Environmental Engineering Reference
In-Depth Information
techniques provide an average estimate for
an entire aquifer or watershed or for a stream
reach, but they may provide little insight into
recharge rates at specific locations. The user
must reconcile project objectives and the spatial
scale of a field site of interest with the spatial
scale inherent in the techniques to be applied.
The spatial scale of each method is discussed in
some detail in this text.
Methods for estimating recharge are asso-
ciated with temporal scales as well as spatial
scales. Some methods, such as the water-table
fluctuation method, can provide an estimate
of recharge for each individual precipitation
event. Most tracer methods, on the other hand,
provide a single estimate of recharge that is
averaged over the time period between tracer
application and tracer sampling. That time
period can extend from a few days to years for
applied tracers to decades or centuries or even
millennia for naturally occurring tracers. In
regions with thick unsaturated zones, recharge
is sometimes assumed to be constant in time
(some methods for estimating recharge are
based on this assumption). On the basis of the
chloride mass-balance method, Scanlon (
1991
)
determined that there has essentially been no
recharge since the Pleistocene in interdrainage
areas of the Chihuahuan Desert in the south-
western United States. As with spatial vari-
ability, the importance of identifying temporal
variability can be determined only by the user.
The user must take care that the methods that
are selected will provide results over the time
frame of interest.
Water-budget methods, Darcy methods,
and other methods can be applied over a var-
iety of time intervals, for example, with daily,
monthly, or annual data; however, results from
application of the same method over differ-
ent time intervals can vary. Consider a simple
water-budget approach for a watershed in a
subhumid climate, where recharge can occur
only when precipitation exceeds evapotrans-
piration. On a monthly basis, evapotranspir-
ation totals exceed precipitation totals for the
months of May through August; therefore,
no recharge would be predicted. Within one
of these months, though, there could be days
when precipitation exceeds evapotranspiration.
Water-budget calculations, when performed
with daily values, could conceivably calculate
recharge on those days.
1.5.3 Expense
Expense is a common limitation for apply-
ing some recharge-estimation methods. Some
methods can be applied by means of a single
field trip to collect and analyze soil and water
samples (this is sometimes the case with use
of tracers); other methods require continuous
monitoring over the course of a year or more.
Analytical costs for tracers such as carbon-14
may appear to be beyond the means of many
recharge studies. But the costs of collecting and
analyzing a single set of samples may be less
than that required for continuous monitoring.
It should be kept in mind that “more expensive”
does not always mean “better or more accu-
rate.” Methods that require large expenses or
hard-to-collect data are seldom applied outside
of research studies. In discussions in the follow-
ing chapters, the relative cost of each method is
addressed. The user must balance cost against
expected improvements in recharge estimates
and attempt to answer the question: how much
will my knowledge of the system be improved
and at what cost?
1.6 Discussion
The selection of methods for estimating
recharge is largely driven by the goals of a
study and the financial and time constraints
placed on that study. Examples of study goals
might be to:
•
obtain a long-term average rate of recharge
for an aquifer,
determine point estimates of recharge in
•
space for assessing aquifer vulnerability to
contamination,
estimate recharge at specific points in time
•
and space to serve as observations for cali-
bration of a model for simulating combined
surface-water/groundwater flow,
