed, the better will be the crop response.

𝑦
𝜂𝑑 = 100 1 −
𝑑

Where
𝜂𝑑 = water distribution efficiency
𝑦 = average numerical deviation in depth of water stored
from average depth stored during irrigation
𝑑 = average depth of water stored during irrigation

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Gross Command Area

➢ The gross command area (GCA) is the total area bounded within the
irrigation boundary of the project that can be irrigated economically
without bothering about the limitation of quantity of water available.

➢ The gross command area includes the cultivable area and uncultivable
area. The examples of the uncultivable area include ponds, residential
areas, reserved forests and roads.

➢ A doab is an area between two drainages. When a canal system lies on

a doab, the irrigation is economical. Here, the gross command area is
the area enclosed by the drainages on both the sides and hence GCA
forms the geographical area of the doab.

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Gross Command Area (cont.)

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Culturable or Cultivable Command Area

➢ Gross command area contains some unfertile barren land, local ponds,
villages etc which are actually unculturable areas. The remaining area on
which crops can be grown satisfactorily is known as Culturable Command
Area (CCA).
CCA = GCA – Unculturable Area

Culturable Cultivated Area

➢ The area on which crop is grown at a particular time or crop season.

Culturable Uncultivated Area

➢ The area on which no crop is grown at a particular time or crop season
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Example 3.11 (Punmia)

An irrigation canal has gross command area of 80,000 hectares out of

which 85% is culturable irrigable. The intensity of irrigation for kharif
season is 30% and for Rabi season 60%. Find the discharge required at
the head of canal if the duty at its head is 800 hectares/cumec for kharif
season and 1700 hectares/cumec for rabi season.

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Example 3.17 (Punmia)

The base period, intensity of irrigation and duty of various crops under a
canal system are given in the table. Find the reservoir capacity if the
canal losses are 20% and reservoir losses are 12%.

Crop Base Period Duty at the field Area under the crop
(days) (hectares/cumec) (hectares)
Wheat 120 1800 4800
Sugarcane 360 800 5600
Cotton 200 1400 2400
Rice 120 900 3200
Vegetables 120 700 1400

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Quality of Irrigation Water

➢ A good irrigation water is the one which performs the required

functions without any side effects which retard the plant growth.
➢ Irrigation water may be said to be unsatisfactory for the intended use
if it contains:
1) Chemicals toxic to plants or the persons using plant as food.
2) Chemicals which react with the soil to produce unsatisfactory
moisture characteristics, and
3) Bacteria injurious to persons or animals eating plants irrigated with
the water.

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Quality of Irrigation Water (cont.)

➢ Irrigation water may contain various types of salts such as sodium, calcium,
magnesium and potassium etc. A high concentration of these salts may prove
to be injurious to the crops.

➢ The water quality for irrigation is determined by three approaches:

1) Salt Concentration
2) Electrical Conductivity
3) Sodium Absorption Ratio (SAR)

1) Salt Concentration
The salt concentration is generally expressed by ppm (parts per million) or
by mg per liter. However, amounts in excess of 700 ppm are harmful to
some plants and more than 2000 ppm is injurious to all crops.

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Quality of Irrigation Water (cont.)

2) Electrical Conductivity
✓ The salt concentration is also measured by determining the
electrical conductivity of water, is expressed in micro mhos per
centimeter.
▪ Low salinity water (C1): The value between 100 to 250 micro
mhos per cm at 25 oC is called as low conductivity water, can be
used for irrigation.
▪ Medium salinity water (C2): The value from 250 to 750 micro
mhos per cm at 25 oC is called as medium conductivity water
can be used for irrigation for normal salt tolerant plants.
(cotton, alfalfa, cereals, grain sorghum).
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Quality of Irrigation Water (cont.)

2) Electrical Conductivity
▪ High salinity water (C3): The value from 750 to 2250 micro mhos per cm at
25 oC is called high conductivity water, can be used for high salt tolerant
crops with proper drainage. (barley and oat)

▪ Very high salinity water (C4): The value above 2250 micro mhos per cm at
25 oC is termed as very high conductivity water, it is prohibited for
irrigation.

3) Sodium Absorption Ratio (SAR)

The proportion of sodium ions present in soil is generally measured by SAR

𝑁𝑎+
𝑆𝐴𝑅 =
𝐶𝑎++ + 𝑀𝑔++
2
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Quality of Irrigation Water (cont.)

➢ Low sodium water (S1): when the value of SAR lies between 0 to 10. It is
suitable for all crops except those which are highly sensitive to sodium.

➢ Medium sodium water (S2): when the value of SAR lies between 10 to 18.
May be used on coarse textured or organic permeable soils. Addition of
gypsum either to water or soil is required for use on fine textured soils. The
soils tend towards alkaline because of increase in pH value.

➢ High sodium water (S3): when the value of SAR lies between 18 to 26. May
be used provided gypsum is added, and good drainage and high leaching is
provided.

➢ Very high sodium water (S4): when the value of SAR is above 26. Generally,
not suitable for irrigation.
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Quality of Irrigation Water (cont.)

Boron Toxicity
➢ Boron is essential to the normal growth of all plants, but the amount
required is low. If it exceeds a certain level of tolerance depending on the
crop, then boron may cause injury.

Class Boron content (mg/l) Suitability for irrigation

Low Below 1.0 Excellent
Medium 1.0 – 2.0 Most crops
High 2.0 – 4.0 For more boron-tolerant crops
Very high Above 4.0 Doubtful for irrigation

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Quality of Irrigation Water (cont.)

Chloride Toxicity
➢ The most common crop toxicity is caused by chlorides in irrigation water.
The chloride (Cl-) anion occurs in all waters; chlorides are soluble and leach
readily to drainage water. Chlorides are necessary for plant growth, though
in high concentrations they can inhibit (restrain) plant growth, and can be
highly toxic to some plant species.

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