WEPP—A Process-Based Hydrology and Erosion Model for Watershed Assessment and Restoration

1. WEPP—A Process-Based Hydrology and Erosion Model for Watershed Assessment and Restoration Joan Wu, Markus Flury, and Shuhui Dun Washington State University, Pullman, WA Don McCool USDA ARS PWA, Pullman, WA Bill Elliot USDA FS RMRS, Moscow, ID Dennis Flanagan USDA ARS NSERL, West Lafayette, IN

2. The Needs • Protecting and improving water quality in agricultural watersheds are major goals of numerous federal, state and local agencies. For many watersheds, sediment is the greatest pollutant • In watershed assessment, it is crucial to understand sedimentation processes and their impacts on water quality • To successfully implement erosion control practices, it is necessary to determine the spatio-temporal distribution of sediment sources and potential long-term effectiveness of sediment reduction by these practices

3. Surface runoff and erosion from undisturbed forests are negligible • Stream formed due to subsurface flow has low sediment

4. Both surface runoff and erosion can increase dramatically due to disturbances • Models are needed as a tool for forest resource management

5. The WEPP Model • WEPP: Water Erosion Prediction Project • a process-based erosion prediction model developed by the USDA ARS to replace the functional model USLE • built on fundamentals of hydrology, plant science, hydraulics, and erosion mechanics • WEPP uses observed or stochastically-generated climate inputs to predict spatial and temporal distributions of soil detachment and deposition on an event or continuous basis, along a hillslope or across a watershed • Equipped with a geospatial processing interface, WEPP is a promising tool in watershed assessment and management

6. The WEPP Modelcont’d • WEPP Windows Interface • WEPP Internet Interface • GeoWEPP

7. Long-term Research Efforts • Goal • Continuously develop, refine and apply the WEPP model for watershed assessment and restoration under different land-use, climatic and hydrologic conditions • Objectives • Improve the subsurface hydrology routines so that WEPP can be used under both infiltration-excess and saturation-excess runoff conditions in crop-, range- and forestlands • Improve the winter hydrology and erosion routines through combined experimentation and modeling so that WEPP can be used for quantifying water erosion in the US PNW and other cold regions where winter hydrology is important • Continually test the suitability of WEPP using data available from different localities within and outside the US

8. Progresses Made • Numerous modifications to WEPP have been made to • Correct the hydraulic structure routines • Improve the water balance algorithms • Incorporate the Penman-Monteith ET method (UN FAO standard) • Improve the subsurface lateral flow routines • Expand and improve winter hydrology routines to better simulate • Freeze-thaw processes • Snow redistribution processes • WEPP new releases accessible at NSERL’s websitehttp://topsoil.nserl.purdue.edu/nserlweb/index.html

9. WEPP Applications

10. DEM Effects on WEPPErosion Modeling • Paradise Creek Watershed, ID Zhang et al. (2008) Int. J. Geogr. Info. Sci.

11. Erosion Effect of Cropping Systems • Experimental plots, University of Bologna, Italy • Cooperative Graduate Exchange program and collaborative research • State-of-the-science research facility Pieri et al. (2006) J. Hydrol

12. WEPP Applicationsfor Watershed Erosion Modeling • Reeder Experimental Watershed at the USDA ARS CPCRC, Pendleton, OR (Dr. John Williams) • Mica Creek Watershed, ID (Dr. Katy Kavanagh) • Centonara Watershed, Bologna, Italy (Drs. Marco Bitteli and Paola Rossi) http://www.cnr.uidaho.edu/micacreek/ Courtesy of D. Mantovani

13. Forest Hydrologic Modeling: Predominance of Subsurface Lateral Flow Dun, S., J.Q. Wu, W.J Elliot, P.R. Robichaud, D.C. Flanagan, J.R. Frankenberger, R.E. Brown, A.C. Xu, 2008. J. Hydrol (in review)

14. Comparison of Processes * Earlier versions of WEPP typically overestimated Dp

15. 2 1 3 Redistributionof Infiltration Water in WEPP

16. Study Site: Hermada Watershed

17. Physical Setting • Located in the Boise National Forest, SE Lowman, ID • Instrumented during 1995−2000 to collect whether, runoff, and erosion data • 5-yr observed data showing an average annual precipitation of 954 mm, among which nearly 30% was runoff

18. Watershed Discretization

19. Model Inputs • Topography • Derived from 30-m DEMs using GeoWEPP • 10-ha in area, 3 hillslopes and 1 channel • 40−60% slope • Soil  Typic Cryumbrept loamy sand 500 mm in depth • underlying weathered granite • Management • 1992 cable-yarding harvest • 1995 prescribed fire • West and North slopes with low-severity burn • South slope and channel unburned • Climate • 11/1995−09/2000 observed data

20. Results

21. Living Biomass and Ground Cover(WEPP v2006.5) * (a) and (b) unburned S slope; (c) and (d) burned W slope

22. Runoff and Erosion: Obs. vs Pre.(WEPP v2004.7 vs v2006.5)

23. Current Efforts • Improvements have been continually made to the WEPP model • Current WEPP has seen broad applicability in watershed assessment, restoration and management • A recently initiated study supported by the US Forest Service Pacific Southwest Research Station is aimed at development of a process-based tool • For objectively evaluating the impact of specific management practices on erosion in the Lake Tahoe basin; and • For assisting in the design and evaluation of erosion control management practices • Through this study, we will continue to better the WEPP model so that it can be used as a reliable and cost-efficient tool under different hydrologic and climatic conditions

24. Thank You! Questions?

25. Code Modification • Provide options for different applications • a flag added to the soil input file • User-specified vertical hydraulic conductivity K for the added restrictive layer • e.g., 0.005 mm/hr • User-specified anisotropy ratio for soil saturated hydraulic conductivity • horizontal Kh  vertical Kv, e.g., Kh/Kv= 25

26. Code Modificationcont’d • Subroutines modified to properly write the “pass” files • WEPP’s approach to passing outputs • Subsurface flow not “passed” previously • Simplified hillslope-channel relation • All subsurface runoff from hillslopes assumed to enter the channel • Flow added and sediment neglected