Entry of Water into Soil

1. Infiltration: Infiltration refers to the downward entry or movement of water into the soil surface. It is a surface characteristic and hence primarily influenced by the condition of the surface soil.Soil surface with vegetative cover has more infiltration rate than bare soil. Warm soils absorb more water than colder ones. Coarse surface texture, granular structure and high organic matter content in surface soil, all help to increase infiltration. Infiltration rate is comparatively lower in wet soils than dry soils.

Factors affecting infiltration
1. Clay minerals
2. Soil Texture
3. Soil structure
4. Moisture content
5. Vegetative cover
6. Topography
2. Percolation: The movement of water through a column of soil is called percolation. It is important for two reasons.

i) This is the only source of recharge of ground water which can be used through wells for irrigation
ii) Percolating waters carry plant nutrients down and often out of reach of plant roots (leaching)
Percolation is dependent of rainfall. In dry region it is negligible and under high rainfall it is high. Sandy soils have greater percolation than clayey soil. Vegetation and high water table reduce the percolation loss
3. Permeability: It indicates the relative ease of movement of water with in the soil. The characteristics that determine how fast air and water move through the soil are known as permeability. The term hydraulic conductivity is also used which refers to the readiness with which a soil transmits fluids through it.
Soil Water Movement

i) Saturated Flow
ii) Unsaturated Flow
iii) Water Vapour Movement
Saturated flow: This occurs when the soil pores are completely filled with water. This water moves at water potentials larger than – 33 k Pa. Saturated flow is water flow caused by gravity’s pull. It begins with infiltration, which is water movement into soil when rain or irrigation water is on the soil surface. When the soil profile is wetted, the movement of more water flowing through the wetted soil is termed percolation.

Hydraulic conductivity can be expressed mathematically as
V = kf
Where,
V = Total volume of water moved per unit time
f = Water moving force
k = Hydraulic conductivity of soil
Factors affecting movement of water
1. Texture, 2.Structure, 3.Amount of organic matter, 4.Depth of soil to hard pan, 5.Amount of water in the soil, 6.temperature and 7. Pressure
Vertical water flow:
The vertical water flow rate through soil is given by Darcy’s law. The law states that the rate of flow of liquid or flux through a porous medium is proportional to the hydraulic gradient in the direction of floe of the liquid.
(dw) At
QW = - k--------------
Ds
Where,
QW = Quantity of water in cm-3
k = rate constant (cm/s)
dw = Water height (head), cm
A = Soil area (cm2))
t = Time
ds = Soil depth (cm)