Geospatial Application of the Water Erosion Prediction Project (WEPP) Model

Abstract

The Water Erosion Prediction Project (WEPP) model is a process-based technology for prediction of soil erosion by water at hillslope profile, field, and small watershed scales. In particular, WEPP utilizes observed or generated daily climate inputs to drive the surface hydrology processes (infiltration, runoff, ET) component, which subsequently impacts the rest of the model, including subsurface hydrology (percolation, subsurface lateral flow), hillslope erosion (interrill & rill detachment, sediment transport & deposition), channel hydrology/erosion (channel flow routing, detachment, sediment transport, deposition), plant growth, and residue decomposition. At the hillslope profile and/or field scale, simple Windows graphical user interfaces (GUIs) have been developed to easily specify the slope, soil, and management inputs. Likewise, simple watershed configurations of a few hillslopes and channels can be easily created and simulated with this GUI. However, as the catchment size increases, the complexity of developing and organizing all WEPP model inputs greatly increases, due to the multitude of potential variations in topography, soils, and land management practices. For these types of situations, numerical approaches and special user interfaces have been developed to allow for easier WEPP setup, utilizing either publicly-available or user-specific geospatial information (Digital Elevation Models (DEMs), Geographic Information System (GIS) soil layers, GIS land-use layers). The basic approach used to automatically generate the slope input files for hillslope profiles is to analyze and process a DEM of an area in three iterations: 1) channel network delineation, 2) watershed boundary and subcatchment delineation, and 3) flow-path and representative hillslope profile determination. The TOPAZ digital landscape analysis tool is used for channel, watershed, and subcatchment delineation. In whichever interface is being used, once a user selects a rectangular region of interest within a DEM, TOPAZ delineates the network of channels within that region. The user then selects the outlet point on a channel for their watershed of interest, after which TOPAZ is run again to delineate the watershed boundary and its subcatchments (that will subsequently be used as WEPP hillslopes). The final step before the actual erosion model simulations is to create the slope, soil and management input files for WEPP, using custom software (called Prepwepp) taking data from the extracted land use, soils, DEM, and TOPAZ watershed structures. Specifically, in regards to the hillslope profile slope inputs, there are two options for running WEPP: 1) creating a single representative hillslope profile slope input for each subcatchment based upon an averaging of all the computed flowpaths within a subcatchment; and/or 2) running WEPP model simulations for all TOPAZ-identified flowpaths within each subcatchment. The model slope inputs for each of the channels within the delineated watershed also are obtained from the TOPAZ output. A user has the option of specifying a single soil and land management for each subcatchment, or utilizing information in soil and land use GIS layers to automatically assign these. Once WEPP runs are completed, output is scanned (by Prepwepp), results interpreted, and maps of spatial soil loss are generated and sent to the GIS for display. These procedures have been used within a number of GIS platforms. GeoWEPP is an ArcView/ ArcGIS extension that was the first to be developed, and which allows experienced GIS users the ability to import and utilize their own detailed DEM, soil, and/or land use information, or to access commonly available spatial datasets. An initial web-based GIS system that uses the MapServer web GIS software for handling and displaying the spatial data and model results was released in 2004. Most recently, the Google Maps and OpenLayers technologies have been integrated into the web GIS software and WEPP model to provide some significant enhancements over the earlier prototype. This presentation will discuss in detail the logic and procedures for developing the WEPP model inputs, a variety of WEPP GIS interfaces, and future directions.