Watershed Application of WEPP and Geospatial Interfaces Dennis C. Flanagan Research Agricultural Engineer USDA-Agricultural Research Service Adjunct Professor Purdue Univ., Dept. of Agric. & Biol. Eng. National Soil Erosion Research Laboratory West Lafayette, Indiana, USA
Presentation Outline Watershed Representation in WEPP Geospatial Application Techniques WEPP Geospatial Interfaces
Real World translated to WEPP World In any watershed simulation, the real world topography must be translated into rectangular hillslopes and channels that WEPP can work with. These screen shots are from a WEPP model simulation using the Windows stand-alone interface, with a background image. The watershed configuration was created by hand, and the user can switch from a polygon view (above) to a rectangular view (right).
Hillslope Model Inputs Climate input file (daily precipitation info, temperatures, radiation, wind) Slope input file (slope profile description length, width, gradients) Soil input file (infiltration and erodibility parameters; soil texture, O.M., etc. by layer) Cropping/management input file (plant, residue, tillage parameters and dates)
Watershed Model Inputs All hillslope model inputs for all hillslopes in the watershed Channel input files (channel description, geometry, roughness, etc.) Impoundment input files (impoundment description, parameters) Watershed structure file (describes all of the linkages for all channels, hillslopes, and impoundments)
Larger, more complex watershed Actual delineated watershed using web-based WEPP GIS system using TOPAZ program output. The large number of subcatchments and channels would be extremely difficult to parameterize by hand.
Geospatial WEPP Techniques Only way to successfully apply the model to larger, complicated watersheds. Utilize commonly available GIS data, particularly USGS topography DEM s, USGS land use data, and NRCS soils data. Custom software to direct accessing of local and remote datasets, running topographic delineation software, processing topographic outputs, setting up WEPP model simulations, and processing and graphing outputs. System has to automatically determine WEPP slope representations.
Geospatial Application Techniques Example delineated watershed showing three subcatchments (hillslopes) and main channel (in blue) for a simple watershed. The inset shows a flowpath derived from a grid-based DEM
Two Types of Simulation Methods Flowpath Method Create a slope input file for WEPP for each flowpath within a watershed subcatchment. Run model simulation for every flow path, and translate soil loss / deposition values along profile back to geo-referenced space (GIS pixels). Hillslope Methods use a single representative profile (hillslope slope input file) to simulate a subcatchment Chanleng method Calcleng method
Representative Slope Profile The representative profile is created by averaging all of the slope values on the flowpaths at distances away from a channel. The weighting utilizes the slope gradient values at the points along the flowpath, the entire contributing area of the flowpath (area of all the grid cells in that flowpath), and the entire length of the flowpath with the equation: S i n s pi p 1 n p 1 S i = weighted slope value at distance i from the channel for all flowpaths in a subcatchment, s pi is the slope gradient of an individual flowpath p at distance i from the channel, k p is the weighting factor for flowpath p, and n is number of flowpaths in the hillslope. The weighting factor is k p = a p * l p, where a p is the contributing area of a flowpath (sum of all contributing grid cell areas) and l p is the entire flowpath length. * k k p p
Representative Hillslope Methods Chanleng method For hillslopes on the left and right side of a channel, the hillslope width is set to the channel length. The representative profile length is computed by dividing the subcatchment area by the hillslope width. Calcleng method For hillslopes at the top of channels, a representative profile length is calculated. The hillslope width is determined by dividing the subcatchment area by the length.
Calcleng profile length For hillslopes that flow to the top of a channel, a representative profile length is computed using the equation: n L R p p 1 n p 1 * a where L R is the representative hillslope profile length, l p is the flowpath length, and a p is the contributing area of a flowpath (sum of all contributing grid cell areas). The representative hillslope width is computed by dividing the total subcatchment area by the L R value. l a p p
Calcleng Method used here Chanleng Method used here Chanleng Method used here
Procedures in all WEPP Geospatial Interfaces Extract channel network from DEM with TOPAZ program. Select outlet point of watershed. Delineate watershed boundary and subcatchments, using TOPAZ. With flowpath output from TOPAZ delineation, determine representative hillslope profiles using custom software (Prepwepp).
Procedures in all WEPP Geospatial Interfaces (cont.) Topographic analysis also provides channel slope information. Set up WEPP model simulations (Prepwepp handles this). User can specify soil & landuse for watershed, or can use GIS layer info to define. Run WEPP for all flowpaths, and for representative hillslopes and channels. Map output from WEPP to GIS layers to display.
GeoWEPP Originally an ArcView 3.2 extension Allows user to access and import commonly available data from the Internet (DEM, soils, landuse). Also allows user to import their own unique and detailed data. Can be difficult for a GIS-novice to understand and apply. Has been updated to ESRI ArcGIS 10.x
GeoWEPP Application Cheesman Lake, Jefferson County, Colorado
ARS Web-based Geospatial WEPP Interface - Generation I Same general procedures described earlier and implemented in GeoWEPP used. Open source MapServer environment used as web GIS. National Elevation Dataset from USGS was preprocessed, clipped by state boundaries, and stored on NSERL servers. NRCS STATSGO soils data and USGS National Land Cover Dataset used. Topo map images and orthophotos retrieved on demand from Terraserver.
ARS Web GIS Generation I
Web-based Geospatial WEPP Interface- Generation II Cooperative project with USDA-Forest Service, ARS, Washington State University PHP, HTML and JavaScript languages used to write main user interface. OpenLayers package used to display image layers in geo-referenced space. Connects to external GIS data servers using Web Mapping Services. MapServer software converts GIS data into images and reprojections compatible with Google Maps image layers.
Web-based Geospatial WEPP Interface- Generation II (cont.) Custom programs used to: Clip the DEM data to the screen view Call the TOPAZ topographic analysis program Process the TOPAZ outputs for: Channel delineation Watershed delineation Subcatchment delineation Flowpath delineation. Invoke the WEPP model simulations Process the WEPP runoff, soil loss, and sediment yield outputs for display in the GIS.
Watershed delineated, flowpaths determined, subcatchments determined, representative hillslopes determined, channel slope inputs determined, spatial land use data can be used, spatial soil properties data can be used, nearest climate can be used. Ready for WEPP model simulations.
WEPP spatial soil loss results
Output Soil loss by pixel and showing other display layer options
Output - Runoff by Hillslope
Summary & Conclusions To conduct WEPP watershed simulations, detailed information on watershed structure, topography, soils and land use are needed for all hillslopes, channels and impoundments. Geospatial interfaces allow for much more rapid, consistent, and unbiased WEPP application. Newest GIS interfaces to WEPP are webbased and require only a PC, an internet connection, and a web browser.
The End
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Example Application of WEPP Web GIS
Zoom to location of interest, build channel network (TOPAZ run first time).
Set outlet point for watershed. TOPAZ run second time, and watershed summary generated (# hillslopes, # channels, landuse, soils, etc.)
Set up WEPP model runs. Run WEPP for all flowpaths, and for watershed simulation of representative hillslopes and channels.
Set up WEPP model runs. Run WEPP for all flowpaths, and for watershed simulation of representative hillslopes and channels. View Erosion Output
Google Maps available. Can select different layers to view on the screen.
Hillslope outputs available runoff, soil loss, sediment yield. Graphical and text.