Environmental Engineering Reference
In-Depth Information
The antecedent recession curve is the trace
that the well hydrograph would have followed in
the absence of the rise-producing precipitation
(
Figure 6.1
). That trace can be drawn manually,
a practice that is time consuming and prone to
subjectivity but one that accommodates hydro-
logic intuition. Automated techniques for gener-
ating recession curves have also been developed
(Crosbie
et al
.,
2005
; Heppner and Nimmo,
2005
;
Delin
et al
.,
2007
). These techniques generally
use long-term hydrograph records to fit linear
or exponential equations that define the rate of
water-table decline as a function of water-table
height. At any particular well, the rate of water-
table decline decreases as the height of the
water table decreases. Automated techniques
are effective for processing long-term water-
level records and removing subjectivity from
the recharge estimation process. Approaches
for representing water-table recession curves
are similar to those employed for base flow in
streams (
Section 4.5
).
The WTF method for estimating ground-
water recharge was applied as early as the 1920s
(Meinzer,
1923
; Meinzer and Stearns,
1929
) and
since then has been used in numerous studies
(e.g. Rasmussen and Andreasen,
1959
; Gerhart,
1986
; Hall and Risser,
1993
; Crosbie
et al
.,
2005
;
Lee
et al
.,
2005
; Coes
et al
.,
2007
; and Delin
et al
.,
2007
). White (
1932
) used diurnal fluctuations
in groundwater levels to estimate evapotrans-
piration in the Escalante Valley of Utah. Weeks
and Sorey (
1973
), Loheide
et al
. (
2005
), Schilling
and Kiniry (
2007
), and others have used water-
table fluctuations to estimate
ET
gw
.
The WTF method is simple and easy to use.
Because no assumptions are made on the mech-
anisms of water movement through the unsat-
urated zone, the presence of preferential flow
paths does not restrict its application. Recharge
estimates by the WTF method are representa-
tive of an area of several to perhaps thousands
of square meters. So the WTF method can be
viewed as less of a point measurement than
those methods that are based on data in the
unsaturated zone (
Chapter 5
).
The WTF method has some limitations.
Recharge rates may vary substantially within a
watershed because of differences in elevation,
geology, land-surface slope, vegetation, and
other factors (Lee
e t a l
.,
2005
). Data from multiple
wells should be used to ensure that recharge esti-
mates are representative of the catchment as a
whole. In the WTF method, recharge is assumed
to occur as discrete events in time, in direct
contrast to methods, such as the unit hydraulic
gradient method (
Section 5.4
), in which a steady
recharge rate is assumed. If the recharge rate to
an aquifer was constant and equal to the drain-
age rate away from the aquifer, the ground-
water levels would not change, and the WTF
method would estimate a recharge rate of zero.
Therefore, the method is not applicable under
these conditions. As discussed in the following
section, not all fluctuations in groundwater
levels indicate recharge. Difficulties in estimat-
ing specific yield also contribute to the overall
uncertainty of the method. In addition, the fre-
quency with which water levels are measured
can affect recharge estimates. Morgan and Stolt
(
2004
) found that magnitudes of water-table
fluctuations determined on the basis of weekly
measurements were 33% less than those deter-
mined on the basis of measurements obtained
at half-hour intervals for the same observa-
tion well and time period. Delin
et al
. (
2007
)
also found that decreasing the frequency of
water-level measurements led to a decrease in
estimated recharge rates; those authors recom-
mended that water levels be measured on a
weekly or higher frequency for application of
the WTF method. In spite of its limitations, the
simplicity of the approach and the widespread
availability of groundwater-level data combine
to make the water-table fluctuation method one
of the most widely used approaches for estimat-
ing recharge.
6.2.1 Causes of water-table fluctuations
in unconfined aquifers
Groundwater levels fluctuate over multiple
time scales. Long-term changes, over periods
of perhaps decades, may be related to changes
in recharge rates due to natural variations in
climate or to anthropogenic effects. Allison
et al
. (
1990
) attributed rising groundwater levels
and increased recharge in the Murray Basin of
Australia to replacement of native eucalyptus
