Environmental Engineering Reference
In-Depth Information
2008
). Application of GSFLOW is complex, and
only the most salient aspects of the application
are presented here. The watershed was deline-
ated into 128 HRUs on the basis of flow planes,
climate, vegetation, and depth to water table.
Daily precipitation and minimum and max-
imum air temperatures were obtained from
sites near the watershed. Precipitation and tem-
peratures were adjusted on the basis of eleva-
tion, which ranged from 1935 to 2653 m above
sea level. Daily streamflow data were available
from a gauge located near the mouth of the
watershed.
The MODFLOW-2005 model consisted of two
model layers, each with 73 rows and 81 columns
of cells that were all 90 m in length and width.
Five hydraulic conductivity zones were identi-
fied on the basis of geology. Hydraulic conduct-
ivity within each zone was uniform and was
determined through model calibration. Other
data required for MODFLOW-2005 included
saturated water content, a parameter for deter-
mining water content within the unsaturated
zone, specific storage, thickness of hydrogeo-
logic units, stream channel dimensions, and
initial water content beneath streams.
Daily time steps were used for both the PRMS
and MODFLOW-2005 components of GSFLOW,
Example: GSFLOW simulation of the
Sagehen Creek watershed
GSFLOW (Markstrom
et al
.,
2008
) is a coupled
watershed/groundwater-flow model created by
linking PRMS with MODFLOW-2005 (Harbaugh,
2005
). The GSFLOW domain consists of three
regions (
Figure 3.14
). Region 1 includes the
plant canopy, snowpack, impervious (surface-
depression) storage, and the soil zone. Region
2 includes streams and lakes. Region 3 is the
subsurface underlying Regions 1 and 2 and
includes both the unsaturated and saturated
zones. Region 1 is simulated with PRMS, and
Regions 2 and 3 are simulated with MODFLOW-
2005. Recharge to Region 3 can occur as gravity
drainage from Region 1 and as head-dependent
leakage from Region 2. Gravity drainage is
simulated with a kinematic wave approxima-
tion to the one-dimensional Richards equation
(Niswonger
et al
.,
2006
). Groundwater discharge
to Region 1 or 2 occurs when the water table
rises into the soil zone or above stream and
lake levels.
The Sagehen Creek watershed lies on the
east slope of the Sierra Nevada mountains
near Truckee, California. The hydrology of
the watershed was simulated with GSFLOW
for the period 1981 to 1995 (Markstrom
et al
.,
Figure 3.14
Schematic diagram
of the exchange of flow among
the three regions in GSFLOW
(Markstrom
et al
.,
2008
).
Surface runoff
Region 1—Plant canopy,
snowpack, surface-
depression storage,
and soil zone
(PRMS)
Interflow
Region 2—Streams
and lakes
(MODFLOW-2005)
Soil-moisture
dependent flow
Groundwater discharge
Groundwater discharge
Soil-moisture or
head-dependent flow
Head-dependent flow
Gravity drainage
Leakage
Region 3—Subsurface
(unsaturated and saturated
zones) beneath soil zone
(MODFLOW-2005)
