WEPP: A Process-Based Watershed Runoff and Erosion Model for Watershed Assessment William Conroy, Joan Wu, Shuhui Dun Dept. Biological Systems Engineering William Elliot Rocky Mountain Research Station USDA Forest Service WEPP: A Process-Based Watershed Runoff and Erosion Model for Watershed Assessment William Conroy, Joan Wu, Shuhui Dun Dept. Biological Systems Engineering Washington State University William Elliot Rocky Mountain Research Station USDA Forest Service

Pristine watercourses are the ultimate goal

This goal is rarely achieved

Crop Production as Non-point Sources of Pollution

Forest Management Can Increase Erosion and Sedimentation Harvesting –Compacts soil –Reduces cover –Increases number of potential flow paths –Reduces ET losses Forest management –Temporally varied –Spatially varied

New road construction exposes mineral soil

BMPs for Reducing Erosion and Sedimentation: Forested Buffers Minimize impact of management activities Reduce surface runoff from disturbed area Reduce water erosion from disturbed area Reduce sedimentation in surface water bodies

Sediment TMDL Evaluation and Monitoring Post-hoc in-stream sediment analysis extremely difficult and problematic SSC vs. discharge often has hysteresis loops In-stream sediment levels disconnected in time and space to disturbances

Models and TMDLs Modeling is an important component of TMDL development, implementation, and monitoring Adequately designed models are useful and efficient for –Estimating water balance components –Estimating erosion rates and load allocations –Evaluating historic and current conditions –Evaluating possible future alternative scenarios

Models for Sediment TMDLs They must evaluate hydrologic processes and erosion processes Empirical models provide a “gross estimate” but do not account for the physical processes as physically-based models do –USLE –SCS Curve Number method –Factor total models

Hydrologic Models That Do not Evaluate Sediment Erosion HEC-RAS, HMS HSPF USGS API ARM SWMM SPUR CASC2D HYDROTEL TR-20 DHSVM SMR

Hydrologic Models That Evaluate Runoff Using SCS CN Method GLEAMS SMA APEX ANSWERS AGNPS SWAT BASINS CREAMS EPIC SWRRB

Hydrologic Models That Evaluate Erosion Using USLE Method GLEAMS SMA ANSWERS SHE/SHESED TOPMODEL KINEROS 2 MIKE-SHE PRMS SWAT BASINS CREAMS EPIC SWRRB AGNPS

Limitations of the Functional Models (SCS, USLE) Do not explicitly account for spatiotemporal variability of processes Based on empirically obtained relationships that may be inappropriate for extrapolation and therefore forecast Have little to no capability of identifying sources and pathways of pollutants Often poorly conceptualized and parameterized so that the resultant model outputs could be erroneous and misleading

Models That Physically Evaluate Erosion and Hydrology CCHE-1D (must be coupled with a physically- based upland erosion model) WEPP (Water Erosion Prediction Project)

KEY COMPONENTS Inter-agency, inter-disciplinary development Does not use Curve Number for runoff calculation Does not use USLE-based erosion technology

WEPP: State-of-the science, Physically- based, Water Erosion The only water erosion model to include physically-based methods for calculating: –Watershed hydrology (infiltration, ET and runoff) –Plant growth on crop-, range-, or forest lands –Spatiotemporally varied soil detachment and deposition WEPP uses –Actual or randomly generated climate inputs –Rill and interrill erosion concepts –Kinematic-wave model for overland transport

Using WEPP for TMDL Development Multiple model runs Varied by slope Varied by cover percent Varied by buffer width Varied by hillslope length Varied by manage- ment intensity Under random climate conditions

Using WEPP for Sediment TMDL BMP Evaluation Multiple manage- ment scenarios Time-sequence analysis Varied buffer widths Varied manage- ment intensity Varied climatic conditions

Some Current WEPP Components Are Limited For Watershed Applications Archaic channel routing algorithms –Rational method or regression equations Inadequate representation of forest hydrology –Forest conditions (trees) not modeled explicitly –Snow hydrology can be improved –SAME AS MOST OTHER MODELS

Current Work to Improve WEPP Current Work to Improve WEPP Incorporate channel routing routines for large watersheds (50 mi 2 ) Incorporate improved snow distribution routines

WEPP’s Current Channel Routing Approaches Currently no explicit channel routing algorithm Empirical peak flow calculations based on –Rational method –Regional regression equations Synthetic hydrographs for “flow routing” –SCS triangular hydrographs –Assuming 24-hr hydrograph duration

Limitations Of Current WEPP Channel Routines Adequate for small (640 ac), agricultural watersheds –with moderate topography –with only a few small channels –without gullies –without permanent streams –dominated by hillslope hydrology

Improved Channel Routing Methods For WEPP Diffusion-wave for backwater, tributary and floodplain flows –Numerous existing hydraulic models use this method for channel routing Muskingum-Cunge for simplified analyses –Popular method used by many hydrologists

Summary WEPP is a physically-based water erosion model intended for evaluating erosion from agricultural lands and forestry WEPP’s comprehensive framework and physicality make it a model of great potential for watershed assessment, such as sediment TMDL development and BMP effectiveness evaluations In the past WEPP was used primarily for small watersheds With modifications, WEPP can be used for large watersheds

Summary cont ’ d Researchers at WSU have recently completed modifying WEPP for improved –Channel-flow routing –Snow-distribution –Groundwater baseflow simulation Modifications are regularly evaluated and incorporated into WEPP for official release by NSERL

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