About one-half of precipitation becomes recharge to the ground water system; the rest flows as surface runoff or is lost through evapotranspiration.

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.” 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, a predevelopment water budget by itself is of limited value in determining the amount of ground water that can be withdrawn on a sustained basis.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

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:

• more water entering the ground water system (increased recharge);

• less water leaving the system (decreased discharge);

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

It is the changes in the system that allows water to be withdrawn. That is, the water pumped must come from some change of flows and from removal of water stored in the predevelopment system. The predevelopment water budget does not provide information on where the water will come from to supply the amount that is withdrawn. Furthermore, the predevelopment water budget only indirectly provides information on the amount of water perennially available, in that it only can 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.

Regardless of the amount of water withdrawn, the system will undergo some drawdown in water levels in pumping wells to induce the flow of water to these wells, which means that some water initially is removed from storage. Thus, the ground water system serves as both a water reservoir and a water-distribution system. For most ground water systems, the change in storage in response to pumping is a transient phenomenon that occurs as the system readjusts to the pumping stress. The relative contributions of changes in storage, changes in recharge and changes in discharge evolve with time. The initial response to withdrawal of water is changes in storage. If the system can come to a new equilibrium, the changes in storage will stop and inflows will again balance outflows.

Thus, the long-term source of water to discharging wells typically is a change in the amount of water entering or leaving the system. How much ground water is available for use depends upon how these changes in inflow and outflow affect the surrounding environment and what the public defines as undesirable effects on the environment.

In determining the effects of pumping and the amount of water available for use, it is critical to recognize that not all the water pumped is necessarily consumed. For example, not all the water pumped for irrigation is consumed by evapotranspiration. Some of the water returns to the ground water system as infiltration (irrigation return flow). Most other uses of ground water are similar in that some of the water pumped is not consumed but is returned to the system. Thus, it is important to differentiate between the amount of water pumped and the amount of water consumed when estimating water availability and developing sustainable management strategies.

The possibilities of severe, long-term droughts and climate change also should be considered (see sidebar, “Through Droughts and Climate Change,” next page). Long-term droughts, which virtually always result in reduced ground water recharge, may be viewed as a natural stress on a ground water system that in many ways has effects similar to ground water withdrawals - namely, reductions in ground water storage and accompanying reductions in ground water discharge to streams and other surface-water bodies. Because a climate stress on the hydrologic system is added to the existing or projected human-derived stress, droughts represent extreme hydrologic conditions that should be evaluated in any long-term management plan.

Hypothetical Examples

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. If the consumptive use were 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 streamflow. 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. These various effects illustrate how the societal issue of what constitutes an undesired result enters into the determination of ground water sustainability. 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.

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

Another potential source of increased recharge is the capture of recharge that originally was rejected because water levels were at or near land surface. 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. This source of water to pumping wells usually is negligible, however, compared to other sources.

In summary, estimation of the amount of ground water that is available for use requires consideration of two key elements. First, the use of ground water and surface water must be evaluated together on a system-wide basis. This evaluation includes the amount of water available from changes in ground water recharge, from changes in ground water discharge, and from changes in storage for different levels of water consumption. Second, because any use of ground water changes the subsurface and surface environment (that is, the water must come from somewhere), the public should determine the tradeoff between ground water use and changes to the environment and set a threshold at which the level of change becomes undesirable. This threshold can then be used in conjunction with a system-wide analysis of the ground water and surface-water resources to determine appropriate limits for consumptive use.

System-wide hydrologic analyses typically use simulations (computer models) to aid in estimating water availability and the effects of extracting water on the ground water and surface water system. Computer models attempt to reproduce the most important features of an actual system with a mathematical representation. If constructed correctly, the model represents the complex relations among the inflows, outflows, changes in storage, movement of water in the system, and possibly other important features. As a mathematical representation of the system, the model can be used to estimate the response of the system to various development options and provide insight into appropriate management strategies. However, a computer model is a simplified representation of the actual system, and the judgment of water-management professionals is required to evaluate model simulation results and plan appropriate actions.
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Droughts represent extreme hydrologic conditions that should be evaluated in any long-term management plan. Photo courtesy of the United Nations Development Programme

Sidebar: Through Droughts and Climate Change

Ground water systems are a possible backup source of water during periods of drought. If ground water storage is large and the effects of existing ground water development are minimal, droughts may have limited, if any, effect on the long-term sustainability of aquifer systems from a storage perspective. In contrast, where ground water storage and heads have been substantially reduced by withdrawals of ground water before a drought occurs, ground water may be less useful as a source of water to help communities and others cope with droughts. Furthermore, previous ground water withdrawals can cause water levels and flows in lakes, streams and other water bodies during droughts to be below limits that would have occurred in the absence of ground water development. Likewise, reduced freshwater discharges to coastal areas during droughts may cause seawater to move beyond previous landward limits, or reduced heads in aquifers may cause renewed land subsidence.

A common response to droughts is to drill more wells. Increased use of ground water may continue after a drought because installation of wells and the infrastructure for delivery of ground water can be a considerable investment. Thus, a drought may lead to a permanent, unanticipated change in the level of ground water development. Use of ground water resources for mitigating the effects of droughts is likely to be most effective with advance planning for that purpose.

Ground water systems tend to respond much more slowly to short-term variability in climate conditions than do surface-water systems. As a result, assessments of ground water resources and related model simulations commonly are based on average conditions, such as average annual recharge or average annual discharge to streams. This use of average conditions may underestimate the importance of droughts.

The effect of potential long-term changes in climate, including changes in average conditions and in climate variability, also merits consideration. Climate change could affect ground water sustainability in several ways, including:

• changes in ground water recharge resulting from changes in average precipitation and temperature or in the seasonal distribution of precipitation;

• more severe and longer lasting droughts;

• changes in evapotranspiration resulting from changes in vegetation;

• possible increased demands for ground water as a backup source of water supply.

Surficial aquifers, which supply much of the flow to streams, lakes, wetlands and springs are likely to be the part of the ground water system most sensitive to climate change; yet, limited attention has been directed at determining the possible effects of climate change on shallow aquifers and their interaction with surface water.

Consideration of climate can be a key - but underemphasized - factor in ensuring the sustainability and proper management of ground water resources. As increasing attention is placed on the interactions of ground water with land and surface-water resources, concerns about the effects of droughts, other aspects of climate variability, and the potential effects of climate change are likely to increase.