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such situation will depend upon the soil characteristics, largely, its grain size,
shape and sorting of the particles, which determine the void space in the soil.
Such void space may be different for different areas and therefore have the
different capacities to absorb water. The maximum rate at which a soil in any
given condition is capable of absorbing water is called infiltration capacity.
Field capacity of soil :
Immediately after rainfall water infiltrates down to reach
the water table zone, but some amount of water is retained on the surface of the
soil grains due to molecular attraction. This water, known as pellicular water,
can not be removed easily. Expressed in terms of depth of water as it is spread
over the basin, this is known as field capacity of the soil and is of importance to
agriculture. A portion of the pellicular water is absorbed by the vegetative root
and is known as available moisture. The remaining unavailable water is known
as hygroscopic water. The plants can extract water from the soil till it reach
wilting point of the plant beyond which the plants wilt permanently. In other words
the available moisture is the moisture between the field capacity (upper limit)
and the wilting point (lower limit).
In the zone above water table the vertical flow of water is significant even though
there is a horizontal flow known as sub-surface flow. When the vertical inflow
enters the water table or in the saturated zone horizontal flow becomes
important. In this zone groundwater flows from higher gradient to lower gradient
till it reaches the outfall area i.e. stream, rivers or oceans. The amount of water
that flow out from a particular basin to rivers etc. is replenished every year by
rainfall and other sources and is known as annual replenishable groundwater
recharge. Annual replenishable groundwater is withdrawn every year for
various purposes, mainly for irrigation. Such replenishable groundwater is also
known as dynamic groundwater resource. When groundwater is withdrawn more
than the dynamic resource, gradual water table recession is observed and the
area is known to be over developed. In case of thick zone of groundwater
saturation only the upper zone is susceptible to change in water table due to
either withdrawal or replenishment, but the zone below this dynamic flow zone
is not affected by such drawal or replenishment and is known as static
groundwater resource.
