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Chapter: Civil : Water Resources and Irrigation Engineering : Irrigation Water Requirements

Water budget and development plan

A ground-water system consists of a mass of water flowing through the pores or cracks below the Earth's surface.

Water budget and development plan


ü        A ground-water system consists of a mass of water flowing through the pores or cracks below the Earth's surface.


ü        This mass of water is in motion.


ü        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.


ü        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.


ü        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.


ü        An accounting of all the inflows, outflows, and changes in storage is called a water budget.

ü        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.


ü        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.


ü        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).


ü        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.


ü        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.


ü        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.


ü        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.


Ground-Water Budgets


Under predevelopment conditions, the ground-water system is in long-term equilibrium.


That is, averaged over some period of time, the amount of water entering or recharging the system is approximately equal to the amount of water leaving or discharging from the system.


ü      Because the system is in equilibrium, the quantity of water stored in the system is constant or varies about some average condition in response to annual or longer-term climatic variations.

This predevelopment water budget is shown schematically


We also can write an equation that describes the water budget of the predevelopment system as:


Recharge (water entering) = Discharge (water leaving)


ü        Humans change the natural or predevelopment flow system by withdrawing (pumping) water for use, changing recharge patterns by irrigation and urban development, changing the type of vegetation, and other activities.

Focusing our attention on the effects of withdrawing ground water, we can conclude that the source of water for pumpage must be supplied by


(1) more water entering the ground-water system (increased recharge),


(2) less water leaving the system (decreased discharge),


(3) removal of water that was stored in the system, or some combination of these three.


Pumpage = Increased recharge + Water removed from storage + Decreased discharge.


ü        It is the changes in the system that allow water to be withdrawn.


ü        That is, the water pumped must come from some change of flows and from removal of water stored in the predevelopment system (Theis, 1940; Lohman, 1972).


ü        The predevelopment water budget does not provide information on where the water will come from to supply the amount withdrawn.


ü        Furthermore, the predevelopment water budget only indirectly provides information on the amount of water perennially available, in that it can only indicate the magnitude of the original discharge that can be decreased (captured) under possible, usually extreme, development alternatives at possible significant expense to the environment.



Ground-Water Systems Change in Response to Pumping


ü        Consider a ground-water system in which the only natural source of inflow is areal recharge from precipitation.


ü        The amount of inflow is thus relatively fixed.


ü        Further consider that the primary sources of any water pumped from this ground-water system are removal from storage, decreased discharge to streams, and decreased transpiration by plants rooted near the water table.


ü        If the above-described ground-water system can come to a new equilibrium after a period of removing water from storage, the amount of water consumed is balanced by less water flowing to surface-water bodies, and perhaps, less water available for transpiration by vegetation as the water table declines.


ü        If the consumptive use is so large that a new equilibrium cannot be achieved, water would continue to be removed from storage. In either case, less water will be available to surface-water users and the ecological resources dependent on stream flow.

ü        Depending upon the location of the water withdrawals, the headwaters of streams may begin to go dry. If the vegetation receives less water, the vegetative character of the area also might change.


ü        These various effects illustrate how the societal issue of what constitutes an undesired result enters into the determination of ground-water sustainability.


ü        The tradeoff between water for consumption and the effects of withdrawals on the environment often become the driving force in determining a good management scheme.


ü        In most situations, withdrawals from ground-water systems are derived primarily from decreased ground-water discharge and decreased ground-water storage.


ü        These sources of water were thus emphasized in the previous example. Two special situations in which increased recharge can occur in response to ground-water withdrawals are noted here.


ü        Pumping ground water can increase recharge by inducing flow from a stream into the ground-water system.


ü        When streams flowing across ground-water systems originate in areas outside these systems, the source of water being discharged by pumpage can be supplied in part by streamflow that originates upstream from the ground-water basin.


ü        In this case, the predevelopment water budget of the ground-water system does not account for a source of water outside the ground-water system that is potentially available as recharge from the stream.


ü        Another potential source of increased recharge is the capture of recharge that was originally rejected because water levels were at or near land surface.


ü        As the water table declines in response to pumping, a storage capacity for infiltration of water becomes available in the unsaturated zone. As a result, some water that previously was rejected as surface runoff can recharge the aquifer and cause a net increase in recharge.


ü        This source of water to pumping wells is usually negligible, however, compared to other sources.


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