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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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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Application to Paradise Creek

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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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Undisturbed

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WEPP “Fitted” Snowmelt

WEPP Simulations

Rain Passes Through Snow pack

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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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Snowmelt Variability with Multiple OFEs

North Facing Slope

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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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Perched Water on a Fragipan soil horizon

Courtesy of Paul McDaniel

Single Hillslope: Runoff

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