Module-20

Irrigation & Drainage

Part-II

20.1 Functions of Irrigation water

The functions of soil moisture in plant growth are very important. Water and nutrients are the

two most important requirements of the crop. Following are the main functions of irrigation

water.

(1) It acts a solvent for the nutrients. Water forms the solution of the nutrients, and this

solution is absorbed by the roots. Thus, water acts as the nutrient carrier.

(2) The irrigation water supplies moisture which is essential for the life of bacteria beneficial

to the plant growth.

(3) Irrigation water supplies moisture which is essential for the chemical action within the

plant leading to its growth.

(4) Some salts present in soil react to produce nourishing food products only in the presence

of water.

(5) Water cools the soil and the atmosphere, and thus makes more favorable environment for

healthy plant growth.

(6) Irrigation water, with controlled supplies, washes out or dilutes salts in the soil.

(7) It reduces the hazard of soil piping.

(8) It softens the tillage pans.

20.2 Preparation of land for irrigation

The land should be properly prepared before irrigation water is applied upon it. This can be done

as follows.

(i) Removal of thick jungle, bushes etc., from the raw land. The roots of the trees should be

extracted and burnt. The land should thereafter be properly cleaned.

(ii) The land should be made level. High patches should be scraped and depression filled.

Unless this is done, water will fill the depression and duty may be too low.

(iii) The land should be provided with regular slope in the direction of falling gradient.

(iv) The land should be divided into suitable plots by small levees according to this

method of irrigation to be practiced.

(v) Proper drainage measures should be adopted where the danger of water logging may

become eminent after the introduction of canal irrigation.

20.3 Classes and availability of soil water

Water present in the soil may be classified

under three heads (figure 20.1).

(1) Gravitational water

(2) Capillary water

(3) Hygroscopic water

• Gravitational water: A soil sample

saturated with water and left to drain

the excess out by gravity holds on to a

certain amount of water. The volume

of water that could easily drain off is

termed as the gravitational water. This

water is not available for plants use as

it drains off rapidly from the root zone.

Capillary water: the water content retained in the soil after the gravitational water has drained off from the soil is known as the capillary water. This water is held in the soil by surface tension. Plant roots

Figure 20.1: Classes of soil water

gradually absorb the capillary water and thus constitute the principle source of water for plant growth.

• Hygroscopic water: the water that an oven dry sample of soil absorbs when exposed to

moist air is termed as hygroscopic water. It is held as a very thin film over the surface of

the soil particles and is under tremendous negative (gauge) pressure. This water is not

available to plants.

The above definitions of the soil water are based on physical factors. Some properties of soil

water are not directly related to the above significance to plant growth. These are discussed next.

Water may also be classified as unavailable, available and superfluous. This classification is

based on the availability of soil water to plants.

20.4 Soil water constants

For a particular soil, certain soil water proportions are defined which dictate whether the water is

available or not for plant growth. These are called the soil water constants, which are described

below.

(1) Saturation capacity: This is the total water content of the soil when all the pores of the

soil are filled with water. It is also termed as the maximum water holding capacity of the

soil. At saturation capacity, the soil moisture tension is almost equal to zero.

(2) Field capacity: This is the water retained by an initially saturated soil against the force of

gravity. Hence, as the gravitational water gets drained off from the soil, it is said to reach

the field capacity. At field capacity, the macro pores of the soil are drained off, but water

is retained in the micro pores. Though the soil moisture tension at field capacity varies

from soil to soil, it is normally between 1/10 (for clayey soils) to 1/3 (for sandy soils)

atmospheres.

(3) Permanent wilting point: Plant roots are able to extract water from a soil matrix, which is

saturated up to field capacity. However, as the water extraction proceeds, the moisture

content diminishes and the negative (gauge) pressure increases. At one point, the plant

cannot extract any further water and thus wilts.

Two stages of wilting points are recognized and they are:

Temporary wilting point: This denotes the soil water content at which the plant wilts

at day time, but recovers during night or when water is added to the soil.

Ultimate wilting point: at such a soil water content, the plant wilts and fails to regain

life even after addition of water to soil.

It must be noted that the above water contents are expressed as percentage of water held in the

soil pores, compared to a fully saturated soil. Figure 20.1 explains graphically, the various soil

constants; the full pie represents the volume of voids in soil.

Figure 20.1 Classification of soil water

As shown in Figure 20.1, the available water for plants is defined as the difference in moisture

content of the soil between field capacity and permanent wilting point.

Field capacity and Permanent wilting point: Although the pie diagrams in Figure 20.1

demonstrate the drying up of saturated soil pores, all the soil constants are expressed as a

percentage by weight of the moisture available at that point compared to the weight of the dried

soil sand sample.

(4) Available Moisture: The difference in water content of the soil between field capacity and

permanent wilting point is known as available water or available moisture.

(5) Readily available moisture: It is that portion of the available moisture that is most easily

extracted by plants, and is approximately 75% of the available moisture.

The above mentioned soil-moisture constants for different types of soils are shown below in a

tabular form.

Types of Soil Wilting Coefficient Field Capacity Available WaterSand 4 9 5Sandy loam 6 14 8Loam 10 22 12Clay loam 13 27 14Silty clay 15 31 16Clay 17 35 18

(6) Moisture equivalent: This is an artificial moisture property of the soil and is used as an

index of the natural properties. Moisture equivalent is used as a single factor to which the

properties of soil can be related within reasonable limits. The moisture equivalent roughly

equals field capacity for a medium textured soil. The relation between these are as

follows

Moisture equivalent = Field capacity

= 1.8 to 2 permanent wilting point

= 2.7 Hygroscopic coefficient

(7) Soil-Moisture Deficiency

Soil-moisture deficiency or field moisture deficiency is the water required to bring the soil

moisture content of the soil to it field capacity.

Depth of water stored in root zone and available to plants:

In order to estimate the depth of water stored in the root zone of soil containing water

upto field capacity, let d be the depth of root zone (in metres) and Fc be the field capacity

(expressed as ratio).

γd = dry unit weight of soil

γw = unit weight of water

Consider unit area (1 Sq. Metre) of soil area. Then

Fc = Wt. of water retained in unit areaWt. of soil of unit area = Wt. of water

retained in nit areaγd.1. d

Wt. of water retained in unit area =

Depth of water stored (in depth d) = metres

A part of this depth of water will be available for evapo-transpiration. Available moisture depth

(dw) is given by

[Field capacity – Wilting coefficient]

Where, Sg = apparent specific gravity of soil =

Fc = Field capacity, expressed as ratio = weight of water held by soil per unit

areaWeight of soil per unit area

Wc = wilting coefficient, expressed as ratio

Example: The root zone of an irrigation soil has dry weight of 15 kN/m3 and a field capacity of

30%. The root zone depth of a certain crop, having permanent wilting percentage of 8% is 0.8m.

Determine (a) depth of moisture in the root zone at field capacity (b) depth of moisture in the

root zone at permanent wilting point, and (c) depth of water available.

Solution:

(a) Depth of water in root zone at field capacity, per metre depth of soil

(b) Depth of water in root zone at permanent wilting point (PWP), per metre depth of soil

(c) Depth of water available in root zone, dw

20.5 Limiting soil moisture conditions

It is essential to maintain readily available water in the soil if crops are to make satisfactory

growth. The plant growth may be retarded if the soil-moisture is either deficient or excessive. If

the soil moisture is only slightly more than wilting coefficient, the plant must expend extra

energy to obtain it and the plant will not grow healthy. Similarly, excessive flooding fills the soil

pores with water, thus driving out air. Since air is essential to satisfactory plant growth, excessive

water supply retards plant growth. The optimum moisture percentage is thus that moisture

corresponding to which optimum growth of plant takes place, as shown in figure 20.2.

Figure 20.2: Limiting soil moisture conditions

20.6 Depth and frequency of irrigation

As explained earlier, available moisture is the moisture between wilting point and the field

capacity. The readily available moisture is that moisture which is easily extracted by the plants,

and is approximately 75% of the available moisture. At any time, therefore, the moisture content

in the soil should be between the field capacity and lower limit (m0), of the readily available

moisture, as shown in figure 20.3.

Figure 20.3 Frequency of irrigation

Thus, m0 is the maximum level upto which the soil moisture may be allowed to be depleted in

the root zone without fall in the yield.

When watering is done, the amount of water supplied should be such that the eater

content should be equal to the field capacity. Water will be gradually utilized consumptively by

plants after the water application, and the soil moisture will start falling. When the water content

in the soil reaches the value m0, fresh doses of irrigation may be done so that water content is

again raised to the field capacity of the soil

The frequency of irrigation is controlled by the amount of available water of contained in

the root zone of the soil and the consumptive use rate. If d is the root zone depth in metres, FC

is

the field capacity and m0 is the lower limit of readily available moisture content, the depth of

water dw to be given during each watering is found from the following expression,

Both Fc and m

0 are the moisture contents to be expressed as the ratio.

If Cu is the daily consumptive use rate, frequency of watering f

w is given by

Time required to irrigate a certain area

Let t be the time required to apply the desired water depth dw to bring the water level in the soil

from m0 to the field capacity F

C, over irrigation field of area A. If q is the discharge in the field

channel, in cumecs, we have

Where A is the area in square metres and dw is the depth of water to be applied in metres, if,

however, area A is expressed in hectares, we have

Example: After how many days will you supply water to soil (clay loam) in order to ensure

efficient irrigation of the given crop, if

(i)Field capacity of soil = 27%

(ii)Permanent wilting point = 14%

(iii)Dry density of soil = 15 kN/m

3

(iv)Effective depth of root zone = 75 cm

(v)Daily consumptive use of water for the given crop = 11 mm.

Solution: Available moisture = field capacity – Permanent wilting point

= 27-14=13%

Let the readily available moisture be 80% of the available moisture.

∴ Readily available moisture = 13x0.8=10.4%

∴ m0 = 27-10.4=16.6%

Hence when irrigation water is applied, moisture is raised from 1.6% to 27%.

∴ Depth of water stored in root zone, during each watering,

Thus, depth of water available for evapo-transpiration = 11.9 cm

Daily consumptive use of water = 1.1 cm

∴ Watering frequency = 11.9/1.1=10.82 days or 10 days

Example: An irrigation field 40 m wide 250 m long has soil which has apparent specific gravity

equal to 1.56 and field capacity equal to 22%. The depth of root zone is 0.6 m. If the irrigation is

started when 70% of the available moisture has been used, compute (a) net depth of irrigation

water required, and (b) time required to irrigate the field if the discharge in the field channel is 20

litres per second.

Solution: (a) Given, FC = 22%=0.22; S

g = 1.56; d=0.6 m

Moisture content before start of irrigation, m= 0.3x0.22=0.066

(b) Discharge, q = 20lps = 20x10-3

x3600 m3

/hour = 72 m3

/hour