Watershed Validation Data
Fifteen watersheds were selected to validate the WEPP watershed version.
These watersheds are:
Chickasha, OK c5
Coshocton, OH 109
Coshocton, OH 191
Coshocton, OH 130
Holly Springs, MS 1
Holly Springs, MS 2
Holly Springs, MS 3
Riesel, TX w-12
Riesel, TX w-13
Riesel, TX sw-12
Tifton, GA z
Watkinsville, GA p-1
Watkinsville, GA p-2
Watkinsville, GA p-3
Watkinsville, GA p-4
Fifteen small watersheds from 0.34 to 5.14 hectares in area from
six locations in the United States were used in this study (Table 1).
Land use for these watersheds included crop rotations, no-till
corn and meadow. To run the WEPP watershed model, information was needed
on climate, soil, management, topography, channel geometry,
and channel control structure. Also, each watershed had to be divided
into different hillslope, channel and impoundment elements,
and the overland flow routing between them had to be
provided to WEPP. All of this information was compiled into
six to eight input files. They are climate, soil, management,
slope, channel, irrigation (if needed), impoundment (if
needed) and watershed structure files.
Because of the small watershed size, we were able to use a single climate
input file for each watershed. All the input files were created using
WEPP version 94.7 file formats.
CLIMATE INPUT FILES
WEPP climate input files contain ten daily parameter values. The four
precipitation parameters were precipitation amount, duration, ratio of
time to rainfall peak / rainfall duration (TP), ratio of maximum rainfall
intensity / average intensity (IP). These four parameters were calculated using
breakpoint precipitation data for all locations except the p3 and p4
watersheds in Watkinsville, Georgia. Since the breakpoint data
were not available for these two watersheds, and they were
about 3.5 kilometers away from the p1 watershed, we used the p1
watershed's TP and IP values for the p3 and p4 watersheds.
The p3 and p4 watersheds were very close, so the same climate file
was used for both locations. The other six daily climate parameters
(maximum temperature, minimum temperature, solar radiation, wind
velocity, wind direction, and dew point temperature were
generated by the WEPP weather generator, CLIGEN (Nicks et al.,
1995). In addition, the rainfall data for the Z watershed in
Tifton, Georgia were 5-minute punch rainfall records. The data
were recorded every 5 minutes. Rainfall amounts could be 0,
0.1, 0.2, 0.3 inches, and so on for each 5 minute time period.
Thus the accuracy of the total storm duration and the ratio of
maximum of intensity / average intensity for each storm were poor
for this location.
SLOPE INPUT FILES
According to the watershed topography and soils, each watershed was
divided into several hillslopes and channel elements. The slope
steepness, slope length, and element width were calculated using the
topographic maps for each hillslope and channel.
MANAGEMENT INPUT FILES
Tillage and crop management information were entered into the
plant/management files according to the field operation notes.
These data included tillage equipment and date, planting
date and what crop was planted, harvest date, residue management
etc. Most of the plant specific parameters used were WEPP default values
at the medium productivity level. If a plant was not available
from the WEPP defaults, we used the Crop Parameter Intelligent Database System
(Deer-Ascough et al., 1995) and other data to estimated the parameters.
SOIL INPUT FILES
Basic soil characteristics, including percentages of sand, clay,
organic matter, rock fragments and cation exchange capacity were
obtained from measured data. The three soil erodibility
parameters (baseline interrill erodibility, baseline rill
erodibility and baseline critical shear stress) were estimated
by using the WEPP default estimations. The effective hydraulic
conductivity of the surface soil was predicted using the WEPP
estimation equations except for the Grenada silt loam soil in
Holly Springs, Mississippi, in which we used the optimized value of
0.47 mm/hr. This optimization, which was made by Risse et al.,
was obtained by using the field fallow plot data (Risse et al., 1995a)
from the Grenada soil.
CHANNEL INPUT FILES
Channel parameters varied from watershed to watershed and from channel to
channel. The EPIC peak runoff calculation method was used.
The friction slope calculation method used the friction slope
equal to the channel bed slope. The channel erodibility and
critical shear stress used WEPP estimated values. The bare soil
and total Manning roughness coefficients used were from the CREAMS document
table II - 28 ( Forst et al., 1980).
IRRIGATION INPUT FILE
Only one watershed in Tifton, Georgia was irrigated. The actual date and
water amount were input in WEPP Fixed-Date Irrigation Scheduling format.
WATERSHED STRUCTURE FILES
The watershed structure files were created according to how the
watersheds were divided and what the overland flow directions were.
These files provide the water and sediment routing linkages for the
WEPP watershed components (i.e. runoff from which element(s) flows into
what other element until finally flow exits at the watershed outlet).
Any Comments Would Be Appreciated
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