Water Resources Management - Practices of Irrigation and On-farm Water Management - page 163

degradation, and many other problems. Under natural conditions before groundwa-
ter development, aquifers are in a state of dynamic equilibrium. The discharge by
new installed wells must be balanced by an increase in recharge of the aquifer, or
by a decrease in the old natural discharge, or by loss of storage in the aquifer, or by
a combination of the above. Thus, to avoid an adverse effect of overexploitation of
groundwater, development of groundwater should be based on safe yield.
Different researchers defined safe yield from different perspectives. In essence,
safe yield is the quantity of water that can be harnessed from an aquifer annually
without producing an adverse or undesirable result. The undesirable results may
include the depletion of the groundwater reserves, degradation of water quality,
intrusion of water of undesirable quality (e.g., saline water intrusion), violation
of existing water rights, deterioration of economic advantages of pumping (i.e.,
increasing pumping cost), excessive depletion or reduction of stream flow by
induced infiltration (or recharge), loss of wetlands and riparian ecosystem, land
subsidence, etc. Often, a misperception among many hydrogeologists and water
resources engineers and managers is that the development of groundwater is “safe”
if the rate of withdrawal does not exceed the rate of recharge. Even with a pumping
rate smaller than the natural recharge (so called “safe yield”), pumping may cause
induced recharge or decreased discharge. The induced recharge may cause deple-
tion of stream flow, and residual discharge may not be sufficient to maintain the
ecosystem. In the 1980s, the concept of sustainability emerged, which has removed
the problem of safe yield determination.

Sustainable Yield

Sustainability is a goal for the long-term welfare of both humans and the envi-
ronment. Sustainable development must meet the needs of the present without
compromising the ability of future generations to meet their own needs (World
Commission on Environment and Development, 1987 ) . Sustainability refers to
renewable natural resources; therefore, sustainability implies renewability. Since
groundwater is neither completely renewable nor completely nonrenewable, it begs
the question of how much groundwater pumping is sustainable. In principle, sustain-
able yield is that which is in agreement with sustainable development. Alley et al.
( 1999 ) defined groundwater sustainability as development and use of groundwater in
a manner that can be maintained for an infinite time without causing unacceptable
environmental, economic, or social consequences. Furthermore, groundwater sus-
tainability must be defined within the context of the complete hydrological system
and with a long-term perspective to the groundwater resources management.
Basin sustainable yield can be defined by the following water balance equation
(Kalf and Woolley, 2005 ) :

P s =

R 0 + R 0 − D R (5.2)

where P s is the sustainable pumping rate, R 0 is the natural recharge, R 0 is the

increased recharge induced by pumping, and D R is the residual discharge.