MOVEMENT OF WATER INTO SOILS
Immediately after irrigation or rainfall, the first action or process of water intake is called infiltration, then percolation and then seepage take place. The movement of water from the surface into the soil is called infiltration.
The infiltration characteristics of the soil are one of the dominant variables influencing irrigation. Infiltration rate is the soil characteristic determining the maximum rate at which water can enter the soil under specific conditions, including the presence of excess water. It has the dimensions of velocity. The actual rate at which water is entering the soil at any given time is termed the Infiltration velocity. The infiltration rate decreases during irrigation.
The rate of decrease is rapid initially and the infiltration rate tends to approach a constant value. The nearly constant rate that develops after some time has elapsed from the start of irrigation is called the basic infiltration rate.
Factors affecting infiltration rate – The major factors affecting the infiltration of water into the soil are the initial moisture content, condition of the soil surface, hydraulic conductivity of the soil profile, texture, porosity, and degree of swelling of soil colloids and organic matter, vegetative cover, duration of irrigation or rainfall and viscosity of water. The antecedent soil moisture content has considerable influence on the initial rate and total amount of infiltration, both decreasing as the soil moisture content rises.
The infiltration rate of any soil is limited by any restraint to the flow of water into and through the soil profile. The soil layer with the lowest permeability, either at the surface or below it, usually determines the infiltration rate. Infiltration rates are also affected by the porosity of the soil, which is changed by cultivation or compaction. Cultivation influences the infiltration rate by increasing the porosity of the surface soil and breaking up the surface seals. The effect of tillage on infiltration usually lasts only until the soil settles back to its former condition of bulk density because of subsequent irrigations. Infiltration rates are generally lower in soils of heavy texture than on soils of light texture.
The influence of water depth over soil on infiltration rate was investigated by many workers. It has been established that in surface irrigation, increased depth increases initial infiltration slightly but the head has negligible effect after prolonged irrigation. Infiltration rates are also influenced by the vegetal cover. Infiltration rate on grassland is substantially higher than bare uncultivated land. Additions of organic matter increase infiltration rate substantially.
The hydraulic conductivity of the soil profile often change during infiltration, not only because of increasing moisture content, but also because of the puddling of the surface caused by reorientation of surface particles and washing of finger materials into the soil. Viscosity of water influences infiltration. The high rate of infiltration in the tropics under otherwise comparable soil conditions is due to the low viscosity of warm water.
Water Movement in Soil Profile
Normally water will move from higher potential to lower potential area in soil profile. Generally the water movement within the soil profile takes place under three conditions.
• Water moves through the water filled pore spaces due to gravity and Hydraulic conductivity or it can also be termed as water movement under saturated condition, i.e., when soil pore spaces are completely filled with water.
• Film of water surrounding the soil particles moves due to the force of surface tension under unsaturated condition or it can be stated as capillary water movement along the potential gradient.
• Water also diffuses as water vapour through the air filled pore spaces along the gradient of decreasing vapour pressure.
Water movement in saturated conditions – Saturated flow occurs when water is in zero or smaller tension or at free water conditions. In this situation, all or most of the pore spaces are completely filled with water and the water moves downwards due to gravitational force. This saturated flow decreases as the soil pore space size decreases i.e., the saturated flow is high in coarse textured soil than fine textured soil. Generally the rate of flow of various texture soils is in the following sequence.
Sand > loam > clay
The theory of water movement in the soil is based on Darcy’s law or generalized form of Darcy’s law.
Darcy’s law – It states that the quantity (volume) of water passing through an unit cross-section of soil is proportional to the gradient of hydraulic head or hydraulic gradient.
Hydraulic gradient – It is the rate of change in hydraulic head with distance.
Hg = Difference in hydraulic head/ Distance i.e. I= h/
where , Hg = Hydraulic gradient = I
Generally, Darcy’s law is used to compute the velocity of flow of water through soil by using the formula.
V = kh/d =ki
V = velocity in cubic centimeter/second/centimeter
h = hydraulic head in centimeters
d = flow length or distance in centimeters.
k = hydraulic conductivity or proportionality constant
This formula can also be written as
V = ki, (since h/d = I)
V = effective flow velocity
k = hydraulic conductivity
i = hydraulic gradient.
Here, the value of ‘k’ depends upon the properties of fluid as well and those of soil. In mathematical expression Darcy’s law can be written as
q = kia
q = volume of flow per unit time (cm3/sec)
i = hydraulic gradient (dimensionless)
a = cross-section of flow area (cm2)
k = hydraulic conductivity (cm/sec)
Water Movement in Unsaturated Condition
The unsaturated soil water movement is also called as capillary movement. In this condition the macro pores are filled with air and only micro pores are filled with water which is held relatively more tightly and water is able to move very slowly. When soil moisture decreases, a part of pore spaces is occupied by soil air and the cross-sectional area for water movement is reduced and three by hydraulic conductivity becomes low. In unsaturated conditions, the conductivity is more in fine soil than coarse textured soil. Hence, the unsaturated hydraulic conductivity is the function of soil moisture content, number, size and continuity of soil pores etc. The rate of unsaturated flow in various soil texture is in the following order. Sand < loam < clay
Water Vapour Movement
It takes place within the soil as well as between soil and atmosphere under dry range. The vaporization under wet range is not taken into account in irrigation practices as it is in negligible range. The finer the soil pores higher is the moisture tension under which maximum water vapour occurs. In the coarse textured soil, at low tension the soil pores become free of liquid water when soil dries out. There is little moisture left for vapour transfer. But fine textured soil retains substantial amount of moisture even at high tensions thus permitting vapour movement in soil occurs before it reaches PWP (Permanent Wilting Point). In this situation water vapour movement contribution is considered for the survival of plants.
Distribution of water in sandy loam and clay loam type of soil is given in figure. In coarse textured sandy loam soil the water distribution is very narrow and it percolates down to 180 cm within 24 hours of time period. At the same time horizontally the water spread to the maximum of 30 cm width. But in clay soil, the water percolates down to a depth of 90-120 cm after 24 hours of irrigation. The water distribution is to a width of more than 60. cm horizontally during the same period. The figure clearly indicates that in finer texture soil, water movement is slow vertically and spread horizontally more than coarse textured soil.
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