Environmental Engineering Reference
In-Depth Information
Whether an estimate can be called a point
estimate in space is a matter of perspective.
From the viewpoint of a watershed that drains
1000 ha, estimates of recharge generated over
areas of 1 ha may be considered a point esti-
mate. If the watershed area is only 1 ha, the
area pertaining to a point estimate decreases
equivalently. For discussion purposes, results
from methods applied over the smallest spa-
tial scale, 1 m
2
, are considered to be point
estimates. Whether a point estimate is appro-
priate for a study should be determined in light
of study objectives. If aquifer vulnerability to
contamination is of concern, point estimates
are useful for describing spatial variability of
recharge and perhaps for identifying areas that
are influenced by preferential flow paths. If an
average recharge rate within a watershed is
required, methods that provide point estimates
of recharge may need to be applied at a number
of points in the watershed.
There are some additional items of note
with regard to
Table 9.2
. The spatial scale for
the stream water-budget method is reported
in units of length of stream channel rather
than in terms of area. This is a focused form
of recharge, and the recharge rate of interest is
the rate of volumetric loss of water to the sub-
surface per unit length of stream channel in
units of L2/T;
2
/T; the concept of contributing area
does not apply. The presence of preferential
flow paths within the unsaturated zone may
adversely affect some methods. Methods that
provide point estimates of recharge are partic-
ularly affected; results produced by them can
vary substantially, depending on whether the
measurement area coincided with a preferen-
tial flow path.
The expense and complexity of applying
a method are presented in a relative sense in
Table 9.2
. Again, these are subjective ratings
based on the author's experiences and should
not be taken as strict rules. The rating system
runs from 1 (lowest) to 5 (highest). This informa-
tion is included to assist the reader in screen-
ing out inappropriate methods. Some methods
can be applied at different levels of complexity
and expense. Water-budget methods can be
simple and inexpensive if using only existing
data, or they can be quite complex if individual
water-budget components are measured in the
field. Similarly, models that have been used for
estimating recharge range from very simple
to quite complex. Models can be expensive to
apply if training is required, but experienced
modelers can often generate results efficiently.
9.4 Recharge characteristics
of groundwater regions of
the United States
Discussion of recharge processes in every con-
ceivable hydrogeologic environment, although
desirable, is beyond the scope of this chapter.
Natural and human-affected hydrologic sys-
tems, and associated conceptual models of
recharge processes, are too diverse and compli-
cated to describe without in-depth analysis. The
aim of this section is to describe one or more
conceptual models of recharge processes that
have been adopted within different ground-
water regions of the United States. The intent is
to illustrate concepts of where, when, and why
recharge occurs in typical hydrologic, geologic,
and climatic settings. Examples point out the
types of methods that have been applied and
provide insight into the range of recharge rates
that have been obtained. Limited space permits
only brief discussion of examples, but many ref-
erences are provided to allow interested readers
to pursue additional information.
Attempts at categorizing hydrogeological
and climatic provinces for purposes of identi-
fying regions of similar hydrologic response
were discussed in
Section 1.4.9
. As concluded
in that section, no system is ideal. For discus-
sion in this section, the classification system
of Thomas (
1952
), which consists of 10 ground-
water regions (
Figure 9.1
), has been adopted,
largely on the basis of its simplicity. This was
a somewhat arbitrary decision; other classifi-
cation systems could equally serve this pur-
pose. Features on which the Thomas (
1952
)
classification system is based, and which are
most pertinent to the topic of groundwater
recharge, include geology, the water-bearing
properties of prominent aquifers, and the
