i) Saturated Flow
ii) Unsaturated Flow
Saturated flow: This occurs when the soil pores are completely filled with water. This water
moves at water potentials larger than – 33 kPa. 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 iswetted, the movement of more water flowing through
the wetted soil is termed percolation.
(ii) Unsaturated Flow
It is flow of water held with water potentials lower than- 1/3 bar. Water willmove toward
the region of lower potential (towards the greater “pulling” force). Ina uniform soil this means that
water moves from wetter to drier areas. The water movement may be in any direction. T h e
rate of flow is greater as the waterpotential gradient (the difference in potential between wet
and dry) increases andas the size of water filled pores also increases. The two forces responsible
for thismovement are the attraction of soil solids for water (adhesion) and capillarity.Under
field conditions this movement occurs when the soil macrospores (non-capillary) pores with filled
with air and the microspores (capillary) pores with waterand partly with air.
Entry of Water into Soil
Infiltration: Infiltration refers to the downward entry or movement of water intothe soil surface
It is a surface characteristic and hence primarily influenced by the conditionof 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 insurface soil,
all help to increase infiltration
Infiltration rate is comparatively lower in wet soils than dry soils
Percolation: The movement of water through a column of soil is calledpercolation. It is
important for two reasons.
i) This is the only source of recharge of ground water which can be used throughwells for
irrigationii) Percolating waters carry plant nutrients down and often out of reach of plantroots
(leaching)
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
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 is known as
permeability. The term hydraulic conductivity is also used which refers to the readiness with
which a soil transmits fluids through it