A sound knowledge about soil and water relationship is very essential for irrigation engineers to improve irrigation practices and economise the use of irrigation water.

The qualities which must be determined to judge the soil-water relationships are :

(1) Infiltration,

(2) Permeability,

(3) Drainability,

(4) Leachability,

(5) Erodability, and

(6) Available moisture holding capacity.

(1) Infiltration:

Infiltration is the entry of water from one medium into another i.e.. air into soil. Water moves into the soil during irrigation applica tion through infiltration. 

This movement of irrigation water into the soil is governed by many complex factors such as:

(i) soil properties 

(ii) hydraulic gradient and 

(iii) initial moisture content of the soil. 

There are different infiltration zones such as:- 

(i) wetting zone 

(ii) transmis sion zone 

(iii) transition zone 

(iv) saturated zone and 

(v) water layer or water table. 

The soil moisture content in the transmission zone is above field capacity but below saturation and at the wetting front is at or near the field capacity. The moisture content immediately below the wetting front is at the moisture content present when irrigation is started.

When irrigation application ceases, the free water disappears from the soil surface and the excess water in the saturated and transmission zones move downward or evaporates from the upper few cm until field capacity is reached throughout the wetted soil. 

The movement of water through unsaturated soils is governed largely by the capillary and absorption characteristics of the soil. The depth to which irrigation water will penetrate is governed by the total amount which enters the surface and the soil-moisture deficiency below field capacity. 

The infiltration rate is rapid in the beginning and decreases till it attains a certain minimum, which is equal to per meability, at which it remains constant. The infiltration rate, expres sed in cm lowering of water surface per hour, is determined by means of infiltrometers.


(2) Permeability:

Permeability is defined as the average velocity of flow which occurs through the total cross sectional area of soil under unit hydraulic gradient. It is expressed as cm/sec or m/day. 

Material like gravels having continuous voids is termed as permeable, while clay is least permeable or impermeable. Permeability is important for studies of ground water flow towards wells, drainage of soils, seepage through dams, safety of hydraulic structures against piping.

Various factors affecting permeability are 

(i) grain size 

(ii) void ratio of the soil 

(iii) properties of pore fluid 

(iv) structural arrangement of soil particles

(v) trapped air and foreign matter 

(vi) absorbed water in clay soils.

Permeability is thus the property to transmit water in the same medium i.e. soil. 

It is a broad term and can be further specified as (i) hydraulic conductivity and (ii) intrinsic permeability.

Hydraulic Conductivity:

Hydraulic conductivity is more impor tant and is defined as the effective flow velocity at unit hydraulic gradient and has the dimensions of the velocity. This does not take into account the properties of fluids.

Intrinsic Permeability: 

Intrinsic permeability is the same as hydraulic conductivity except that the properties of fluids such as viscosity and density are also taken into account.

The hydraulic conductivity will change with the quality of water but the value of the intrinsic permeability of a porous medium remains constant for any fluid.

(3) Drainability:

The soil should be capable of draining the excess water easily without raising the water-table within the root zone. 

The primary factors which affect the drainability of a soil are its permeability (hy draulic conductivity) and hydraulic gradient. 

Hydraulic conductivity is a soil quality related to its texture, clay minerology, composition, porosity and many other factors. 

Hydraulic gradient may or may not be related to the soil. It depends upon the depth and spacing of drains and on soil slope. It is also related to the depth and thickness of permeable or impermeable zone in the soil strata.

(4) Leachability:

Leachability is accomplished by passing good quality water through the soil and thereby removing soluble salts. 

For most soils, it is directly related to drainability. In other words, if the soil can be drained or will drain naturally, it can be leached.

(5) Erodability:

The erosion hazards that will arise under irrigation should be predicted. This will influence the kind of preventive measures needed for soil erosion and soil development.

(6) Available moisture holding capacity:

Judicious irrigation implies optimum utilisation of water for maximum benefit of crop growth which in turn depends on the capa city of a soil to hold water in the root zone and pass it on to the root system of plants for their healthy growth. Characteristics of a good soil are:

(i) To hold the maximum quantity of water, without losing it by gravitational flow to the subsoil,

(ii) To permit the nutrients added in water to move easily through the pore spaces, and

(iii) Good structure with optimum pore size distribution.

The characteristics that influence greatly the total available water capacity of a soil are:

(i) Depth of the soil layer

(ii) Texture of the soil

(iii) Structure of the soil

(iv) Type of clay mineral

(v) Storage capacity of the soil.

With other factors remaining equal, soils with high available water holding capacity are found to be better suited for irrigation.

The available water-holding capacity of a soil can be calculated from the relation

AWHC = Pvd/100

AWHC = Available water-holding capacity of a soil in cm of water/cm of soil.

p = Percent moisture of the soil held between field capacity and wilting point.

v = Volume weight of the soil expressed in gm/cm³.

d = Depth of the soil in cm.

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