WEPP

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WEPPA Process-Based Hydrology and Erosion Model
for Watershed Assessment and Restoration

• Joan Q. Wu, Markus Flury, Shuhui Dun, R. Cory
  Greer
• Washington State University, Pullman, WA
• Donald K. McCool
• USDA ARS PWA, Pullman, WA
• William J. Elliot
• USDA FS RMRS, Moscow, ID
• Dennis C. Flanagan
• USDA ARS NSERL, West Lafayette, IN

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Major Funding Agencies

• USDA National Research Initiatives (NRI) Programs
• US Forest Service Rocky Mountain Research Station
  (RMRS)
• US Geological Survey/State of Washington Water
  Research Center
• In-house funding from various collaborating
  research institutes

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The Needs

• Protecting and improving water quality in
  agricultural watersheds are major goals of the
  USDA NWQ and NRI Programs
• For many watersheds, sediment is the greatest
  pollutant
• In watershed assessment, it is crucial to
  understand sedimentation processes and their
  impacts on water quality
• To successfully implement erosion control
  practices, it is necessary to determine the
  spatiotemporal distribution of sediment sources
  and potential long-term effectiveness of sediment
  reduction by these practices

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• Surface runoff and erosion from undisturbed
  forests are negligible
• Stream formed due to subsurface flow has low
  sediment

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• Both surface runoff and erosion can increase
  dramatically due to disturbances
• Models are needed as a tool for forest resource
  management

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The WEPP Model

• WEPP Water Erosion Prediction Project
• a process-based erosion prediction model
  developed by the USDA ARS
• built on fundamentals of hydrology, plant
  science, hydraulics, and erosion mechanics
• WEPPs unique advantage it models
  watershed-scale spatial and temporal
  distributions of soil detachment and deposition
  on event or continuous basis
• Equipped with a geospatial processing interface,
  WEPP has great potential as a reliable and
  efficient tool for watershed assessment

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The WEPP Model contd

• WEPP Windows Interface
• WEPP Internet Interface
• GeoWEPP

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Long-term Research Efforts

• Goal
• Continuously refine and apply the WEPP model for
  watershed assessment and restoration under
  different land-use, climatic and hydrologic
  conditions
• Objectives
• Improve the subsurface hydrology routines so that
  WEPP can be used under both infiltration-excess
  and saturation-excess runoff conditions in crop-,
  range- and forestlands
• Improve the winter hydrology and erosion routines
  through combined experimentation and modeling so
  that WEPP can be used for quantifying water
  erosion in the US PNW and other areas where
  winter hydrology is important
• Continually test the suitability of WEPP using
  data available from different localities across
  the world

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Progresses

• Numerous modifications to WEPP have been made to
• Correct the hydraulic structure routines
• Improve the water balance algorithms
• Incorporate the Penman-Monteith ET method (FAO
  standard)
• Improve the subsurface runoff routines
• Expand and improve winter hydrology routines to
  better simulate
• Freeze-thaw processes
• Snow redistribution processes
• WEPP newest release accessible at NSERLs website
  http//topsoil.nserl.purdue.edu/nserlweb/index.htm
  l

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Comparison of Processes
Previous version of WEPP typically
overestimated Dp
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Redistributionof Infiltration Water in WEPP
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Code Modification

• Provide options for different applications
• a flag added to the soil input file
• User-specified vertical hydraulic conductivity K
  for the added restrictive layer
• e.g., 0.005 mm/hr
• basalt (Domenico and Schwartz, 1998)
• User-specified anisotropy ratio for soil
  saturated hydraulic conductivity
• horizontal Kh ? vertical Kv, e.g., Kh/Kv 25

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Code Modification contd

• Subroutines modified to properly write the pass
  files
• WEPPs approach to passing outputs
• subsurface flow not passed previously
• Simplified hillslope-channel relation
• all subsurface runoff from hillslopes assumed to
  enter the channel
• flow added and sediment neglected

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A Case ApplicationModeling Forest Runoff and
Erosion
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Study Site Hermada Watershed
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Physical Setting

• Located in the Boise National Forest, SE Lowman,
  ID
• Instrumented during 1995-2000 to collect whether,
  runoff, and erosion data
• 5-yr observed data showing an average annual
  precipitation of 860 mm, among which nearly 20
  was runoff

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Watershed Discretization
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Model Inputs

• Topography
• Derived from 30-m DEMs using GeoWEPP
• 10-ha in area, 3 hillslopes and 1 channel
• 40-60 slope
• Soil
• ? Typic Cryumbrept loamy sand 500 mm in depth
• underlying weathered granite
• Management
• 1992 cable-yarding harvest
• 1995 prescribed fire on W and N slopes
• Climate
• 11/1995-09/2000 observed data

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Results
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Living Biomass and Ground Cover
(a) unburned, (b) burned
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Runoff and Erosion Obs vs Pre
Observation Period 11/3/1995-9/30/2000
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Water year 19971998
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Summary

• Modifications were made in the approach to, and
  algorithms for modeling deep percolation of soil
  water and subsurface lateral flow
• The refined model has the ability to more
  properly partition infiltration water between
  deep percolation and subsurface lateral flow
• For the Hermada forest watershed
• Vegetation growth and ground cover were described
  realistically
• WEPP-simulated watershed discharge for 19982000
  was compatible with field observation however,
  the agreement was poor for first two years
• Overall, predicted annual watershed discharge and
  sediment yield were not significantly different
  from the observed (paired t-test)
• Nash-Sutcliffe model efficiency coefficient for
  daily runoff was 1.7, suggesting improvement
  needed

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Ongoing Efforts
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Palouse Conservation Field Station (PCFS),
Pullman, WA, USA

• Laboratory and field experimentation on runoff
  and erosion as affected by freezing and thawing
  of soils

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Tilting flume at PCFS
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Experimental plots at PCFS
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Collaborating Research Units

• Testing WEPP using data collected at
• USDA ARS CPCRC, Pendleton, OR, USA (Dr. John
  Williams)
• Ag Exp Farm, University of Bologna, Italy (Dr.
  Paola Rossi Pisa)

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Other PNW Watersheds

• Testing and applying WEPP for evaluating DEM
  effect on soil erosion prediction
• Paradise Creek Watershed, ID, USA (Dr. Jan Boll)
• Mica Creek Watershed, ID, USA (Dr. Tim Link)

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Thank You!
Questions?
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Important Parameters
Parameters Values
Surface Soil Effective K, mm/hr 16.6
Bedrock K, mm/hr 1.0E-2
Anisotropy Ratio 25