Overview & Applicability
The Water Erosion Prediction Project (WEPP), is a physically-based soil erosion prediction technology. WEPP has a number of customized interfaces developed for common applications such as roads, managed forests, forests following wildfire, and rangelands. It also has a large database of cropland soils and vegetation scenarios. The WEPP model is a distributed parameter, continuous simulation model, and is able to describe a given erosion concern in great detail for an experienced user.
The WEPP model integrates hydrology, plant science, hydraulics and erosion mechanics to predict erosion at the hillslope and watershed scale. It has the capability to model and assess a variety of land uses, climates and hydrologic conditions. WEPP models hillslope erosion processes (sheet and rill erosion), including snow accumulation and melt, deep percolation of soil water, and subsurface lateral flow.
Numerous interfaces have been developed for the WEPP model. It can be run offline on personal computers supporting Windows. GeoWEPP, an ArcMap wizard, accesses readily available spatially distributed soil and vegetation databases (in the U.S.) and provides a geospatial representation of the WEPP output. An online interface focusing on individual hillslopes or road segments was developed by the US Forest Service (FSWEPP). It offers a number of modules meant to answer specific management questions. These include: WEPP:ROAD (sediment prediction associated to road construction and management), ERMiT (sediment delivery prediction for hillslopes affected by wildfire), Disturbed WEPP (predicts soil erosion after a disturbance on a hillside), FuME (predicts soil erosion associated with fuel management practices), Tahoe Basin ( developed specifically for soils and weather in the Lake Tahoe Basin); Peak Flow Calculator (estimates peak flow following wildfire), and Rock:Clim (creates climate datasets for use with WEPP or other applications). Batch processers in Excel are available for WEPP:Road, Disturbed WEPP and ERMiT allowing users to carry out runs for hundreds of individual scenarios. This feature is particularly useful if linked to GIS tools. An online GIS watershed application is under development and can be useful for populating the Disturbed WEPP and ERMiT batch processers. On another web site, WEPP CAT, a climate assessment tool, is available to compare the effects of different climate scenarios on soil erosion.
The WEPP online interfaces can be used by anyone, regardless of past modeling experience; however, the WEPP windows and GIS software are best for advanced users who have experience in resource planning and modeling.
The Water Erosion Prediction Project (WEPP) is an interagency model supported by the USFS-Moscow Forest Science Lab (Soil, Water and Engineering group), the USDA Agricultural Research Service’s National Soil Erosion Research Laboratory, West Lafayette, IN, and numerous universities. The WEPP software took shape in the late 1980’s and went into wide use in the late 1990s. WEPP has been improved and refined over the years, and has benefited from contributions by scientists and researchers from the USDA, USDI, USFS as well as university and other institutions. Originally WEPP was created for agricultural modeling, but it is now widely used in forestry, fisheries, rangeland and mining studies and projects.
Inputs and outputs
Model inputs include:
- Watershed Topography - Digital Elevation Model (DEM) for a GIS tool, or slope length, steepness along the slope, and aspect for a single hillslope.
- Soil - Soil texture, depth, and erodibility. In forests and rangelands, erodibility is dependent on texture and disturbance. Soil databases of typical disturbances are available online and are distributed with WEPP windows.
- Land use and management - Depending on the interface, a large database of land use options are provided, including numerous forest, rangeland, and road scenarios as well as a large database of cropland plants and management systems.
- Climate - Both online and windows applications include a database of 2600 U.S. climate stations. Both the Windows application and the online FSWEPP applications have the ability to allow users to build custom climates for anywhere in the world. The FSWEPP interfaces can also access a 4-km grid of mean monthly precipitation amount from the PRISM database. The online watershed version accesses the PRISM 800-m grid for monthly precipitation and maximum and minimum temperatures. The user can also format his/her historic or future climate or climate statistics for the standalone Windows version of WEPP. In WEPP Windows, a single storm can also be described in detail for validation studies or flood flow analyses.
Outputs from the Windows interface include a water balance as a text file or graphic, the distribution of soil detachment and deposition along a complex hillslope, and sediment delivery from a hillslope or a channel segment within a watershed. In addition, in the windows interface, output of daily vegetation growth and senescence, and the accumulation and decomposition of leaf litter are also available. The GeoWEPP ArcGIS wizard provides a geospatial representation of upland erosion and sediment delivery.
Applications to future climates
The likely impact of climate change on forest climates dominated by snowmelt processes is more related to snowmelt timing and rates than to precipitation amounts. Also, when storms do occur, they are likely to be more severe. WEPP has the ability to adjust the monthly precipitation amounts, the monthly number of wet days (for “intensification” of storms), and the monthly maximum and minimum temperatures. The Rock:Clime interface builds weather files to run the online WEPP hillslope modules, or can write a climate file to run WEPP that is modified to describe future climates. One online hillslope tool was specifically written to allow users the ability to evaluate the impacts of future climates, the WEPP Climate Assessment Tool (WEPP CAT). WEPP CAT uses the same climate modification screens as the FSWEPP interfaces, but has the ability to compare different climate scenarios for the same soil and vegetation. The WEPP CAT database includes cropland vegetation and soils as well as forest vegetation and soils.
Restrictions and limitations
The primary limitation of WEPP is the small area it models (< 2.6 km2). In larger watersheds, users will often need to use advanced modeling skills to incorporate climate variability within the watershed and to combine results from runs on numerous small sub watersheds within the study area.
The soil databases for running some versions of WEPP may not cover some of the more remote forest or rangeland watersheds.
WEPP does not model gully erosion or mass wasting processes.
Currently the WEPP standalone software operates on the Windows operating system. The wepp.exe fortran program, however can operate on any system for which it has been compiled, including the LINUX system that supports the online interfaces. The wepp.exe program can also be run without an interface, allowing advanced users to incorporate it into their own customized interface or spreadsheet applications.
Unusual erosion applications, such as construction sites, recreational areas or ATV trails are not included in the online or windows soil and vegetation databases. Users will have to develop their own databases and templates for these applications.
As with all models, the quality of the model output is going to depend on the quality of the input data. Because of the high degree of natural variablity in soil erodibility properties, the accuracy of any erosion prediction model, including WEPP, is about plus or minus 50 percent of the predicted value.
Accessing the tool and additional information
To download the windows software: http://www.ars.usda.gov/Research/docs.htm?docid=18084
To access the online FSWEPP interface for forest hillslopes: http://forest.moscowfsl.wsu.edu/fswepp/
To download GeoWEPP: http://geowepp.geog.buffalo.edu/
To access the online watershed interface: http://cals-wepponlinegis.ag.uidaho.edu/gl/
To access the WEPP Climate Assessment Tool: http://typhoon.tucson.ars.ag.gov/weppcat/index.php
More information about the WEPP individual modules:
FSWEPP online Hillslope Tools
WEPP: ROAD allows users to evaluate the erosion associated with a number of different management strategies from forest road segments. A log of all runs can be developed.
ERMiT allows users to predict the probability of a given amount of sediment delivery from the base of a hillslope following wildfire with or without some common mitigation practices for forest, rangeland and chaparral plant communities in each of five years following the fire.
Disturbed WEPP allows users to describe numerous disturbed forest and rangeland erosion conditions at a hillslope scale. The interface presents the average annual runoff, erosion, and sediment delivery and the probability of a given level of erosion occurring the year following a disturbance.
Batch processors are available online to allow users to run hundreds of hillslopes with Disturbed WEPP and ERMiT, and hundreds of road segments with WEPP:Road. The current limitation is 200 runs, but this limit can be increased if requested.
FuME (Fuel Management) - The FS WEPP FuMe interface predicts soil erosion associated with fuel management practices including prescribed fire, thinning, and a road network, and compares that prediction to erosion from wildfire.
Tahoe Basin – This interface was created with a soil database to suit the Lake Tahoe Basin. It has the same capabilities as Disturbed WEPP, but includes estimates of fine sediment and phosphorus delivery from a hillslope. This interface also incorporates two future climate scenarios (A2 and B1) for eight locations within the basin for the remainder of this century. The user selects the starting year between 2013 and 2080, and the interface then runs for the model for a 20 year period for the daily climate predicted for that period.
Rock:Clim - The Rocky Mountain Climate Generator creates a daily weather file using the ARS CLIGEN weather generator. The file is intended to be used with the online interfaces and the WEPP Windows and GeoWEPP interfaces. It also can be a source of daily weather data for any application. It creates up to 200 years of simulated weather values from a database of more than 2600 weather stations and the PRISM 4-km grid of precipitation data.The WEPP Windows interface can generate up to 999 years of stochastic weather. More information about CLIGEN: http://www.ars.usda.gov/Research/docs.htm?docid=18094
WEPP CAT online Hillslope Climate Assessment Tool
WEPP CAT was designed specifically to compare different climate scenarios on forests, rangelands and croplands. It also has the ability to allow users to increase the amount of precipitation that occurs on a wet day to account for the intensification of storms that is anticipated with future climates. It has input screens similar to the RockClime interface, including access to the PRISM 4-km monthly precipitation database.
Online Watershed Tools
Peak Flow Calculator: Estimates peak flow rates for small watersheds using Curve Number technology. It has the ability to alter both the curve number to predict runoff amount from a given storm for the specified watershed condition, and to alter the Time of Concentration to incorporate the delayed watershed response that is typical of unburned forests where runoff is dominated by subsurface lateral flow.
The Great Lakes watershed interface is a GIS tool that can be used to populate the Disturbed WEPP and ERMiT batch processers, and to provide watershed topographic information for the Peak Flow calculator. The interface can also be used to identify hillslopes at risk to erosion following wildfire or fuel treatments, and if time permits, to carry out cumulative watershed effects analyses. Enhancements to use the interface following wildfire, and for road networks are under development. The current soils database needs to be updated and until it is, the interface tends to overpredict soil erosion on the hillslopes and in the channels.