Sustainability of Ground-Water Resources--Circular 1186
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Sustainability, and Water Budgets
A ground-water system consists of a mass of water flowing through the pores or cracks
below the Earth's surface. This mass of water is in motion. Water is constantly added to
the system by recharge from precipitation, and water is constantly leaving the system as
discharge to surface water and as evapotranspiration. Each ground-water system is unique
in that the source and amount of water flowing through the system is dependent upon
external factors such as rate of precipitation, location of streams and other surface-water
bodies, and rate of evapotranspiration. The one common factor for all ground-water
systems, however, is that the total amount of water entering, leaving, and being stored in
the system must be conserved. An accounting of all the inflows, outflows, and changes in
storage is called a water budget.
Human activities, such as ground-water withdrawals and irrigation, change the natural
flow patterns, and these changes must be accounted for in the calculation of the water
budget. Because any water that is used must come from somewhere, human activities
affect the amount and rate of movement of water in the system, entering the system, and
leaving the system.
Some hydrologists believe that a pre-development water budget for a ground-water
system (that is, a water budget for the natural conditions before humans used the water)
can be used to calculate the amount of water available for consumption (or the safe
yield). In this case, the development of a ground-water system is considered to be "safe"
if the rate of ground-water withdrawal does not exceed the rate of natural recharge. This
concept has been referred to as the "Water-Budget Myth" (Bredehoeft and others, 1982).
It is a myth because it is an oversimplification of the information that is needed to
understand the effects of developing a ground-water system. As human activities change
the system, the components of the water budget (inflows, outflows, and changes in
storage) also will change and must be accounted for in any management decision.
Understanding water budgets and how they change in response to human activities is an
important aspect of ground-water hydrology; however, as we shall see, a predevelopment
water budget by itself is of limited value in determining the amount of ground water that
can be withdrawn on a sustained basis.