Environmental Engineering Reference
In-Depth Information
4
Methods based on surface-water data
traditional streamflow hydrograph analyses
(including hydrograph separation and reces-
sion-curve displacement), and chemical and
isotopic hydrograph separation techniques.
Some of these methods are designed specific-
ally for estimating focused recharge; others are
for estimating diffuse recharge. Discussions
are centered on groundwater movement to or
from streams, but the principles discussed and
the methods described are equally applicable
for groundwater exchange with other surface-
water bodies, such as lakes, reservoirs, and
wetlands. Proper application of any method
requires a good conceptual model of the hydro-
logic system and a solid understanding of
underlying assumptions. Prior to presentation
of individual methods, background discussions
are given on the exchange of groundwater and
surface water and on the relationship between
base flow and recharge. These discussions illus-
trate assumptions inherent to the methods and
provide some guidelines for assessing the valid-
ity of those assumptions.
4.1 Introduction
Streamflow data are commonly used to esti-
mate recharge rates in humid and subhumid
regions, in part because of the abundance of
streamflow data and the availability of com-
puter programs for analyzing those data. Most
of the methods described in this chapter are
easy to use, but application of any of the meth-
ods should be accompanied by a careful analysis
of the underlying assumptions. The methods
estimate exchange rates between ground-
water and surface-water bodies. That exchange
can represent focused recharge from a losing
stream, or, as in the case of groundwater dis-
charge to a stream, the exchange can reflect
diffuse recharge that occurs over widespread
areas. Some of these methods may be unfamil-
iar to groundwater hydrologists because they
were not developed specifically for the study
of groundwater recharge; instead, they were
developed for purposes such as sizing of cul-
verts and bridge openings, predicting low-flow
rates in streams, or developing an understand-
ing of stream-water quality and the ability of a
stream to assimilate solutes and contaminants.
The fact that base-flow or recharge estimates
are generated as byproducts of these methods
does not diminish the usefulness or applicabil-
ity of the methods in recharge studies.
Techniques presented herein include the
stream water-budget method, seepage meters,
Darcy methods, streamflow duration curves,
4.1.1 Groundwater/surface-water
exchange
Exchange of groundwater and surface water
occurs in most watersheds and is governed by
the difference between water-table and surface-
water elevations (Winter
et al
.,
1998
). If the
water table is higher than stream stage, ground-
water discharges to the stream and the stream
is referred to as a
gaining stream
(
Fig ure 4.1a
).
If stream stage is higher than the water table,
