Modeling Variable Source Area Hydrology With WEPP
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 838437Graduate Research Assistant, Forest Resources, Univ. of Idaho, Moscow
Winter Erosion Processes and Modeling MeetingUSDA-ARS National Soil Erosion Research Laboratory
West Lafayette, IndianaMay 1-3, 2007
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)
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
Water Balance
with lateral flow (2-Dim
Flow)
In VSA Hydrology,Steeper slopes generate less runoff, than flat slopes
Percolation
LateralFlow out
Surface runoff
Percolation
perchedlayer
ET P
Surface runoff
LateralFlow in
Soil Saturation and runoffin convergingzones
Lateral Flow Drives Spatial Variability
High lateral flow,minimal runoffIn steep, divergingareas
3 Dim Flow
Perched Water Tables
STATSGO
Perched Water Tables
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
Single Hillslope: Lateral flow, Runoff, Erosion and Deposition
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1 3 5 7 9 11 13 15 17 19Distance (# of OFEs)
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Net Soil LossRunoffLateral Flow
Small lateralflow at the outlet doesnot mean
lateral flow is not important!!
Paradise Creek
Watershed
- 28 km2 (Ag+Forest)- WW-SG-Legume- 556 Hillslopes- Up to 19 OFEs on each hillslope
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
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11/30/02 12/30/02 1/29/03 2/28/03 3/30/03 4/29/03
stre
amflo
w (m
m)
Sim 1Sim 2Sim 3
Application to Paradise Creek
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amflo
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m)
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el. (
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Sim. StreamflowObs. StreamflowObs. Del. Sed.Sim. Del. Sed.
Paradise Creek Watershed
Grass Direct Seed Mulch Till Conventional
Winter WheatSpring BarleySpring PeasRotation
SedimentDelivery
by Hillslope
0.9 tons/ac10,000 tons
0.07 Tons/ac614 Tons
0.1 tons/ac1100 tons
2.5 tons/ac24,000 tons
***30 year Averages
Application to Mica CreekNested forested watershedSnowmelt Dominated- 2-12 km2 sub-watersheds
Soil Moisture Routing Model (Frankenberger et al., 1999)
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10/1/1991 2/12/1993 6/27/1994 11/9/1995 3/23/1997
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Flume 3
Nash Sutcliffe = 0.796
Undisturbed
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SW
E (m
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WEPP “Fitted” Snowmelt
WEPP Simulations
Rain Passes Through Snow pack
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SWE
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Observed95% Canopy25% Canopy
Simulation on a 39% North Facing Slope
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
• 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
Hubbart et al. work in Mica Creek
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
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14-Jan17-Apr19-AprSoil Surface
Snowmelt Variability with Multiple OFEs
North Facing Slope
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Distance (m)
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14-Jan17-Mar25-MarSoil Surface
South Facing Slope
Snowmelt Variability with Multiple OFEs
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
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
EXTRA SLIDES
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
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OFE 1OFE 8OFE 19
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Perched Water on a Fragipan soil horizon
Courtesy of Paul McDaniel
Single Hillslope: Runoff
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