Environmental Engineering Reference
In-Depth Information
in a water-budget equation provides these
methods with additional flexibility. Models are
linked with water-budget methods at multiple
levels. Most hydrological models are water-
budget models, being based on some variation
of Equations (
2.1
), (
2.8
), or (
2.9
). In many water-
budget studies, models are used to evaluate one
or more components in a water-budget equa-
tion. As such, there is a certain amount of over-
lap between this chapter on water budgets and
the following chapter on modeling methods.
The major limitation of the residual water-
budget approach for estimating recharge is
that the accuracy of the recharge estimate
is dependent on the accuracy with which the
other components in the water-budget can be
determined. This limitation is important when
the magnitude of recharge is small relative to
that of the other variables. In this case, small
inaccuracies in values for those variables may
result in large uncertainties in recharge.
Future applications of water-budget methods
should see improved accuracy as new tools for
measuring and estimating water-budget com-
ponents evolve. New developments in remote
sensing, geophysical techniques, and modeling
should be particularly helpful.
2.6 Discussion
Water budgets are the fundamental building
blocks upon which conceptual models of hydro-
logic systems are constructed. Water-budget
equations for an aquifer, a watershed, a stream,
even a column of soil offer more than just an
approach for estimating recharge. Water-budget
equations provide insight to the entire hydrol-
ogy of a system, supplying information not
only on recharge, but also on the interrelation-
ship between recharge and other water-budget
components. Water budgets serve another
important role; as will be shown in following
chapters, water budgets provide a framework
within which the assumptions and limitations
of other techniques for estimating recharge can
be analyzed.
Many, if not most, methods for estimating
recharge are based on some form of a water-
budget equation. The universal nature of
water budgets allows water-budget methods
to be applied over the wide range of space and
time scales encountered in hydrologic stud-
ies. The lack of assumptions on the mecha-
nisms that drive the individual components