Root Zone Soil Water Relationships | Irrigation

Any given volume, V of soil (Fig. 3.1) consists of:

(i) Volume of solids V_s

(ii) Volume of liquid (water) V_w, and

(iii) Volume of gas (air) V_a

The porosity n, the volumetric moisture content w and the saturation s are defined as

N = V_v/V

Obviously, w = s.n.

Further, if the weight of water in a wet soil sample is W_w and the dry weight of the sample is W_s, then the dry weight moisture fraction, W is expressed as

The bulk density (or the bulk specific weight or the bulk unit weight) of a soil mass is then dry weight of the soil per unit bulk volume, i.e., W_s/V (Fig. 3.1). The specific weight (or the unit weight) of the soil particles is the ratio of dry weight of the soil particles W_s to the volume of the soil particles V_s, i.e., W_s/V_s. Thus,

Here, y_w is the unit weight of water and G_b and G_s are, respectively, bulk specific gravity of the soil and relative density of soil grains. Further,

Considering a soil of root zone depth d and surface area A (i.e., bulk volume = Ad),

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.

Crop growth (or yield) is directly affected by the soil-moisture content. Both excessive water (which results in waterlogging) and deficient water retard the crop growth and reduce the crop yield.

Soil water can be divided into three categories (Fig. 3.2):

(i) Gravity (or free) water.

(ii) Capillary water.

(iii) Hygroscopic water.

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 the 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 soil 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. This moisture can be removed by heat only. 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 gravitational water.

The water remaining in the soil after removal of gravitational water is called the field capacity. Field capacity of a soil is defined as the moisture fraction, W_fc (= W_w/W_s) of the soil when free drainage (at rapid rate) has essentially stopped and further drainage, if any, occurs at a very slow rate.

An irrigated soil may take approximately one (in case of sandy soil) to three (in case of clayey soil) days for the rapid drainage to stop. Obviously, the field capacity depends on porosity and soil-moisture tension. The volumetric mois­ture content at the field capacity, W_fc becomes equal to G_b W_fc.

A plant wilts when it is incapable of extracting sufficient moisture from the soil to meet its water requirements. Permanent wilting point is defined as the soil-moisture fraction, W_wp at which the plant leaves wilt (or droop) permanently and applying additional water after this stage will not relieve the wilted condition.

The moisture content at the permanent wilting condition will be higher in the hot climate than in the 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, w_wp becomes G_b. W_wp Figure 3.2 shows the different stages of soil-moisture content and the corresponding conditions.

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 as the available moisture. It represents the maximum moisture which can be stored in the soil for the 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 W_t (in terms of depth) for a plant (or soil) is given by

W_t = (W_fc – W_wp).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. This permissible amount of depletion is referred to as the management allowed deficit D_m which primarily depends on the type of crop and its stage of growth. Thus,

D_m = f_m W_t (3.10)

in which f_m 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 D_s is given as

Ds = (w_fc– w).d (3.11)

Example 3.1: For the following data, calculate the total available water and the soil-moisture deficit:

Total available water = 259.725 mm ≈ 260 mm

and the soil moisture deficit = 108.9 mm ≈ 109 mm