Modeling Variable Source Area Hydrology With WEPP

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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
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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)

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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

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In VSA Hydrology, Steeper slopes generate less
runoff, than flat slopes
Water Balance with lateral flow (2-Dim Flow)
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Lateral Flow Drives Spatial Variability
3 Dim Flow
Soil Saturation and runoff in converging zones
High lateral flow, minimal runoff In steep,
diverging areas
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Perched Water Tables
STATSGO
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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

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Single Hillslope Lateral flow, Runoff, Erosion
and Deposition
Small lateral flow at the outlet does not mean
lateral flow is not important!!
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Paradise Creek Watershed

• 28 km2 (AgForest)
• WW-SG-Legume
• - 556 Hillslopes
• - Up to 19 OFEs on each hillslope

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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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Application to Paradise Creek
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Paradise Creek Watershed
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Grass
Direct Seed
Mulch Till
Conventional
0.07 Tons/ac 614 Tons
2.5 tons/ac 24,000 tons
0.9 tons/ac 10,000 tons
0.1 tons/ac 1100 tons
Sediment Delivery by Hillslope
Winter Wheat Spring Barley Spring Peas Rotation
30 year Averages
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Application to Mica Creek
Nested forested watershed Snowmelt Dominated -
2-12 km2 sub-watersheds
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Soil Moisture Routing Model (Frankenberger et
al., 1999)
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WEPP Fitted Snowmelt
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WEPP Simulations
Rain Passes Through Snow pack
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Simulation on a 39 North Facing Slope
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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

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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

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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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Snowmelt Variability with Multiple OFEs
North Facing Slope
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Snowmelt Variability with Multiple OFEs
South Facing Slope
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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

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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

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EXTRA SLIDES
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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 6980.05 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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Perched Water on a Fragipan soil horizon
Courtesy of Paul McDaniel
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Single Hillslope Runoff