Geology Reference
In-Depth Information
●
AGNPS (http://www.wsi.nrcs.usda.gov/produ
cts/w2q/h&h/tools_models/agnps/)
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ways. The Watershed version of WEPP was used.
The Watershed version includes the Hillslope
version for hillslope erosion and sediment deliv-
ery estimates to channels, and then, the channels
that detach, transport and deposit sediment are
modelled in the Watershed version. The water-
shed could have been modelled using GeoWEPP,
but the watershed as modelled is simple (Fig. 9.3)
and could easily be parameterized. For more com-
plex watersheds, GeoWEPP provides the neces-
sary information to run the WEPP Watershed
version. This information includes hillslope
delineation, channel location, and slopes and
dimensions of channels and hillslopes.
There are four kinds of information used by
WEPP. These are topography, climate, soil, and
management.
EUROSEM
(http://www.cranfield.ac.uk/sas/
nsri/index.jsp)
●
GeoWEPP (http://www.geog.buffalo.edu/
∼
ren
sch/geowepp/)
Each of these models has its strengths and weak-
nesses. A major weakness for several of the mod-
els listed above is the use of USLE technology for
erosion prediction. A major strength of several of
the models above is their application to large
watersheds. WEPP could not be applied to the
entire Osage river watershed, but several of the
above models might be suitable for that purpose;
however, they would not capture the detail of the
particular watershed studied here. RUSLE is not a
watershed model, but it could make reasonable
estimates of average annual soil erosion on each
of the hillslopes in the small watershed here, but
not sediment delivery or daily soil erosion values.
GeoWEPP (Renschler
et al
., 2002) is a geospatial
interface for WEPP, using available digital geo-
referenced information as model input to model
watersheds. GeoWEPP can provide all the infor-
mation that the WEPP Watershed version sup-
plies but not the detailed analysis of hillslopes.
WEPP was chosen because it could work at the
level of detail for this watershed to give informa-
tion about the individual hillslopes and channels,
the erosion from each and the deposition in each.
These are the levels at which individual farmers
operate, and where they make their decisions.
Also, it has the necessary tools for easy use in the
US, and particularly easily accessible databases
and management files. Additionally, it was able
to make estimates of the impact of climate
change, and provide return period information
useful in design for every scenario.
9.4.1 Topography
The small square watershed is about 0.8 km on
each side. The individual hillslopes are modelled
in either three or four slope length segments, flat-
ter at the upper and lower ends than in the mid-
section. The slopes and slope lengths are typical
of those found in the upland areas of eastern
Kansas and western Missouri. All hillslopes had
slope segments of 2% and 4%. All channels were
modelled as having 2% slopes, fairly typical of
these upland areas. The hillslopes were modelled
as being uniform of soil and management (only
one overland flow element per hillslope), and
channels were assumed to be planted through,
rather than set aside in a permanent waterway, a
typical practice in this region when channels are
not severely eroded. The entire watershed was
farmed as a single field, typical of the larger farms
and the larger equipment used in this region.
Except for the two upper hillslopes, H1 and
H2, the area between channels drains to adjacent
channels to the east or west. H1 and H2 drain to
the west. Delivery of runoff and eroded material
to the channels is assumed to be uniform along
the channel. Topographic information for all
hillslopes and channels is given in Table 9.1. This
information includes the channel and hillslope
dimensions, average slope, and the channels
9.4
WEPP Setup and Implementation
The particular small watershed chosen for this
example is in western Missouri, US (longitude
94.4° W, latitude 37.9° N). It is modelled in its
state of about 30 years ago before extensive
conservation work involving terraces and water-
