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

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. Sediment Delivery by Hillslope
Grass
Direct Seed
Mulch Till
Conventional
0.07 Tons/ac
614 Tons
0.1 tons/ac
1100 tons
0.9 tons/ac
10,000 tons
2.5 tons/ac
24,000 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
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