Environmental Engineering Reference
In-Depth Information
model, or some similar approach. These models
can be calibrated by matching model-calculated
groundwater levels to measured values. These
models are discussed in some detail in
Sections
3.3.1 and 3.3.2
.
Su (
1994
) developed a method for estimat-
ing recharge from knowledge of the time ser-
ies of water levels in an observation well. His
approach assumes one-dimensional ground-
water flow in an unconfined aquifer of uni-
form properties overlying a sloping impervious
base with spatially uniform diffuse recharge.
An analytical solution was developed for the
Boussinesq equation and from it an equation
was derived for estimating recharge as a func-
tion of head, the derivative of head with respect
to time, specific yield, and aquifer thickness,
slope, and hydraulic conductivity. For the field
example described by Su (
1994
), most terms in
the analytical solution were negligible, and the
solution took the form:
2001
; Jan
et al
.,
2007
). These time series mod-
els make no attempt to represent the physics
of water storage and movement in the sub-
surface. Although the models were not devel-
oped specifically for estimating recharge, the
water-table fluctuation method can be applied
with any time series of predicted groundwater
levels. These empirical models can be useful
in management of groundwater resources in
regions where little aquifer information is
available (Coulibaly
et al
.,
2001
).
6.5 Other methods
Other approaches based on groundwater level
data have been used to study aquifer recharge
and discharge. Streamflow hydrograph reces-
sion analysis (
Chapter 4
) is a popular approach
for determining base flow, but the analysis of
groundwater hydrographs has received much
less attention. Shevenell (
1996
) analyzed water-
level recession curves to determine values of
specific yield for different elements of a karst
aquifer. Salama
et al
. (
1994a
) used groundwater
hydrograph separation techniques to estimate
evapotranspiration rates. Ketchum
et al
. (
2000
)
used spring discharge measurements to sep-
arate a groundwater hydrograph and estimate
recharge in a small watershed. Schicht and
Walton (
1961
) combined stream discharge meas-
urements and measured groundwater levels to
develop a rating curve for estimating base flow
as a function of groundwater level. Moench and
Barlow (
2000
) and Barlow
et al
. (
2000
) describe a
method for using groundwater level and stage
data for a fully penetrating stream to estimate
bank storage and discharge and basinwide
recharge rates (
Section 4.3.3
).
In the work of van der Kamp and Schmidt
(
1997
), water levels in fine-grained geologic
units were found to respond to changes in soil-
water storage much like weighing lysimeters.
Water-level measurements in fine-grained
units, or in confined aquifers (Sophocleous
et al
.,
2006
; Bardsley and Campbell,
2007
;
Rasmussen and Mote,
2007
), can be used
to estimate changes in water storage. The
method, sometimes referred to as a geological
R
=
SH
(
∆∆
H
/
t b
)/
(6.18)
y
where b is average aquifer thickness, and H is
measured relative to the base of the aquifer.
When there is little variation in aquifer thick-
ness, Equation (
6.18
) reverts to Equation (
6.2
).
Ostendorf
et al
. (
2004
), using assumptions
similar to those of Su (
1994
), developed a method
for predicting steady, monthly, and annual
recharge rates by using analytical solutions to
the one-dimensional groundwater-flow equa-
tion. Dickinson
et al
. (
2004
) describe a method
for predicting focused recharge on the basis of
groundwater level fluctuations at different dis-
tances from the point of recharge; assumptions
included one-dimensional flow from the point
of focused recharge, uniform aquifer proper-
ties, and no additional recharge to the aquifer
along the flow line. Dickinson
et al
. (
2004
) used
an analytical solution to the groundwater-flow
equation.
Predictions of depth to water table and
groundwater-level fluctuations have also
been made with empirical time series mod-
els on the basis of daily or monthly records
of water level, hydraulic gradient, precipita-
tion, or precipitation intensity (Viswanathan,
1984
; Tankersley
et al
.,
1993
; Coulibaly
et al
.,
