Environmental Engineering Reference
In-Depth Information
where relative hydraulic conductivity is writ-
ten as a function of volumetric water content
instead of pressure head. This form of the equa-
tion can be solved by the method of characteris-
tics (Smith,
1983
; Charbeneau,
1984
; Niswonger
et al
.,
2006
) with much less computational
effort than that required for Equation (
3.4
).
This approach cannot account for layered soils,
however; the simulated soil column must have
uniform properties.
160
140
1m
5m
10 m
120
100
80
60
40
20
Example: Maryland agricultural field
Webb
et al
. (
2008
) described a hypothetical study
of recharge and pesticide leaching at a field in
Maryland that was planted with a corn and soy-
bean rotation. Recharge was calculated for 1995
through 2004 with LEACHM, a one-dimensional
unsaturated-zone flow model based on the
Richards equation (Hutson and Wagenet,
1992
).
Model input included daily precipitation (meas-
ured at the site in 2003-2004 and from a nearby
National Weather Service site for 1995-2002)
and potential evapotranspiration (calculated by
the Hamon (
1963
) equation on the basis of daily
air temperature).
Simulations were run to determine the effect
of unsaturated-zone thickness on recharge
rates and on travel times for water movement
from land surface to the water table. Three sets
of simulations were run, with unsaturated-zone
thicknesses of 1, 5, and 10 m that span the range
of thicknesses actually measured in the field.
The top of the simulated soil column repre-
sented land surface and the bottom represented
the water table; computational cells within the
column were 100 mm in height. Optimum van
Genuchten parameters for the water-retention
and hydraulic-conductivity curves (Equations
(
3.5
) and (
3.6
)) were determined by simulat-
ing a tracer experiment that was conducted at
the site and matching observed and simulated
rates of tracer movement. As expected, average
recharge rates decreased with increasing depth
to the water table. Annual rates of recharge
were 159, 124, and 111 mm for unsaturated-
zone thicknesses of 1, 5, and 10 m, respectively.
These estimates range from 9 to 13% of the
annual precipitation (about 1200 mm). Thicker
unsaturated zones also led to longer lags
0
Figure 3.5
Simulated recharge for unsaturated zone
thicknesses of 1, 5, and 10 m at 3-month intervals for an
agricultural field in Maryland from 1995 to 2004 (Webb
et al
.,
2008
).
between infiltration and arrival of recharge
at the water table (
Figure 3.5
); peak rates of
recharge occurred in winter for the 1-m thick
unsaturated zone, but not until the following
fall for the 10-m thick unsaturated zone.
3.4 Watershed models
Watershed models were developed to evaluate
the effects of climate and land use on watershed
hydrology. Affected processes include evapo-
transpiration, surface and subsurface water stor-
age, and groundwater recharge and discharge;
but for many if not most applications, the pro-
cess of primary interest is streamflow. The
first watershed model, the Stanford Watershed
Model (Crawford and Linsley,
1962
), was devel-
oped to aid design of flood-control projects.
Newer watershed models have improved upon
early models in terms of sophistication with
which processes are represented and GIS input
and output processing capabilities. These
improved models, along with increased avail-
ability of online data, have led to an expansion
in the types of studies in which watershed mod-
els are used. Watershed models are now used in
studies of groundwater recharge, surface-water
chemistry, groundwater chemistry, biology,
