Another potential source of increased
recharge is the capture of recharge that was
originally 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
is usually 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 systemwide 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 groundwater
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 systemwide analysis of
the ground-water and surface-water resources to
determine appropriate limits for consumptive use.
Systemwide hydrologic analyses typically
use simulations (that is, computer models) to aid
in estimating water availability and the effects of
Hypothetical Examples of How Ground-Water
Systems Change in Response to Pumping
23
extracting water on the ground-water and surfacewater
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. We return to the use of
models in the final chapter of this report, “Meeting
the Challenges of Ground-Water Sustainability