Environmental Engineering Reference
In-Depth Information
can be expensive to obtain. Hydraulic conduc-
tivity of geologic materials varies over a wide
range. Hydraulic conductivity of gravels, frac-
tured basalts, and karst limestones can be as
high as 1000 m/d, whereas values for shales,
marine clays, and glacial tills can be as low as
10
- 7
m/d (Heath,
1983
). Variability in hydraulic
conductivity is typically the largest source of
uncertainty in estimating groundwater flow.
Hydraulic conductivity can be measured in the
laboratory on undisturbed soil-core samples or
repacked samples (Reynolds
et al
.,
2002
); these
samples usually have lengths of tens to hun-
dreds of millimeters. Hydraulic conductivity
can be determined over larger spatial scales by
applying field techniques such as single well
slug tests (Butler,
1997
) and multiwell aquifer
pumping tests (Walton,
2007
).
Groundwater flow models (
Section 3.5
) are
often used as an alternative to Equation (
2.25
)
for estimating groundwater flow rates because
of the complex nature of many groundwater
systems. Prior to the development of numerical
flow models, groundwater flow rates were esti-
mated with flow nets (
Section 6.3.3
). Additional
information on groundwater flow is available
in texts such as Freeze and Cherry (
1979
) and
Todd and Mays (
2005
).
Groundwater withdrawal or injection may
be an important part of the water budget of
a watershed or aquifer. Reporting rules for
groundwater users vary by governing entity.
In some areas, data on water use are easily
obtained. In other areas, with many domestic
or irrigation wells, data on water use may be
difficult to obtain. Most of these wells are not
equipped for monitoring flow rates. For large
capacity irrigation wells, pumping rates can be
estimated on the basis of power consumption
(Hurr and Litke,
1989
).
Precipitation was measured with two tip-
ping-bucket gauges and one weighing gauge.
Evapotranspiration was estimated by the ener-
gy-balance Bowen ratio method. Change in
unsaturated-zone storage was determined by
borehole logging with a gamma-attenuation
tool at three locations on a weekly to biweekly
schedule. Runoff to three intermittent streams
was measured with a Parshall flume or a V-notch
weir. Drainage was calculated by the residual
method, using Equation (
2.26
). Drainage was
also determined directly by applying the
unsaturated-zone Darcy method (
Section 5.4
) at
two of the three logging locations. Therefore,
this study provides an opportunity to compare
the residual water-budget method with another
method.
Results for the first year of the study (
Figure
2.7
) illustrate the variability in time of indi-
vidual water-budget components as well as
the interdependency among all components.
Total precipitation of 927 mm is similar to
the long-term average of 890 mm for the site;
it was distributed fairly evenly across the year
with slightly lower amounts in winter months.
Evapotranspiration rates followed the seasonal
pattern in net radiation, highest in late spring
and summer and close to 0 during winter
months. Runoff occurred only after large pre-
cipitation events. Drainage, as calculated from
Equation (
2.26
), was episodic, occurring in
response to large rainfalls, mostly in spring and
early summer when evapotranspiration was
low and soil-water contents were high. Negative
values for drainage were calculated for many
days. These values do not indicate groundwater
discharge (the water table was at a depth of 10 m
or more). Instead, they are attributed to the fol-
lowing factors: the time lag between measure-
ments of Δ
S
uz
(the rate of change was assumed
uniform between measurements), instrument
inaccuracy, and an insufficient number of sam-
pling points particularly for Δ
S
uz
and runoff.
Drainage calculated for the year by Equation
(
2.26
) was 126 mm, whereas that calculated
by the Darcy method was 216 mm (Healy
et al
.,
1989
).
Several factors may have contributed to
the discrepancy between the two drainage
Example: Northwestern Illinois
The water budget for an 8-hectare waste-dis-
posal site in northwestern Illinois was studied
from 1982 to 1984 (Healy
et al
.,
1989
). A modi-
fied form of Equation (
2.9
) was used to deter-
mine drainage below the zero-flux plane (
D
):
(2.26)
D
=− − −
P
ET
uz
∆
S
uz
R
off
