Environmental Engineering Reference
In-Depth Information
recharge processes at individual sites to ensure
that assumptions underlying the techniques are
consistent with conceptual models. As such, the
text should serve as a resource to which hydrol-
ogists can refer for making informed decisions
on the selection and application of methods. A
thorough understanding of methods also pro-
vides a framework for the analysis of implica-
tions of modifying methods or applying them
under less-than-ideal conditions.
A conceptual model of recharge processes
attempts to answer the questions of where,
when, and why recharge occurs. The model
will thus identify the prominent recharge
mechanisms, perhaps provide initial estimates
of recharge rates, and serve as a guide for the
selection of methods and for deciding on loca-
tions and time frames for data collection. The
importance of matching methods for estimat-
ing recharge with conceptual models cannot be
overemphasized. Development of a sound con-
ceptual model is imperative for selecting proper
methods and obtaining meaningful recharge
estimates, but this process can be difficult, com-
plicated by both natural and anthropogenic fac-
tors. A conceptual model often evolves over time
as data are collected and interpreted; there may
be a dynamic feedback effect - recharge esti-
mates may support revision of the conceptual
model or suggest the application of alternative
methods.
Nature is complex, and each study site is
unique. Although conceptual models of recharge
processes are important, the development of a
conceptual model is not the main focus of this
book. Because of the great complexity and lim-
itless variability in hydrologic systems, it is
beyond the scope of this text to provide more
than general guidelines for developing a con-
ceptual model of recharge processes. It is sim-
ply not practical to describe or examine every
scenario under which a method will be applied.
Section 1.4
provides a general review of critical
components of a conceptual model. For illus-
trative purposes, typical recharge processes in
groundwater regions of the United States are
briefly discussed in the final chapter.
This text is not intended as a cookbook
that provides a recipe for estimating recharge
for any and all situations; application of any
method requires some hydrologic analysis.
However, many of the methods described are
simple enough that all the details required for
their application are contained herein. Other
methods, such as the use of complex models,
require training that is beyond the scope of this
text. Information is provided on these methods
to assist the reader in deciding whether the cost
of such training will be balanced by the benefits
gained from applying the methods. Applications
are illustrated with examples to highlight ben-
efits and limitations. Many references are pro-
vided to allow the interested reader to pursue
more details on any of the methods discussed.
Most of the discussion in this text is directed
toward quantifying rates of natural recharge;
however, many methods can and have been used
to estimate recharge from artificial recharge
operations, irrigated areas, and human-made
drainage features, such as canals and urban
water-delivery systems. In addition, many of
the methods can be used to provide qualitative
information on recharge rates (i.e. identifying
areas of high and low relative recharge rates)
for purposes of determining aquifer vulnerabil-
ity to contamination from surface sources.
Numerous journal articles and reports
describe the theory and details of the vari-
ous techniques for estimating recharge.
Applications of methods are discussed in many
other papers. Given the importance of the sub-
ject matter, the paucity of textbooks devoted
to this topic is surprising. Lerner
et al
. (
1990
)
is the most thorough of these publications in
terms of method descriptions. That text pro-
vides generic descriptions of physical controls
that influence recharge in different hydrogeo-
logical provinces and discussion of techniques
based on source of recharge water (i.e. precipi-
tation, rivers, irrigation, and urbanization).
Wilson (
1980
), Simmers (
1997
), and Kinzelbach
et al
. (
2002
) provide informative discussions on
recharge processes in arid and semiarid regions
and the techniques that are applicable in those
regions. Simmers (
1988
) is a compendium of
papers associated with a conference devoted to
groundwater recharge. Hogan
et al
. (
2004
) and
Stonestrom
et al
. (
2007
) each comprise a series
