Environmental Engineering Reference
In-Depth Information
the complexities of some models, detailed model
descriptions are avoided. Instead, examples are
used to highlight capabilities of complex mod-
els, resources required for model application,
and benefits and limitations of using models
to generate estimates of recharge. Empirical
equations, which are widely used for predict-
ing recharge, are also described, as are regres-
sion and geostatistical techniques for upscaling
point estimates of recharge to obtain average
values for an aquifer or watershed.
Chapter 4
addresses physical methods that
are based on surface-water data. Included
are stream water-budget methods, seepage
meters, streamflow-duration curves, stream-
flow hydrograph analysis (hydrograph sepa-
ration), and chemical or isotopic hydrograph
separation.
Chapter 5
describes physical methods that
can be applied on the basis of data collected in
the unsaturated zone. These methods include
the zero-flux plane, the Darcy method, and the
use of lysimeters. Physical methods based on
data collected in the saturated zone form the
basis of
Chapter 6
. The primary method in this
group is the water-table fluctuation method.
The Darcy method and methods based on time
series of measured groundwater levels are also
discussed.
Chapters 7
and
8
are devoted to the use of
tracers for estimating recharge. Chemical and
isotopic tracer methods are described in
Chapter
7
. Tracers can be naturally occurring (e.g. chlo-
ride and isotopes of carbon and hydrogen), can
occur as an indirect outcome of anthropogenic
activity (e.g. tritium, chlorine-36, and chlo-
rofluorocarbon gases), or can be intentionally
applied to the surface or subsurface for experi-
mental purposes (e.g. bromide, fluorescent
dyes). Tracers can be used to study water from
any source. Use of heat as a tracer for estimat-
ing recharge is described in
Chapter 8
.
The final chapter,
Chapter 9
, attempts to
link conceptual models of recharge processes
with estimation methods. It begins with a dis-
cussion of considerations important in selecting
methods. Figures and tables are presented to
compare methods in terms of spatial and tem-
poral scales of applicability. Typical recharge
processes and methods that have been used to
study these processes are described for ground-
water regions of the United States. This dis-
cussion is not an attempt at a comprehensive
summary of recharge processes and studies;
such an attempt is neither practical nor feasible.
Rather, the idea is to illustrate how conceptual
models of recharge processes can be formed
and used to select appropriate methods. The
closing section presents some final thoughts on
good practices for any recharge study.
1.4 Developing a conceptual model
of recharge processes
The development of a conceptual model of
recharge processes (
Figure 1.2
) is an import-
ant step in any recharge study. The conceptual
model should be developed at the beginning of
a study; it can be revised and adjusted as add-
itional data and analyses provide new insights
to the hydrologic system (Zheng and Bennett,
2002
; Bredehoeft,
2005
). Although this topic is
focused on methods, the reader should bear in
mind the importance of a conceptual model
when reviewing various methods. This section
provides some discussion on factors that can
influence a conceptual model - climate, geol-
ogy, topography, hydrology, vegetation, and
land use. The contents of this section are by no
means comprehensive; the intent is to illustrate
some of the factors that can help to shape a con-
ceptual model.
Water budgets are fundamental compo-
nents of any conceptual model of a hydrologic
system, providing a link between recharge
processes and other processes in the hydrologic
cycle. Water-budget equations can be derived
for one or more control volumes, such as an
aquifer, a watershed, a stream, or even a col-
umn of soil (Healy
et al
.,
2007
). A water-budget
equation allows consideration of the entire
hydrology of the system under study, providing
information not only on recharge, but also on
interrelationships among recharge, discharge,
and change in storage. Preliminary water budg-
ets can be readily constructed with existing
data and refined as various measurements and
