WEPP: A Process-Based Watershed
Runoff and Erosion Model
for Watershed Assessment
William Conroy, Joan Wu, Shuhui Dun
Dept. Biological Systems Engineering
Washington State University
William Elliot
Rocky Mountain Research Station
USDA Forest Service
Pristine watercourses are the
ultimate goal
This goal is rarely achieved
Crop Production as Non-point
Sources of Pollution
Forest Management Can Increase
Erosion and Sedimentation
• Harvesting
– Compacts soil
– Reduces cover
– Increases number of
potential flow paths
– Reduces ET losses
• Forest management
– Temporally varied
– Spatially varied
New road construction exposes
mineral soil
BMPs for Reducing Erosion and
Sedimentation: Forested Buffers
• Minimize impact of
management activities
• Reduce surface runoff
from disturbed area
• Reduce water erosion
from disturbed area
• Reduce sedimentation
in surface water
bodies
Sediment TMDL Evaluation
and Monitoring
• Post-hoc in-stream
sediment analysis
extremely difficult
and problematic
Turbidity vs. Discharge Taken at 30-minute Intervals
1400
1200
• SSC vs. discharge
often has hysteresis
loops
• In-stream sediment
levels disconnected
in time and space to
disturbances
Turbidity (NTU)
1000
800
600
400
200
0
0
50
100
150
200
Discharge (cfs)
250
300
350
Models and TMDLs
• Modeling is an important component of TMDL
development, implementation, and monitoring
• Adequately designed models are useful and
efficient for
– Estimating water balance components
– Estimating erosion rates and load allocations
– Evaluating historic and current conditions
– Evaluating possible future alternative scenarios
Models for Sediment TMDLs
• They must evaluate hydrologic processes
and erosion processes
• Empirical models provide a “gross estimate”
but do not account for the physical processes
as physically-based models do
– USLE
– SCS Curve Number method
– Factor total models
Hydrologic Models That Do not
Evaluate Sediment Erosion
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HEC-RAS, HMS
HSPF
USGS
API
ARM
SWMM
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SPUR
CASC2D
HYDROTEL
TR-20
DHSVM
SMR
Hydrologic Models That Evaluate
Runoff Using SCS CN Method
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SWAT
BASINS
CREAMS
EPIC
SWRRB
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GLEAMS
SMA
APEX
ANSWERS
AGNPS
Hydrologic Models That Evaluate
Erosion Using USLE Method
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PRMS
SWAT
BASINS
CREAMS
EPIC
SWRRB
AGNPS
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GLEAMS
SMA
ANSWERS
SHE/SHESED
TOPMODEL
KINEROS 2
MIKE-SHE
Limitations of the Functional
Models (SCS, USLE)
• Do not explicitly account for spatiotemporal
variability of processes
• Based on empirically obtained relationships
that may be inappropriate for extrapolation
and therefore forecast
• Have little to no capability of identifying
sources and pathways of pollutants
• Often poorly conceptualized and
parameterized so that the resultant model
outputs could be erroneous and misleading
Models That Physically Evaluate
Erosion and Hydrology
• CCHE-1D (must be coupled with a physicallybased upland erosion model)
• WEPP (Water Erosion Prediction Project)
KEY COMPONENTS
• Inter-agency, inter-disciplinary development
• Does not use Curve Number for runoff calculation
• Does not use USLE-based erosion technology
WEPP: State-of-the
science, Physicallybased, Water Erosion
• The only water erosion model to include
physically-based methods for calculating:
– Watershed hydrology (infiltration, ET and runoff)
– Plant growth on crop-, range-, or forest lands
– Spatiotemporally varied soil detachment and
deposition
• WEPP uses
– Actual or randomly generated climate inputs
– Rill and interrill erosion concepts
– Kinematic-wave model for overland transport
Using WEPP for TMDL
Development
• Multiple model runs
• Varied by slope
• Varied by cover
percent
• Varied by buffer
width
• Varied by hillslope
length
• Varied by management intensity
• Under random
climate conditions
Using WEPP for Sediment
TMDL BMP Evaluation
• Multiple management scenarios
• Time-sequence
analysis
• Varied buffer
widths
• Varied management intensity
• Varied climatic
conditions
Some Current WEPP
Components Are Limited
For Watershed Applications
• Archaic channel routing algorithms
– Rational method or regression equations
• Inadequate representation of forest hydrology
– Forest conditions (trees) not modeled explicitly
– Snow hydrology can be improved
– SAME AS MOST OTHER MODELS
Current Work to Improve WEPP
• Incorporate channel
routing routines for large
watersheds (50 mi2)
• Incorporate improved
snow distribution routines
WEPP’s Current Channel
Routing Approaches
• Currently no explicit channel routing algorithm
• Empirical peak flow calculations based on
– Rational method
– Regional regression equations
• Synthetic hydrographs for “flow routing”
– SCS triangular hydrographs
– Assuming 24-hr hydrograph duration
Limitations Of Current WEPP
Channel Routines
• Adequate for small (640
ac), agricultural watersheds
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with moderate topography
with only a few small channels
without gullies
without permanent streams
dominated by hillslope
hydrology
Improved Channel Routing
Methods For WEPP
• Diffusion-wave for backwater, tributary and
floodplain flows
– Numerous existing hydraulic models use this method
for channel routing
• Muskingum-Cunge for simplified analyses
– Popular method used by many hydrologists
Summary
• WEPP is a physically-based water erosion
model intended for evaluating erosion from
agricultural lands and forestry
• WEPP’s comprehensive framework and
physicality make it a model of great potential for
watershed assessment, such as sediment TMDL
development and BMP effectiveness evaluations
• In the past WEPP was used primarily for small
watersheds
• With modifications, WEPP can be used for large
watersheds
Summary cont’d
• Researchers at WSU have recently
completed modifying WEPP for improved
– Channel-flow routing
– Snow-distribution
– Groundwater baseflow simulation
• Modifications are regularly evaluated and
incorporated into WEPP for official release
by NSERL
THANK YOU!
• Questions?
• Comments?