31-03-2012, 01:35 PM
SOIL–WATER RELATIONS AND IRRIGATION METHODS
SOILS
Soil mainly consists of finely divided organic matter and minerals (formed due to disintegration
of rocks). It holds the plants upright, stores water for plant use, supplies nutrients to the
plants and helps in aeration. Soils can be classified in many ways, such as on the basis of size
(gravel, sand, silt, clay, etc.), geological process of formation, and so on. Based on their process
of formation (or origin), they can be classified into the following categories:
(i) Residual soils: Disintegration of natural rocks due to the action of air, moisture, frost,
and vegetation results in residual soils.
(ii) Alluvial soils: Sediment material deposited in bodies of water, deltas, and along the
banks of the overflowing streams forms alluvial soils.
(iii) Aeolian soils: These soils are deposited by wind action.
(iv) Glacial soils: These soils are the products of glacial erosion.
(v) Colluvial soils: These are formed by deposition at foothills due to rain wash.
(vi) Volcanic soil: These are formed due to volcanic eruptions and are commonly called as
volcanic wash.
The soils commonly found in India can be classified as follows:
(i) Alluvial Soils: Alluvial soils include the deltaic alluvium, calcareous alluvial soils,
coastal alluvium, and coastal sands. This is the largest and most important soil group of India.
The main features of the alluvial soils of India are derived from the deposition caused by
rivers of the Indus, the Ganges, and the Brahmaputra systems. These rivers bring with them
the products of weathering of rocks constituting the mountains in various degrees of fineness
and deposit them as they traverse the plains. These soils vary from drift sand to loams and
from fine silts to stiff clays. Such soils are very fertile and, hence, large irrigation schemes in
areas of such soils are feasible. However, the irrigation structures themselves would require
strong foundation.
(ii) Black Soils: The black soils vary in depth from a thin layer to a thick stratum. The
typical soil derived from the Deccan trap is black cotton soil. It is common in Maharashtra,
western parts of Madhya Pradesh, parts of Andhra Pradesh, parts of Gujarat, and some parts
of Tamil Nadu. These soils may vary from clay to loam and are also called heavy soils. Many
black soil areas have a high degree of fertility but some, especially in the uplands, are rather
poor. These are suitable for the cultivation of rice and sugarcane. Drainage is poor in such
soils.
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SOIL–WATER RELATIONS AND IRRIGATION METHODS
SOIL-WATER RELATIONS AND IRRIGATION METHODS 93
(iii) Red Soils: These are crystalline soils formed due to meteoric weathering of the ancient
crystalline rocks. Such soils are found in Tamil Nadu, Karnataka, Goa, south-eastern
Maharashtra, eastern Andhra Pradesh, Madhya Pradesh, Orissa, Bihar, and some districts of
West Bengal and Uttar Pradesh. Many of the so-called red soils of south India are not red. Red
soils have also been found under forest vegetation.
(iv) Lateritic Soils: Laterite is a formation peculiar to India and some other tropical
countries. Laterite rock is composed of a mixture of the hydrated oxides of aluminium and iron
with small amounts of manganese oxides. Under the monsoon conditions, the siliceous matter
of the rocks is leached away almost completely during weathering. Laterites are found on the
hills of Karnataka, Kerala, Madhya Pradesh, the estern Ghats of Orissa. Maharashtra, West
Bengal, Tamil Nadu, and Assam.
(v) Desert Soils: A large part of the arid region belonging to western Rajasthan, Haryana,
and Punjab lying between the Indus river and the Aravalli range is affected by desert and
conditions of geologically recent origin. This part is covered with a mantle of the blown sand
which, combined with the arid climate, results in poor soil development. The Rajasthan desert
is a vast sandy plain including isolated hills or rock outcrops at places. The soil in Rajasthan
improves in fertility from west and north-west to east and north-east.
(vi) Forest Soils: These soils contain high percentage of organic and vegetable matter
and are also called humus. These are found in forests and foothills.
Soils suitable for agriculture are called arable soils and other soils are non-arable.
Depending upon their degree of arability, these soils are further subdivided as follows:
(i) Class I: The soils in class I have only a few limitations which restrict their use for
cultivation. These soils are nearly level, deep, well-drained, and possess good water-holding
capacity. They are fertile and suitable for intensive cropping.
(ii) Class II: These soils have some limitations which reduce the choice of crops and
require moderate soil conservation practices to prevent deterioration, when cultivated.
(iii) Class III: These soils have severe limitations which reduce the choice of crops and
require special soil conservation measures, when cultivated.
(iv) Class IV: These soils have very severe limitations which restrict the choice of crops
to only a few and require very careful management. The cultivation may be restricted to once
in three or four years.
Soils of type class I to class IV are called arable soils. Soils inferior to class IV are grouped
as non-arable soils. Irrigation practices are greatly influenced by the soil characteristics. From
agricultural considerations, the following soil characteristics are of particular significance.
(i) Physical properties of soil,
(ii) Chemical properties of soil, and
(iii) Soil-water relationships.
3.2. PHYSICAL PROPERTIES OF SOIL
The permeability of soils with respect to air, water, and roots are as important to the growth of
crop as an adequate supply of nutrients and water. The permeability of a soil depends on the
porosity and the distribution of pore spaces which, in turn, are decided by the texture and
structure of the soil.
94 IRRIGATION AND WATER RESOURCES ENGINEERING
3.2.1. Soil Texture
Soil texture is determined by the size of soil particles. Most soils contain a mixture of sand
(particle size ranging from 0.05 to 1.00 mm in diameter), silt (0.002 to 0.05 mm) and clay
(smaller than 0.002 mm). If the sand particles dominate in a soil, it is called sand and is a
coarse-textured soil. When clay particles dominate, the soil is called clay and is a fine-textured
soil. Loam soils (or simply loams) contain about equal amount of sand, silt, and clay and are
medium-textured soils.
The texture of a soil affects the flow of water, aeration of soil, and the rate of chemical
transformation all of which are important for plant life. The texture also determines the water
holding capacity of the soil.
3.2.2. Soil Structure
Volume of space (i.e., the pores space) between the soil particles depends on the shape and size
distribution of the particles. The pore space in irrigated soils may vary from 35 to 55 per cent.
The term porosity is used to measure the pore space and is defined as the ratio of the volume
of voids (i.e., air and water-filled space) to the total volume of soil (including water and air).
The pore space directly affects the soil fertility (i.e., the productive value of soil) due to its
influence upon the water-holding capacity and also on the movement of air, water, and roots
through the soil.
Soils of uniform particle size have large spaces between the particles, whereas soils of
varying particle sizes are closely packed and the space between the particles is less. The particles
of a coarse-grained soil function separately but those of fine-grained soils function as granules.
Each granule consists of many soil particles. Fine-textured soils offer a favourable soil structure
permitting retention of water, proper movement of air and penetration of roots which is essential
for the growth of a crop.
The granules are broken due to excessive irrigation, ploughing or working under too
wet (puddling) or too dry conditions. Such working affects the soil structure adversely. The
structure of the irrigated soil can be maintained and improved by proper irrigation practices
some of which are as follows (1):
(i) Ploughing up to below the compacted layers,
(ii) After ploughing, allowing sufficient time for soil and air to interact before preparing
the seed bed or giving pre-planting irrigation,
(iii) The organic matter spent by the soil for previous crops should be returned in the form
of fertilisers, manures, etc.,
(iv) Keeping cultivation and tillage operations to a minimum, and
(v) Adopting a good crop rotation.
Green manures keep the soil fertility high. Crops like hamp, gwar, moong etc. are grown
on the fields. When these plants start flowering, ploughing is carried out on the fields so that
these plants are buried below the ground surface. Their decomposition makes up for the soil
deficiencies.
The tendency of cultivators to grow only one type of crop (due to better returns) should
be stopped as this cultivation practice leads to the deficiency in the soil of those nutrients
which are needed by the crop. If the land is not used for cultivation for some season, the soil
recoups its fertility. Alternatively, green manures can be used. Rotation of crops (which means
growing different crops on a field by rotation) is also useful in maintaining soil fertility at a
satisfactory level.
SOIL-WATER RELATIONS AND IRRIGATION METHODS 95
3.2.3. Depth of Soil
The importance of having an adequate depth of soil for storing sufficient amount of irrigation
water and providing space for root penetration cannot be overemphasised. Shallow soils require
more frequent irrigations and cause excessive deep percolation losses when shallow soils overlie
coarse-textured and highly permeable sands and gravels. On the other hand, deep soils would
generally require less frequent irrigations, permit the plant roots to penetrate deeper, and
provide for large storage of irrigation water. As a result, actual water requirement for a given
crop (or plant) is more in case of shallow soils than in deep soils even though the amount of
water actually absorbed by the crop (or plant) may be the same in both types of soils. This is
due to the unavoidable water losses at each irrigation.
3.3. CHEMICAL PROPERTIES OF SOIL
For satisfactory crop yield, soils must have sufficient plant nutrients, such as nitrogen, carbon,
hydrogen, iron, oxygen, potassium, phosphorus, sulphur, magnesium, and so on. Nitrogen is
the most important of all the nutrients. Nitrogenous matter is supplied to the soil from barnyard
manure or from the growing of legume crops as green manures, or from commercial fertilisers.
Plants absorb nitrogen in the form of soluble nitrates.
Soils having excess (greater than 0.15 to 0.20 per cent) soluble salts are called saline
soils and those having excess of exchangeable sodium (more than 15 per cent or pH greater
than 8.5) are called alkaline (or sodic) soils. Excessive amounts of useful plant nutrients such
as sodium nitrate and potassium nitrate may become toxic to plants. Saline soils delay or
prevent crop germination and also reduce the amount and rate of plant growth because of the
high osmotic pressures which develop between the soil-water solution and the plants. These
pressures adversely affect the ability of the plant to absorb water.
Alkaline (or sodic) soils tend to have inferior soil structure due to swelling of the soil
particles. This changes the permeability of the soil. Bacterial environment is also an important
feature of the soil-water-plant relationship. The formation of nitrates from nitrogenous
compounds is accelerated due to favourable bacterial activity. Bacterial action also converts
organic matter and other chemical compounds into forms usable by the plants. Bacterial activity
is directly affected by the soil moisture, soil structure, and soil aeration. Compared to humid
climate soils, arid soils provide better bacterial environment up to much greater depths because
of their open structure. Besides, due to low rainfall in arid regions, leaching (i.e., draining
away of useful salts) is relatively less and the arid soils are rich in mineral plant food nutrients,
such as calcium and potassium.
Soils become saline or alkaline largely on account of the chemical composition of rocks
weathering of which resulted in the formation of soils. Sufficient application of water to the
soil surface through rains or irrigation helps in carrying away the salts from the root-zone
region of the soil to the rivers and oceans. When proper drainage is not provided, the irrigation
water containing excessive quantities of salt may, however, render the soil unsuitable for
cultivation. Saline and alkaline soils can be reclaimed by: (i) adequate lowering of water
table, (ii) leaching out excess salts, and (iii) proper management of soil so that the amount of
salt carried away by the irrigation water is more than the amount brought in by irrigation
water.
96 IRRIGATION AND WATER RESOURCES ENGINEERING
3.4. SOIL–WATER RELATIONSHIPS
Any given volume V of soil (Fig. 3.1) consists of : (i) volume of solids Vs , (ii) volume of liquids
(water) Vw, and (iii) volume of gas (air) Va. Obviously, the volume of voids (or pore spaces) Vv =
Vw + Va. For a fully saturated soil sample, Va = 0 and Vv = Vw . Likewise, for a completely dry
specimen, Vw = 0 and Vv = Va. The weight of air is considered zero compared to the weights
of water and soil grains. The void ratio e, the porosity n, the volumetric moisture content w,
and the saturation S are defined as
Therefore, w = Sn …(3.1)
Gas (Air)
Water Ww
Ws
Soil particles
Fig. 3.1 Occupation of space in a soil sample
It should be noted that the value of porosity n is always less than 1.0. But, the value of
void ratio e may be less, equal to, or greater than 1.0.
Further, if the weight of water in a wet soil sample is Ww and the dry weight of the
sample is Ws , then the dry weight moisture fraction, W is expressed as (2)
(3.2)
The bulk density (or the bulk specific weight or the bulk unit weight) γb of a soil mass is
the total weight of the soil (including water) per unit bulk volume,
in which, WT = Ws + Ww
The specific weight (or the unit weight) of the solid particles is the ratio of dry weight of
the soil particles Ws to the volume of the soil particles Vs, i.e., Ws/Vs. Thus,
SOIL-WATER RELATIONS AND IRRIGATION METHODS 97
and Gs γw =
= (3.3)
Here, γw is the unit weight of water and Gb and Gs are, respectively, the bulk specific
gravity of soil and the relative density of soil grains. Further,
1 – n = 1–
∴ w = GbW (3.5)
and w = Gs(1 – n)W
Considering a soil of root-zone depth d and surface area A (i.e., bulk volume = Ad),
Ws = VsGsγw = Ad (1 – n) Gsγw
Therefore, the dry weight moisture fraction, W =
Therefore, the volume of water in the root-zone soil,
Vw = W Ad (1 – n) Gs (3.6)
This volume of water can also be expressed in terms of depth of water which would be
obtained when this volume of water is spread over the soil surface area A.
∴ Depth of water, dw =
V
A
w
dw = Gs (1 – n) Wd (3.7)
or dw = w d (3.8)
Example 3.1 If the water content of a certain saturated soil sample is 22 per cent and
the specific gravity is 2.65, determine the saturated unit weight γsat, dry unit weight γd, porosity
n and void ratio e.
Solution:
Ws = 2.65 γw Vs
and WWs = Ww
= 0.22 × 2.65 γwVs
98 IRRIGATION AND WATER RESOURCES ENGINEERING
= 0.22 × 2.65 × Vs = 0.583 Vs
Total volume V = Vs + Vw (as Va = 0 since the sample is saturated)
= Vs (1 + 0.583)
= 1.583 Vs
0 583
1583
.
.
= 36.8%
(since Vv = Vw as the soil sample is saturated)
and e =
= 0.583 = 58.3%
and total weight W = Ww + Ws
= 0.22 × 2.65 × γwVs + 2.65 γwVs
= 3.233 γwVs
γsat =
3 233
1583
.
.
γ
= 20.032 kN/m3 (since γw = 9810 N/m3)
2 65
1583
= 16.422 kN/m3.
Example 3.2 A moist clay sample weighs 0.55 N. Its volume is 35 cm3. After drying in
an oven for 24 hours, it weights 0.50 N. Assuming specific gravity of clay as 2.65, compute the
porosity n, degree of saturation S, original moist unit weight, and dry unit weight.
Solution:
WT = 0.55 N
Ws = 0.50 N
Ww = 0.05 N
= 1.923 × 10–5 m3 = 19.23 cm3
Vw =
Ww
γ w
= 0 05
9810
.
= 5.1 × 10–6 m3 = 5.10 cm3
Vv = V – Vs = 35 – 19.23
= 15.77 cm3
Porosity, n =
= 45.06%
Degree of saturation, S =
= 32.34%
Example 3.3 A moist soil sample has a volume of 484 cm3 in the natural state and a
weight of 7.94N. The dry weight of the soil is 7.36 N and the relative density of the soil particles
is 2.65. Determine the porosity, soil moisture content, volumetric moisture content, and degree
of saturation.
ROOT-ZONE SOIL WATER
Water serves the following useful functions in the process of plant growth:
(i) Germination of seeds,
(ii) All chemical reactions,
(iii) All biological processes,
(iv) Absorption of plant nutrients through their aqueous solution,
(v) Temperature control,
(vi) Tillage operations, and
(vii) Washing out or dilution of salts.
Crop growth (or yield) is directly affected by the soil moisture content in the root zone.
The root zone is defined as the volume of soil or fractured rock occupied or occupiable by roots
of the plants from which plants can extract water (3). Both excessive water (which results in
waterlogging) and deficient water in the root-zone soil retard crop growth and reduce the crop
yield.
Soil water can be divided into three categories:
(i) Gravity (or gravitational or free) water,
(ii) Capillary water, and
(iii) Hygroscopic water.
100 IRRIGATION AND WATER RESOURCES ENGINEERING
Gravity water is that water which drains away under the influence of gravity. Soon
after irrigation (or rainfall) this water remains in the soil and saturates the soil, thus preventing
circulation of air in void spaces.
The capillary water is held within soil pores due to the surface tension forces (against
gravity) which act at the liquid-vapour (or water-air) interface.
Water attached to soil particles through loose chemical bonds is termed hygroscopic
water. This water can be removed by heat only. But, the plant roots can use a very small
fraction of this moisture under drought conditions.
When an oven-dry (heated to 105°C for zero per cent moisture content) soil sample is
exposed to atmosphere, it takes up some moisture called hygroscopic moisture. If more water
is made available, it can be retained as capillary moisture due to surface tension (i.e.,
intermolecular forces). Any water, in excess of maximum capillary moisture, flows down freely
and is the gravitational (or gravity) water.
The water remaining in the soil after the removal of gravitational water is called the
field capacity. Field capacity of a soil is defined as the moisture content of a deep, permeable,
and well-drained soil several days after a thorough wetting. Field capacity is measured in
terms of the moisture fraction, Wfc = (Ww/Ws) of the soil when, after thorough wetting of the
soil, free drainage (at rapid rate) has essentially stopped and further drainage, if any, occurs
at a very slow rate. An irrigated soil, i.e., adequately wetted soil, may take approximately one
(in case of sandy soil) to three (in case of clayey soil) days for the rapid drainage to stop. This
condition corresponds to a surface tension of one-tenth bar (in case of sandy soils) to one-third
bar (in case of clayey soils). Obviously, the field capacity depends on porosity and soil moisture
tension. The volumetric moisture content at the field capacity wfc becomes equal to Gb Wfc.
Plants are capable of extracting water from their root-zone soil to meet their transpiration
demands. But, absence of further addition to the soil moisture may result in very low availability
of soil water and under such a condition the water is held so tightly in the soil pores that the
rate of water absorption by plants may not meet their transpiration demands and the plants
may either wilt or even die, if not supplied with water immediately and well before the plants
wilt. After wilting, however, a plant may not regain its strength and freshness even if the soil
is saturated with water. Permanent wilting point is defined as the soil moisture fraction, Wwp
at which the plant leaves wilt (or droop) permanently and applying additional water after this
stage will not relieve the wilted condition. The soil moisture tension at this condition is around
15 bars (2). The moisture content at the permanent wilting condition will be higher in a hot
climate than in a cold climate. Similarly, the percentage of soil moisture at the permanent
wilting point of a plant will be larger in clayey soil than in sandy soil. The permanent wilting
point is, obviously, at the lower end of the available moisture range and can be approximately
estimated by dividing the field capacity by a factor varying from 2.0 (for soils with low silt
content) to 2.4 (for soils with high silt content). The permanent wilting point also depends
upon the nature of crop. The volumetric moisture content at the permanent wilting point, wwp
becomes GbWwp. Figure 3.2 shows different stages of soil moisture content in a soil and the
corresponding conditions.
SOIL-WATER RELATIONS AND IRRIGATION METHODS 101
Saturation
Field capacity
Permanent
wilting point
Oven-dry
condition
Gravitational
water
(Rapid drainage)
Capillary
water
(Slow drainage)
Hygroscopic
water
(No drainage)
Fig. 3.2 Different stages of soil moisture content in a soil
The difference in the moisture content of the soil between its field capacity and the
permanent wilting point within the root zone of the plants is termed available moisture. It
represents the maximum moisture which can be stored in the soil for plant use. It should be
noted that the soil moisture content near the wilting point is not easily extractable by the
plants. Hence, the term readily available moisture is used to represent that fraction of the
available moisture which can be easily extracted by the plants. Readily available moisture is
approximately 75% of the available moisture.
The total available moisture dt (in terms of depth) for a plant (or soil) is given by
dt = (wfc – wwp) d (3.9)
in which, d is the depth of the root zone.
It is obvious that soil moisture can vary between the field capacity (excess amount would
drain away) and the permanent wilting point. However, depending upon the prevailing
conditions, soil moisture can be allowed to be depleted below the field capacity (but not below
the permanent wilting point in any case), before the next irrigation is applied. The permissible
amount of depletion is referred to as the management allowed deficit Dm which primarily
depends on the type of crop and its stage of growth (2). Thus,
Dm = fm dt (3.10)
in which, fm is, obviously, less than 1 and depends upon the crop and its stage of growth. At a
time when the soil moisture content is w, the soil-moisture deficit Ds is given as
Ds = (wfc – w) d (3.11)
Example 3.4 For the following data, calculate the total available water and the soil
moisture defict.
Soil depth (cm) Gb Wfc Wwp W
0-15 1.25 0.24 0.13 0.16
15-30 1.30 0.28 0.14 0.18
30-60 1.35 0.31 0.15 0.23
60-90 1.40 0.33 0.15 0.26
90-120 1.40 0.31 0.14 0.28
102 IRRIGATION AND WATER RESOURCES ENGINEERING
Solution:
Depth of soil wfc = Gb Wfc wwp = Gb Wwp dt = d × w = Gb W Ds = d ×
layers, d (wfc – wwp) (wfc – w)
(mm) (mm) (mm)
150 0.3 0.1625 20.625 0.2 15.0
150 0.364 0.182 27.300 0.234 19.5
300 0.4185 0.2025 64.800 0.3105 32.4
300 0.462 0.21 75.600 0.364 29.4
300 0.434 0.196 71.400 0.392 12.6
Total 259.725 108.9
Example 3.5 The field capacity and permanent wilting point for a given 0.8 m root-zone
soil are 35 and 10 per cent, respectively. At a given time, the soil moisture in the given soil is
20 per cent when a farmer irrigates the soil with 250 mm depth of water. Assuming bulk
specific gravity of the soil as 1.6, determine the amount of water wasted from the consideration
of irrigation.
Solution:
At the time of application of water,
Soil moisture deficit, Ds = (Wfc – W) d Gb
= (0.35 – 0.20) (0.8) (1.6)
= 0.192 m
Therefore, the amount of water wasted
= 0.250 – 0.192
= 58 mm
=
58
250
× 100 = 23.2%
INFILTRATION
Infiltration is another important property of soil which affects surface irrigation. It not only
controls the amount of water entering the soil but also the overland flow. Infiltration is a
complex process which depends on: (i) soil properties, (ii) initial soil moisture content, (iii)
previous wetting history, (iv) permeability and its changes due to surface water movement, (v)
cultivation practices, (vi) type of crop being sown, and (vii) climatic effects. In an initially dry
soil, the infiltration rate is high at the beginning of rain (or irrigation), but rapidly decreases
with time until a fairly steady state infiltration is reached (Fig. 3.3). This constant rate of
infiltration is also termed the basic infiltration rate and is approximately equal to the
permeability of the saturated soil.
The moisture profile under ponded infiltration into dry soil, Fig. 3.4, can be divided into
the following five zones (4):
SOIL-WATER RELATIONS AND IRRIGATION METHODS 103
0 Infiltration time, hours 20
Infiltration rate, in cm/hr I
Cumulative infiltration, Z in cm
Variation of infiltration rate, I and cumulative infiltration, Z with time
Soil depth
Initial water content
θ θs O i Water content
Transition zone
Saturated zone
Transmission
zone
Wetting zone
Saturated water content
Wetting
front
Soil-moisture profile during ponded infiltration
(i) The saturated zone extending up to about 1.5 cm below the surface and having a
saturated water content.
(ii) The transition zone which is about 5 cm thick and is located below the saturated
zone. In this zone, a rapid decrease in water content occurs.
(iii) The transmission zone in which the water content varies slowly with depth as well as
time.
(iv) The wetting zone in which sharp decrease in water content is observed.
(v) The wetting front is a region of very steep moisture gradient. This represents the
limit of moisture penetration into the soil.
Table 3.1 lists the ranges of porosity, field capacity, permanent wilting point, and basic
infiltration rate (or permeability) for different soil textures.
104 IRRIGATION AND WATER RESOURCES ENGINEERING
Table 3.1 Representative properties of soil
Soil texture Porosity (%) Field capacity Permanent Basic infiltration
(%) wilting point (%) rate (cm/hr)
Sand 32-42 5-10 2-6 2.5-25
Sandy loam 40-47 10-18 4-10 1.3-7.6
Loam 43-49 18-25 8-14 0.8-2.0
Clay loam 47-51 24-32 11-16 0.25-1.5
Silty clay 49-53 27-35 13-17 0.03-0.5
Clay 51-55 32-40 15-22 0.01-0.1
3.7. CONSUMPTIVE USE (OR EVAPOTRANSPIRATION)
The combined loss of water from soil and crop by vaporisation is identified as evapotranspiration
(3). Crops need water for transpiration and evaporation. During the growing period of a crop,
there is a continuous movement of water from soil into the roots, up the stems and leaves, and
out of the leaves to the atmosphere. This movement of water is essential for carrying plant
food from the soil to various parts of the plant. Only a very small portion (less than 2 per cent)
of water absorbed by the roots is retained in the plant and the rest of the absorbed water, after
performing its tasks, gets evaporated to the atmosphere mainly through the leaves and stem.
This process is called transpiration. In addition, some water gets evaporated to the atmosphere
directly from the adjacent soil and water surfaces and from the surfaces of the plant leaves
(i.e., the intercepted precipitation on the plant foliage). The water needs of a crop thus consists
of transpiration and evaporation and is called evapotranspiration or consumptive use.
Consumptive use refers to the water needs of a crop in a specified time and is the sum of
the volume of transpirated and evaporated water. Consumptive use is defined as the amount of
water needed to meet the water loss through evapotranspiration. It generally applies to a crop
but can be extended to a field, farm, project or even a valley. Consumptive use is generally
measured as volume per unit area or simply as the depth of water on the irrigated area.
Knowledge of consumptive use helps determine irrigation requirement at the farm which should,
obviously, be the difference between the consumptive use and the effective precipitation.
Evapotranspiration is dependent on climatic conditions like temperature, daylight hours,
humidity, wind movement, type of crop, stage of growth of crop, soil moisture depletion, and
other physical and chemical properties of soil. For example, in a sunny and hot climate, crops
need more water per day than in a cloudy and cool climate. Similarly, crops like rice or sugarcane
need more water than crops like beans and wheat. Also, fully grown crops need more water
than crops which have been just planted.
While measuring or calculating potential evapotranspiration, it is implicitly assumed
that water is freely available for evaporation at the surface. Actual evapotranspiration, in the
absence of free availability of water for evaporation will, obviously, be less and is determined
by: (i) the extent to which crop covers the soil surface, (ii) the stage of crop growth which
affects the transpiration and soil surface coverage, and (iii) soil water supply.
Potential evapotranspiration is measured by growing crops in large containers, known
as lysimeters, and measuring their water loss and gains. Natural conditions are simulated in
SOIL-WATER RELATIONS AND IRRIGATION METHODS 105
these containers as closely as possible. The operator measures water added, water retained by
the soil, and water lost through evapotranspiration and deep percolation. Weighings can be
made with scales or by floating the lysimeters in water. Growth of roots in lysimeters confined
to the dimensions of lysimeters, the disturbed soil in the lysimeters and other departures from
natural conditions limit the accuracy of lysimeter measurements of potential evapotranspiration.
Potential evapotranspiration from a cropped surface can be estimated either by
correlating potential evapotranspiration with water loss from evaporation devices or by
estimations based on various climatic parameters. Correlation of potential evapotranspiration
assumes that the climatic conditions affecting crop water loss (Det) and evaporation from a free
surface of water (Ep) are the same. Potential evapotranspiration Det can be correlated to the
pan evaporation Ep as (3),
Det = KEp (3.12)
in which, K is the crop factor for that period. Pan evaporation data for various parts of India
are published by the Meteorological Department. The crop factor K depends on the crop as
well as its stage of growth (Table 3.2). The main limitations of this method are the differences
in physical features of evaporation surfaces compared with those of a crop surface.
Table 3.2 Values of crop factor K from some major crops
Percentage of crop Maize, cotton, Wheat,
growing season potatoes, peas Sugarcane Rice barley and
since sowing and sugarbeets other small
grains
In the absence of pan evaporation data, the consumptive use is generally computed as
follows:
(i) Compute the seasonal (or monthly) distribution of potential evapotranspiration, which
is defined as the evapotranspiration rate of a well-watered reference crop which completely
shades the soil surface (2). It is thus an indication of the climatic evaporation
demand of a vigorously growing crop. Usually, grass and alfalfa (a plant with leaves
like that of clover and purple flowers used as food for horses and cattle) are taken as
reference crops.
(ii) Adjust the potential evapotranspiration for the type of crop and the stage of crop
growth. Factors such as soil moisture depletion are ignored so that the estimated
values of the consumptive use are conservative values to be used for design purposes.
Thus, evapotranspiration of a crop can be estimated by multiplying potential
evapotranspiration by a factor known as crop coefficient.
Potential evapotranspiration can be computed by one of the several methods available
for the purpose. These methods range in sophistication from simple temperature correlation
(such as the Blaney-Criddle formula) to equations (such as Penman’s equation) which account
106 IRRIGATION AND WATER RESOURCES ENGINEERING
for radiation energy as well. Blaney-Criddle formula for the consumptive use has been used
extensively and is expressed as (1)
u = kf (3.13)
in which, u = consumptive use of crop in mm,
k = empirical crop consumptive use coefficient (Table 3.3), and
f = consumptive use factor.
The quantities u, k, and f are determined for the same period (annual, irrigation season,
growing season or monthly). The consumptive use factor f is expressed as
f =
p
t
100
(1.8 + 32) (3.14)
in which, t = mean temperature in °C for the chosen period, and
p = percentage of daylight hours of the year occurring during the period.
Table 3.4 lists the values of p for different months of a year for 0° north latitude. The
value of the consumptive use is generally determined on a monthly basis and the irrigation
system must be designed for the maximum monthly water needs. It should be noted that Eq.
(3.13) was originally in FPS system with appropriate values of k. Similarly, Eq. (3.14) too had
a different form with t in Fahrenheit.
Table 3.3 Consumptive use coefficient for some major crops (1)
Crop
Lenght of normal Consumptive use coefficient, k
growing season or For the growing Monthly (maximum
period period* value)*
Corn (maize) 4 months 19.05 to 21.59 20.32 to 30.48
Cotton 7 months 15.24 to 17.78 19.05 to 27.94
Potatoes 3-5 months 16.51 to 19.05 21.59 to 25.40
Rice 3-5 months 25.40 to 27.94 27.94 to 33.02
Small grains 3 months 19.05 to 21.59 21.59 to 25.40
Sugarbeet 6 months 16.51 to 19.05 21.59 to 25.40
Sorghums 4-5 months 17.78 to 20.32 21.59 to 25.40
Orange and lemon 1 year 11.43 to 13.97 16.21 to 19.05
*The lower values are for more humid areas and the higher values are for more arid climates.
* Dependent upon mean monthly temperature and stage of growth of crop.
Table 3.4 Per cent daylight hours for northern hemispere (0-50° latitude) (1)
Latitude
Jan. Fab. March April May June July Aug. Sep. Oct. Nov. Dec.
North
(in degrees)
Table 3.5 gives typical values of the water needs of some major crops for the total growing
period of some of the crops (5). This table also indicates the sensitivity of the crop to water
shortages or drought. High sensitivity to drought means that the crop cannot withstand water
shortages, and that such shortages should be avoided.
Table 3.5 Indicative values of crop water needs and sensitivity to drought (5)
Crop Crop water need Sensitivity of drought
(mm/total growing period)
Alfalfa 800 - 1600 low - medium
Banana 1200 - 2200 high
Barley/oats/wheat 450 - 650 low - medium
Bean 300 - 500 medium - high
Cabbage 350 - 500 medium - high
Citrus 900 - 1200 low - medium
Cotton 700 - 1300 low
Maize 500 - 800 medium - high
Melon 400 - 600 medium - high
Onion 350 - 550 medium - high
Peanut 500 - 700 low - medium
Pea 350 - 500 medium - high
Pepper 600 - 900 medium - high
Potato 500 - 700 high
Rice (paddy) 450 - 700 high
Sorghum/millet 450 - 650 low
Soybean 450 - 700 low - medium
Sugarbeet 550 - 750 low - medium
Sugarcane 1500 - 2500 high
Sunflower 600 - 1000 low - medium
Tomato 400 - 800 medium - high
108 IRRIGATION AND WATER RESOURCES ENGINEERING
Example 3.6 Using the Blaney-Criddle formula, estimate the yearly consumptive use
of water for sugarcane for the data given in the first four columns of Table 3.6.
Solution:
According to Eqs. (3.13) and (3.14),
u = k
p
t
100
(1.8 + 32)
Values of monthly consumptive use calculated from the above formula have been
tabulated in the last column of Table 3.6. Thus, yearly consumptive use = Σu = 1.75 m.
Table 3.6 Data and solution for Example 3.6
Month
Mean monthly Monthly crop Per cent Monthly
temperature, coefficient, k sunshine consumptive
t°C hours, p use, u (mm)
January 13.10 19.05 7.38 78.14
February 15.70 20.32 7.02 85.96
March 20.70 21.59 8.39 125.46
April 27.00 21.59 8.69 151.22
May 31.10 22.86 9.48 190.66
June 33.50 24.13 9.41 209.58
July 30.60 25.40 9.60 212.34
August 29.00 25.40 9.60 205.31
September 28.20 24.13 8.33 166.35
October 24.70 22.86 8.01 140.01
November 18.80 21.59 7.25 103.06
December 13.70 19.05 7.24 78.15
3.8. IRRIGATION REQUIREMENT
Based on the consumptive use, the growth of all plants can be divided into three stages, viz.,
vegetative, flowering, and fruiting. The consumptive use continuously increases during the
vegetative stage and attains the peak value around the flowering stage; thereafter, the
consumptive use decreases. It should be noted that different crops are harvested during different
stages of crop growth. For example, leafy vegetables are harvested during the vegetative stage
and flowers are harvested during the flowering stage. Most crops (such as potatoes, rice, corn,
beans, bananas, etc.) are harvested during the fruiting stage.
At each precipitation, a certain volume of water is added to the crop field. Not all of the
rainfall can be stored within the root zone of the soil. The part of the precipitation which has
gone as surface runoff, percolated deep into the ground or evaporated back to the atomosphere
does not contribute to the available soil moisture for the growth of crop. Thus, effective
precipitation is only that part of the precipitation which contributes to the soil moisture available
for plants. In other words, the effective rainfall is the water retained in the root zone and is
obtained by subtracting the sum of runoff, evaporation, and deep percolation from the total
rainfall.
SOIL-WATER RELATIONS AND IRRIGATION METHODS 109
If, for a given period, the consumptive use exceeds the effective precipitation, the
difference has to be met by irrigation water. In some cases irrigation water has to satisfy
leaching requirements too. Further, some of the water applied to the field necessarily flows
away as surface runoff and/or percolates deep into the ground and/or evaporates to the
atmosphere. Therefore, irrigation requirement is the quantity of water, exclusive of precipitation
and regardless of its source, required by a crop or diversified pattern of crops in a given period
of time of their normal growth under field conditions. It includes evapotranspiration not met
by effective precipitation and other economically unavoidable losses such as surface runoff
and deep percolation. Irregular land surfaces, compact impervious soils or shallow soils over a
gravel stratum of high permeability, small or too large irrigation streams, absence of an
attendant during irrigation, long irrigation runs, improper land preparation, steep ground
slopes and such other factors contribute to large losses of irrigation water which, in turn,
reduce irrigation efficiency. Irrigation efficiency is the ratio of irrigation water consumed by
crops of an irrigated field to the water diverted from the source of supply. Irrigation efficiency
is usually measured at the field entrance (3). Water application efficiency is the ratio of the
average depth added to the root-zone storage to the average depth applied to the field. Obviously,
irrigation efficiency measured at the field and the water application efficiency would be the
same. Thus, the field irrigation requirement FIR is expressed as (2)
FIR =
D D D
E
et p pl
a
− ( − )
(3.15)
in which, Det
= depth of evapotranspiration,
Dp = depth of precipitation,
Dpl = depth of precipitation that goes as surface runoff and/or infiltrates into the
ground and/or intercepted by the plants,
and Ea = irrigation efficiency or application efficiency.
In the absence of any other information, the following values can be used as a guide for
Ea in different methods of surface irrigation for different types of soils:
Soil class Irrigation efficiency (%)
Sand 60
Sandy loam 65
Loam 70
Clay loam 75
Heavy clay 80
If no other information is available, the following formulae can be used to estimate the
effective rainfall depth, Dpe provided that the ground slope does not exceed 5%.
Dpe = 0.8 Dp – 25 if Dp > 75 mm/month
Dpe = 0.6 Dp – 10 if Dp < 75 mm/month
Dpe is always equal to or greater than zero and never negative. Both Dp ad Dpe are in
mm/month in the foregoing formulae.
Example 3.7 Using the data given in the first four columns of Table 3.7 for a given crop,
determine the field irrigation requirement for each month assuming irrigation efficiency to be
60 per cent.
110 IRRIGATION AND WATER RESOURCES ENGINEERING
Month Crop Pan Effective rain- Consumpfactor,
K evaporation, fall, D FIR (mm) p – Dpl tive use,
Ep (mm) (mm) Det (mm)
November 0.20 118.0 6.0 23.60 29.33
December 0.36 96.0 16.0 34.56 30.93
January 0.75 90.0 20.0 67.50 79.17
February 0.90 105.0 15.0 94.50 132.50
March 0.80 140.0 2.0 112.00 183.33
Solution:
According to Eqs. (3.12) and (3.15)
Det = KEp
and FIR =
D D D
E
et p pl
a
− ( − )
Given Ea = 0.6
Field irrigation requirement calculated for each month of the crop-growing season has
been tabulated in the last column of Table 3.7.
3.9. FREQUENCY OF IRRIGATION
Growing crops consume water continuously. However, the rate of consumption depends on the
type of crop, its age, and the atmospheric conditions all of which are variable factors. The aim
of each irrigation is to fulfil the needs of the crop for a period which may vary from few days to
several weeks. The frequency of irrigation primarily depends on: (i) the water needs of the
crop, (ii) the availability of water, and (iii) the capacity of the root-zone soil to store water.
Shallow-rooted crops generally require more frequent irrigation than deep-rooted crops. The
roots of a plant in moist soil extract more water than the roots of the same plant in drier soil.
A moderate quantity of soil moisture is beneficial for good crop growth. Both excessive
and deficient amount of soil moisture retard the crop growth and thus the yield. Excessive
flooding drives out air which is essential for satisfactory crop growth. In case of deficient
moisture, the plant has to spend extra energy to extract the desired amount of water.
Many of the crops have an optimum soil moisture content at which the yield is maximum;
if the moisture content is less or more than this amount, the yield reduces. Wheat has a welldefined
optimum moisture content of around 40 cm. However, there are other crops in which
the yield initially increases at a much faster rate with the increase in the soil moisture content
and the rate of increase of the yield becomes very small at higher moisture content. In such
cases, the soil moisture is kept up to a level beyond which the increase in production is not
worth the cost of the additional water supplied.
It should be noted that, because of the capacity of a soil to store water, it is not necessary
to apply water to the soil every day even though the consumptive use takes place continuously.
The soil moisture can vary between the field capacity and the permanent wilting point. The
average moisture content will thus depend on the frequency of irrigation and quantity of water
applied. As can be seen from Fig. 3.5, frequent irrigation (even of smaller depths) keeps the
average mositure content closer to the field capacity. On the other hand, less frequent irrigation
of larger depths of water will keep the average moisture content on the lower side.
SOIL-WATER RELATIONS AND IRRIGATION METHODS 111
Field capacity
Average
moisture content
PWP
Soil moisture content
Time
(a) MORE FREQUENT IRRIGATION
Field
capacity
Average
moisture content
PWP
Soil moisture content
Time
(b) LESS FREQUENT IRRIGATION
Fig. 3.5 Effect of frequency of irrigation on average moisute content
For most of the crops, the yield remains maximum if not more than 50 per cent of the
available water is removed during the vegetative, flowering, and the intitial periods of the
fruiting stage. During the final period of the fruiting stage, 75 per cent of the available moisture
can be depleted without any adverse effect on the crop yield.
The frequency of irrigation (or irrigation interval) is so decided that the average moisture
content is close to the optimum and at each irrigation the soil moisture content is brought to
the field capacity. Alternatively, the frequency of irrigation can be decided so as to satisfy the
daily consumptive use requirement which varies with stage of growth. Thus, frequency of
irrigation is calculated by dividing the amount of soil moisture which may be depleted (i.e.,
allowable depletion below field capacity and well above permanent wilting point) within the
root-zone soil by the rate of consumptive use. Thus,
Frequency of irrigation =
Allowable soil moisture depletion
Rate of consumptive use
(3.16)
The depth of watering at each irrigation to bring the moisture content w to the field
capacity wfc in a soil of depth d can be determined from the following relation:
Depth of water to be applied =
(w wd
E
fc
a
− )
(3.17)
Example 3.8 During a particular stage of the growth of a crop, consumptive use of
water is 2.8 mm/day. Determine the interval in days between irrigations, and depth of water
to be applied when the amount of water available in the soil is: (i) 25%, (ii) 50% (iii) 75%, and
112 IRRIGATION AND WATER RESOURCES ENGINEERING
(iv) 0% of the maximum depth of available water in the root zone which is 80 mm. Assume
irrigation efficiency to be 65%.
Solution:
(i) Frequency of irrigation =
80 1 0 25
28
× ( − . )
.
= 21.43 days
= 21 days (say)
(ii) Depth of water to be applied =
80 1 0 25
065
× ( − . )
.
= 92.31 mm
= 93.00 mm (say).
Other calculations have been shown in the following table:
Amount of soil moisture depleted to
25% 50% 75% 0%
Frequency of irrigation (days) 21 14 7 28
Depth of water to be applied (mm) 93 62 31 124
3.10. METHODS OF IRRIGATION
Any irrigation system would consist of the following four subsystems (2):
(i) The water supply subsystem which may include diversion from rivers or surface
ponds or pumped flow of ground water.
(ii) The water delivery subsystem which will include canals, branches, and hydraulic
structures on these.
(iii) The water use subsystems, which can be one of the four main types, namely, (a)
surface irrigation, (b) subsurface irrigation, © sprinkler irrigation, and (d) trickle
irrigation.
(iv) The water removal system i.e., the drainage system.
In this section, the water use subsystems have been described.
3.10.1. Water Use Subsystems
Irrigation water can be applied to croplands using one of the following irrigation methods (1):
(i) Surface irrigation which includes the following:
(a) Uncontrolled (or wild or free) flooding method,
(b) Border strip method,
© Check method,
(d) Basin method, and
(e) Furrow method.
(ii) Subsurface irrigation
(iii) Sprinkler irrigation
(iv) Trickle irrigation
SOIL-WATER RELATIONS AND IRRIGATION METHODS 113
Each of the above methods has some advantages and disadvantages, and the choice of
the method depends on the following factors (2):
(i) Size, shape, and slope of the field,
(ii) Soil characteristics,
(iii) Nature and availability of the water supply subsystem,
(iv) Types of crops being grown,
(v) Initial development costs and availability of funds, and
(vi) Preferences and past experience of the farmer.
The design of an irrigation system for applying water to croplands is quite complex and
not amenable to quantitative analysis. Principal criteria for the design of a suitable irrigation
method are as follows (3):
(i) Store the required water in the root-zone of the soil,
(ii) Obtain reasonably uniform application of water,
(iii) Minimise soil erosion,
(iv) Minimise run-off of irrigation water from the field,
(v) Provide for beneficial use of the runoff water,
(vi) Minimise labour requirement for irrigation,
(vii) Minimise land use for ditches and other controls to distribute water,
(viii) Fit irrigation system to field boundaries,
(ix) Adopt the system to soil and topographic changes, and
(x) Facilitate use of machinery for land preparation, cultivating, furrowing, harvesting,
and so on.
3.10.2. Surface Irrigation
In all the surface methods of irrigation, water is either ponded on the soil or allowed to flow
continuously over the soil surface for the duration of irrigation. Although surface irrigation is
the oldest and most common method of irrigation, it does not result in high levels of performance.
This is mainly because of uncertain infiltration rates which are affected by year-to-year changes
in the cropping pattern, cultivation practices, climatic factors, and many other factors. As a
result, correct estimation of irrigation efficiency of surface irrigation is difficult. Application
efficiencies for surface methods may range from about 40 to 80 per cent.
(a) Uncontrolled Flooding
When water is applied to the cropland without any preparation of land and without any
levees to guide or restrict the flow of water on the field, the method is called ‘uncontrolled’,
wild or ‘free’ flooding. In this method of flooding, water is brought to field ditches and then
admitted at one end of the field thus letting it flood the entire field without any control.
Uncontrolled flooding generally results in excess irrigation at the inlet region of the
field and insufficient irrigation at the outlet end. Application efficiency is reduced because of
either deep percolation (in case of longer duration of flooding) or flowing away of water (in case
of shorter flooding duration) from the field. The application efficiency would also depend on
the depth of flooding, the rate of intake of water into the soil, the size of the stream, and
topography of the field.
114 IRRIGATION AND WATER RESOURCES ENGINEERING
Obviously, this method is suitable when water is available in large quantities, the land
surface is irregular, and the crop being grown is unaffected because of excess water. The
advantage of this method is the low initial cost of land preparation. This is offset by the
disadvantage of greater loss of water due to deep percolation and surface runoff.
(b) Border Strip Method
Border strip irrigation (or simply ‘border irrigation’) is a controlled surface flooding
method of applying irrigation water. In this method, the farm is divided into a number of
strips which can be 3-20 metres wide and 100-400 metres long. These strips are separated by
low levees (or borders). The strips are level between levees but slope along the length according
to natural slope. If possible, the slope should be between 0.2 and 0.4 per cent. But, slopes as
flat as 0.1 per cent and as steep as 8 per cent can also be used (1). In case of steep slope, care
should be taken to prevent erosion of soil. Clay loam and clayey soils require much flatter
slopes (around 0.2%) of the border strips because of low infiltration rate. Medium soils may
have slopes ranging from 0.2 to 0.4%. Sandy soils can have slopes ranging from 0.25 to 0.6%.
Water from the supply ditch is diverted to these strips along which it flows slowly towards
the downstream end and in the process it wets and irrigates the soil. When the water supply is
stopped, it recedes from the upstream end to the downsteam end.
The border strip method is suited to soils of moderately low to moderately high intake
rates and low erodibility. This method is suitable for all types of crops except those which
require prolonged flooding which, in this case, is difficult to maintain because of the slope.
This method, however, requires preparation of land involving high initial cost.
© Check Method
The check method of irrigation is based on rapid application of irrigation water to a
level or nearly level area completely enclosed by dikes. In this method, the entire field is
divided into a number of almost levelled plots (compartments or ‘Kiaries’) surrounded by levees.
Water is admitted from the farmer’s watercourse to these plots turn by turn. This method is
suitable for a wide range of soils ranging from very permeable to heavy soils. The farmer has
very good control over the distribution of water in different areas of his farm. Loss of water
through deep percolation (near the supply ditch) and surface runoff can be minimised and
adequate irrigation of the entire farm can be achieved. Thus, application efficiency is higher
for this method. However, this method requires constant attendance and work (allowing and
closing the supplies to the levelled plots). Besides, there is some loss of cultivable area which is
occupied by the levees. Sometimes, levees are made sufficiently wide so that some ‘row’ crops
can be grown over the levee surface.
(d) Basin Method
This method is frequently used to irrigate orchards. Generally, one basin is made for
one tree. However, where conditions are favourable, two or more trees can be included in one
basin.
(e) Furrow Method
In the surface irrigation methods discussed above, the entire land surface is flooded
during each irrigation. An alternative to flooding the entire land surface is to construct small
channels along the primary direction of the movement of water and letting the water flow
through these channels which are termed ‘furrows’, ‘creases’ or ‘corrugation’. Furrows are
small channels having a continuous and almost uniform slope in the direction of irrigation.
Water infiltrates through the wetted perimeter of the furrows and moves vertically and then
laterally to saturate the soil. Furrows are used to irrigate crops planted in rows.
SOIL-WATER RELATIONS AND IRRIGATION METHODS 115
Furrow lengths may vary from 10 metres to as much as 500 metres, although, 100 metres
to 200 metres are the desirable lengths and more common. Very long furrows may result in
excessive deep percolation losses and soil erosion near the upstream end of the field. Preferable
slope for furrows ranges between 0.5 and 3.0 per cent. Many different classes of soil have been
satisfactorily irrigated with furrow slope ranging from 3 to 6 per cent (1). In case of steep
slopes, care should be taken to control erosion. Spacing of furrows for row crops (such as corn,
potatoes, sugarbeet, etc.) is decided by the required spacing of the plant rows. The furrow
stream should be small enough to prevent the flowing water from coming in direct contact
with the plant. Furrows of depth 20 to 30 cm are satisfactory for soils of low permeability. For
other soils, furrows may be kept 8 to 12 cm deep.
Water is distributed to furrows from earthen ditches through small openings made in
earthen banks. Alternatively, a small-diameter pipe of light weight plastic or rubber can be
used to siphon water from the ditch to the furrows without disturbing the banks of the earthen
ditch.
Furrows necessitate the wetting of only about half to one-fifth of the field surface. This
reduces the evaporation loss considerably. Besides, puddling of heavy soils is also lessened
and it is possible to start cultivation soon after irrigation. Furrows provide better on-farm
water management capabilities for most of the surface irrigation conditions, and variable and
severe topographical conditions. For example, with the change in supply conditions, number
of simultaneously supplied furrows can be easily changed. In this manner, very high irrigation
efficiency can be achieved.
The following are the disadvantages of furrow irrigation:
(i) Possibility of increased salinity between furrows,
(ii) Loss of water at the downstream end unless end dikes are used,
(iii) The necessity of one extra tillage work, viz., furrow construction,
(iv) Possibility of increased erosion, and
(v) Furrow irrigation requires more labour than any other surface irrigation method.
3.10.3. Subsurface Irrigation
Subsurface irrigation (or simply subirrigation) is the practice of applying water to soils directly
under the surface. Moisture reaches the plant roots through capillary action. The conditions
which favour subirrigation are as follows (1):
(i) Impervious subsoil at a depth of 2 metres or more,
(ii) A very permeable subsoil,
(iii) A permeable loam or sandy loam surface soil,
(iv) Uniform topographic conditions, and
(v) Moderate ground slopes.
In natural subirrigation, water is distributed in a series of ditches about 0.6 to 0.9 metre
deep and 0.3 metre wide having vertical sides. These ditches are spaced 45 to 90 metres apart.
Sometimes, when soil conditions are favourable for the production of cash crops (i.e.,
high-priced crops) on small areas, a pipe distribution system is placed in the soil well below the
surface. This method of applying water is known as artificial subirrigation. Soils which permit
IRRIGATION AND WATER RESOURCES ENGINEERING
free lateral movement of water, rapid capillary movement in the root-zone soil, and very slow
downward movement of water in the subsoil are very suitable for artificial subirrigation. The
cost of such methods is very high. However, the water consumption is as low as one-third of
the surface irrigation methods. The yield also improves. Application efficiency generally varies
between 30 and 80 per cent.
. Sprinkler Irrigation
Sprinkling is the method of applying water to the soil surface in the form of a spray which is
somewhat sililar to rain. In this method, water is sprayed into the air and allowed to fall on the
soil surface in a uniform pattern at a rate less than the infiltration rate of the soil. This method
started in the beginning of this century and was initially limited to nurseries and orchards. In
the beginning, it was used in humid regions as a supplemental method of irrigation. This
method is popular in the developed countries and is gaining popularity in the developing
countries too.
Rotating sprinkler-head systems are commonly used for sprinkler irrigation. Each
rotating sprinkler head applies water to a given area, size of which is governed by the nozzle
size and the water pressure. Alternatively, perforated pipe can be used to deliver water through
very small holes which are drilled at close intervals along a segment of the circumference of a
pipe. The trajectories of these jets provide fairly uniform application of water over a strip of
cropland along both sides of the pipe. With the availability of flexible PVC pipes, the sprinkler
systems can be made portable too.
Sprinklers have been used on all types of soils on lands of different topography and
slopes, and for many crops. The following conditions are favourable for sprinkler irrigation (1):
(i) Very previous soils which do not permit good distribution of water by surface methods,
(ii) Lands which have steep slopes and easily erodible soils,
(iii) Irrigation channels which are too small to distribute water efficiently by surface
irrigation, and
(iv) Lands with shallow soils and undulating lands which prevent proper levelling required
for surface methods of irrigation.
Besides, the sprinkler system has several features. For example, small amounts of water
can be applied easily and frequently by the sprinkler system. Light and frequent irrigations
are very useful during the germination of new plants, for shallow-rooted crops and to control
soil temperature. Measurement of quantity of water is easier. It causes less interference in
cultivation and other farming operations. While sprinkler irrigation reduces percolation losses,
it increases evaporation losses. The frequency and intensity of the wind will affect the efficiency
of any sprinkler system. Sprinkler application efficiencies should always be more than 75 per
cent so that the system is economically viable.
The sprinkler method is replacing the surface/gravity irrigation methods in all developed
countries due to its higher water application/use efficiency, less labour requirements,
adaptability to hilly terrain, and ability to apply fertilizers in solution. In India too, the gross
area under sprinkler irrigation has increased from 3 lakh hectares in 1985 to 5.80 lakh hectares
in 1989. The total number of sprinkler sets in India now exceeds one lakh.
SOIL-WATER RELATIONS AND IRRIGATION METHODS 117
3.10.5. Trickle Irrigation
Trickle irrigation (also known as drip irrigation) system comprises main line (37.5 mm to 70
mm diameter pipe), submains (25 mm to 37.5 mm diameter pipe), laterals (6 mm to 8 mm
diameter pipe), valves (to control the flow), drippers or emitters (to supply water to the plants),
pressure gauges, water meters, filters (to remove all debris, sand and clay to reduce clogging of
the emitters), pumps, fertiliser tanks, vacuum breakers, and pressure regulators. The drippers
are designed to supply water at the desired rate (1 to 10 litres per hour) directly to the soil.
Low pressure heads at the emitters are considered adequate as the soil capillary forces cause
the emitted water to spread laterally and vertically. Flow is controlled manually or set to
automatically either (i) deliver desired amount of water for a predetermined time, or (ii) supply
water whenever soil moisture decreases to a predetermined amount. A line sketch of a typical
drip irrigation system is shown in Fig. 3.6. Drip irrigation has several advantages. It saves
water, enhances plant growth and crop yield, saves labour and energy, controls weed growth,
causes no erosion of soil, does not require land preparation, and also improves fertilizer
application efficiency. However, this method of irrigation does have some economic and technical
limitations as it requires high skill in design, installation, and subsequent operation.
Porous pipe
Multi-outlet
distributors
Submain
Nutrient tank
Gate valve Filter
Check valve
with bypass
From pump or
pressure supply
Pressure regulator
Sublateral
loop
Lateral Emitters
Gate valve
Main line
Pressure
control valve
Line sketch of a typical drip irrigation system
Trickle irrigation enables efficient water application in the root zone of small trees and
widely spaced plants without wetting the soil where no roots exist. In arid regions, the irrigation
efficiency may be as high as 90 per cent and with very good management it may approach the
ideal value of 100 per cent. The main reasons for the high efficiency of trickle irrigation are its
capability to produce and maintain continuously high soil moisture content in the root zone
and the reduction in the growth of weeds (due to limited wet surface area) competing with the
crop for water and nutrients. Insect, disease, and fungus problems are also reduced by
minimising the wetting of the soil surface.
Due to its ability to maintain a nearly constant soil moisture content in the root zone,
Fig. 3.7, trickle irrigation results in better quality and greater crop yields. Fruits which contain
considerable moisture at the time of harvesting (such as tomatoes, grapes, berries, etc.) respond
very well to trickle irrigation. However, this method is not at all suitable (from practical as
well as economic considerations) for closely planted crops such as wheat and other cereal grains.
118 IRRIGATION AND WATER RESOURCES ENGINEERING
Field
capacity
Moisture content
Wilting
point
Drip
method
Sprinkler
method
Surface
method
Days
Moisture availability for crops in different irrigation methods
One of the major problems of trickle irrigation is the clogging of small conduits and
openings in the emitters due to sand and clay particles, debris, chemical precipitates, and
organic growth. In trickle irrigation, only a part of the soil is wetted and, hence it must be
ensured that the root growth is not restricted. Another problem of trickle irrigation is on account
of the dissolved salt left in the soil as the water is used by the plants. If the rain water flushes
the salts near the surface down into the root zone, severe damage to the crop may result. In
such situations, application of water by sprinkler or surface irrigation may become necessary.
Because of the obvious advantages of water saving and increased crop yield associated
with the drip irrigation, India has embarked on a massive programme for popularising this
method. The area under drip irrigation in India is about 71000 hectares against a world total
of about 1.8 million hectares (6). The area coverage is th