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exhausted by continuous withdrawal, a balance between withdrawal and
replenishment can be maintained, provided groundwater is used judiciously,
keeping the withdrawal within 'safe yield' of the basin. The safe yield of a
groundwater basin can be defined as the amount of water which can be
withdrawn from it without producing an undesired result. Any draft (groundwater
withdrawal) in excess of safe yield is overdraft. Overdraft can cause reduced
water-supply, deteriorate the quality of groundwater (as in some parts of Kolkata),
economics of groundwater withdrawal, legal and social problems affecting
water rights.
Water Balance Equation
Groundwater is a component of 'Hydrologic Cycle' - a natural cycle which
controls the entire water resources based on the law of conservation of matter.
In its simplest form it can be represented as :-
[Surface inflow + Sub-surface inflow + Precipitation + Imported water +
Decrease in surface storage + Decrease in Groundwater storage] = [Surface
outflow + Sub-surface outflow + consumptive use + Exported water + Increase
in surface storage + Increase in groundwater storage].
In this form, the equation includes all waters - surface and sub-surface -
entering and leaving a basin.
The groundwater balance of a basin or an area, for an inventory period, may be
expressed in the form of an equation :
Groundwater inflow - Groundwater outflow = Change in Groundwater storage.
In an expanded form, to include most usual sources of recharge and discharge,
the equation can be written in the form :-
