Modeling Variable Source Area Hydrology With WEPP

1. Modeling Variable Source Area Hydrology With WEPP Winter Erosion Processes and Modeling Meeting USDA-ARS National Soil Erosion Research Laboratory West Lafayette, Indiana May 1-3, 2007 E.S. Brooks1, B. Crabtree2 S. Dun4, J.A. Hubbart7, J.Boll3, J. Wu5, W.J. Elliot6 1Research Support Scientist, 2Graduate Research Assistant, 3Associate Professor, Biol.&Agr. Engr., Univ. of Idaho, Moscow, ID 83844-2060 4Graduate Research Assistant, 5Associate Professor, Biol. Systems Engineering, Washington State University, Pullman, WA 99164 6Research Leader, Rocky Mtn. Res. Station, USDA-FS, Moscow, ID 83843 7Graduate Research Assistant, Forest Resources, Univ. of Idaho, Moscow

2. Research Direction • Evaluation of conservation practices in 10-100 km2 watersheds (USDA-CEAP) • Assessing the cumulative effects of land management practices on sediment loading at the watershed outlet • Both Ag. and Forested watersheds (Paradise Creek and Mica Creek watersheds)

3. Variable Source Area Hydrology • Runoff producing areas are directly related to the local soil water storage capacity (i.e. saturation excess runoff) • Extent varies by season, event • Where is it important? • Shallow soils (i.e. perched WTs) • Steep converging slopes (e.g. toe slopes) • Low intensity rainfall and/or snowmelt

4. P ET Lateral Flow in Surface runoff Surface runoff perched Lateral Flow out layer Percolation Percolation In VSA Hydrology, Steeper slopes generate less runoff, than flat slopes Water Balance with lateral flow (2-Dim Flow)

5. Lateral Flow Drives Spatial Variability 3 Dim Flow Soil Saturation and runoff in converging zones High lateral flow, minimal runoff In steep, diverging areas

6. Perched Water Tables STATSGO

7. Perched Water Tables

8. VSA Hydrology in WEPP • Lateral flow is calculated in WEPP by OFE • Convergence of lateral flow along a hillslope can only be simulated in WEPP with multiple OFEs • Convergence of lateral flow drives the distribution of VSA runoff on a hillslope

9. Single Hillslope: Lateral flow, Runoff, Erosion and Deposition Small lateral flow at the outlet does not mean lateral flow is not important!!

10. Paradise Creek Watershed • 28 km2 (Ag+Forest) • WW-SG-Legume • - 556 Hillslopes • - Up to 19 OFEs on each hillslope

11. Applying WEPP to Large Watersheds • Use GEOWEPP to generate single OFE slope, soil, management files, and hillslope/channel structure • Convert single OFE files to multiple OFE files • Run the program as a batch file • Extract hillslope output (including percolation) to generate streamflow

12. Application to Paradise Creek

13. Paradise Creek Watershed

14. Grass Direct Seed Mulch Till Conventional 0.07 Tons/ac 614 Tons 2.5 tons/ac 24,000 tons 0.1 tons/ac 1100 tons 0.9 tons/ac 10,000 tons Sediment Delivery by Hillslope Winter Wheat Spring Barley Spring Peas Rotation ***30 year Averages

15. Application to Mica Creek Nested forested watershed Snowmelt Dominated - 2-12 km2 sub-watersheds

16. Soil Moisture Routing Model (Frankenberger et al., 1999)

17. WEPP “Fitted” Snowmelt

18. WEPP Simulations Rain Passes Through Snow pack

19. Simulation on a 39% North Facing Slope

20. Hubbart et al. work in Mica Creek • Measured variability in snow accumulation • 2006 peak snow water equivalent • 57 cm clear cut • 30 cm partial cut • 12-22 cm full canopy cover • Measured variability in snow melt rates • 1.08 cm/day clear cut • 0.67 cm/day partial cut • 0.47 cm/day full canopy • Persistent Inverse Air Temperature lapse rates

21. Hubbart et al. work in Mica Creek • Fitting Peak Snow Pack with WEPP • Simulated effective precipitation • 875 mm clear cut • 380 mm partial cut • 190 mm full canopy cover • Fitting Snowmelt Rates with WEPP • Fitted canopy cover • 55% canopy cover for clear cut • 73% canopy cover for partial cut • 81% canopy cover for full canopy

22. WEPP Snowmelt • Primary limitations in high elevation, forests • Does not simulate snow pack temperature (i.e. cold content) • Rain assumed to pass through the snow pack • Maximum snow density is 350 kg/m3 • Snow settling rates too small • Over-sensitivity to canopy cover/solar radiation (i.e. Melt A) • Ignores topographic shading • Ignores snow interception, sublimation, and drifting • A daily model applied on an hourly time step - Modifications by Hendricks to the US Army Corps Engineers approach assumed applicable on an hourly time step

23. Snowmelt Variability with Multiple OFEs North Facing Slope

24. Snowmelt Variability with Multiple OFEs South Facing Slope

25. VSA Hydrology Summary • The spatial distribution of VSA runoff highly correlated with converging subsurface lateral flow • Simulation of VSA Hydrology requires multiple OFEs • Multiple OFEs yield more realistic runoff distribution maps and hydrograph recessions

26. Snowmelt Recommendations • Need research on the effects of canopy on interception, drifting, sublimation • Add in an hourly, physically based approach • snow pack temperature algorithms • Improve snow pack density relationships • Incorporate snow liquid water holding capacity

27. EXTRA SLIDES

28. Snow Water Holding Capacity • Initial Dec.29th 1996 Snow Pack: • 698 mm SWE, 117 kg/m3 snow density, 6.0 m snow depth • Next 4 Days: 135 mm Rain, 17 mm Snow, No Snowmelt • Estimated Water Holding Capacity • (350 kg/m3 – 117 kg/m3)/1000 * 6.0 m Snow = 1.4 m • Final Snow Pack: • 698 mm + 17 mm = 715 mm SWE, 118 kg/m3 snow density • Assuming all water retained in the snow pack and no compaction occurs final snow density should be 140 kg/m3 • Assuming 5% WHC then 698*0.05 = 35 mm • Assuming Tsnow = -0.7 then 31 mm • Total water passing through 135 mm – 35 mm – 31 mm = 69 mm

31. Perched Water on a Fragipan soil horizon Courtesy of Paul McDaniel

32. Single Hillslope: Runoff