Geology Reference
In-Depth Information
developed by Cochrane and Flanagan (1999) and
implemented in GeoWEPP (Renschler
et al
.,
2002; Renschler, 2003), but within an interface
that only requires a web-browser and Internet
connection on the user's computer (a 'thin-client'
application). GIS Viewer software allows users to
specify an area of interest to model with WEPP,
then digital elevation model (DEM) data for the
area are sent to topographic parameterization
software to delineate watersheds, channels and
hillslopes. The DEM data are processed on the
server side, and then images of the delineated
watershed and hillslopes are passed to the user's
web-browser. Once the hillslopes and channels
have been located, WEPP model simulations of
representative hillslope profiles and channels,
and/or all flowpaths in the watershed, are con-
ducted. The simulated soil erosion results in
graphical format are sent as images to the client
computer. Subsequent model simulations using
different land management practices can help to
show the impact of conservation practices on
hillslope runoff and erosion.
Plate 14 provides a schematic of the WEPP-
GIS application. The application uses the open
source MapServer environment from the
University of Minnesota (http://mapserver.gis.
umn.edu) as the basic Web GIS. The TOPAZ
(Topographic PArameteriZation) (Garbrecht &
Martz, 1997) digital landscape analysis tool is
used for channel, watershed and sub-basin (hill-
slope) delineation. There are six major software
components of the Internet-based WEPP GIS
application. Users can select a US State of inter-
est. They then can zoom in to find their specific
area of interest. The data for display are obtained
from the TerraServer site (http://terraservice.net)
and from local spatial data on the National Soil
Erosion Research Laboratory (NSERL) server.
Image data are sent from the MapServer software
(1 in Plate 14) to the client's web-browser, and
MapServer also handles requests for zooming and
panning in the display. After the location of inter-
est has been identified, TopazPrep software (2 in
Plate 14) extracts a region of the DEM to process
with TOPAZ. TopazPrep is custom software
coded in C++ and PHP. PHP is an open-source
scripting language used for web development and
it can be inserted into HTML (HyperText Markup
Language).
The TOPAZ software (3 in Plate 14) is run at
least twice. The first time is to delineate the
entire network of channels within the displayed
region of the DEM. Once the delineated channels
are visible, the user can either accept them,
or alter the critical source area and minimum
channel length parameters and rerun TOPAZ
until a satisfactory representation of the channel
network is obtained. The user must then select
the outlet point for the watershed of interest,
after which TOPAZ is run a second time to delin-
eate the watershed boundary and sub-basins (i.e.
hillslope regions). The area the user can model is
currently limited to 0.25 degrees latitude by 0.25
degrees longitude, in order to ensure that TOPAZ
can handle the extracted DEM and have a reason-
able response time. Once an acceptable water-
shed has been delineated, the WeppPrep (4 in
Plate 14) program (custom software also written
in C++ and PHP) generates WEPP inputs from the
extracted DEM, land use, soils and TOPAZ water-
shed configuration. WeppPrep also executes the
CLIGEN (5 in Plate 14) weather generator (Nicks
et al
., 1995) to create a climate input file for
WEPP. Finally, the WEPP model (6 in Plate 14) is
run on the hillslopes/channels and/or flowpaths.
Once the WEPP simulations are completed,
WeppPrep prepares the output files, interprets the
results and produces maps which are sent to the
client using MapServer.
The WEPP-GIS application represents a client/
server system (2- or 3-tier architecture) if only the
NSERL server is used to provide spatial data.
However, if users utilize topographic map images
(digital raster graphs) and aerial photography to
assist in locating the area where they wish to
apply the WEPP watershed model, they are using
a distributed system (n-tier architecture) since
the data is retrieved on demand from TerraServer
USA (http://terraservice.net) using a Web Map-
ping Services protocol. The WEPP-GIS is a 'thin-
client', where the client (i.e. user) only needs a
web browser to access the application. All other
data processing is accomplished on the host
