WEPP: A Process-Based Watershed Runoff and Erosion Model for Watershed Assessment William Conroy,...
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Transcript of WEPP: A Process-Based Watershed Runoff and Erosion Model for Watershed Assessment William Conroy,...
WEPP: A Process-Based Watershed WEPP: A Process-Based Watershed Runoff and Erosion ModelRunoff and Erosion Modelfor Watershed Assessmentfor Watershed Assessment
William Conroy, Joan Wu, Shuhui DunWilliam Conroy, Joan Wu, Shuhui DunDept. Biological Systems EngineeringDept. Biological Systems Engineering
Washington State University
William ElliotWilliam ElliotRocky Mountain Research StationRocky Mountain Research Station
USDA Forest ServiceUSDA Forest Service
Pristine watercourses are thePristine watercourses are theultimate goalultimate goal
This goal is rarely achievedThis goal is rarely achieved
Crop Production as Non-point Crop Production as Non-point Sources of PollutionSources of Pollution
Forest Management Can Increase Forest Management Can Increase Erosion and SedimentationErosion 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 New road construction exposes mineral soilmineral soil
BMPs for Reducing Erosion and BMPs for Reducing Erosion and Sedimentation: Forested BuffersSedimentation: 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
Turbidity vs. Discharge Taken at 30-minute Intervals
0
200
400
600
800
1000
1200
1400
0 50 100 150 200 250 300 350
Discharge (cfs)
Turbi
dity (
NTU)
Sediment TMDL EvaluationSediment TMDL Evaluationand Monitoringand Monitoring
• Post-hoc in-stream sediment analysis extremely difficult and problematic
• SSC vs. discharge often has hysteresis loops
• In-stream sediment levels disconnected in time and space to disturbances
Models and TMDLsModels 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 TMDLsModels 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 Hydrologic Models That Do not Evaluate Sediment ErosionEvaluate Sediment Erosion
• HEC-RAS, HMS• HSPF• USGS• API• ARM• SWMM
• SPUR• CASC2D• HYDROTEL• TR-20• DHSVM• SMR
Hydrologic Models That Evaluate Hydrologic Models That Evaluate Runoff Using SCS CN MethodRunoff Using SCS CN Method
• GLEAMS• SMA• APEX• ANSWERS• AGNPS
• SWAT• BASINS• CREAMS• EPIC• SWRRB
Hydrologic Models That Evaluate Hydrologic Models That Evaluate Erosion Using USLE MethodErosion Using USLE Method
• GLEAMS• SMA• ANSWERS• SHE/SHESED• TOPMODEL• KINEROS 2• MIKE-SHE
• PRMS• SWAT• BASINS• CREAMS• EPIC• SWRRB• AGNPS
Limitations of the Functional Limitations of the Functional Models (SCS, USLE)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 EvaluateModels That Physically EvaluateErosion and HydrologyErosion and Hydrology
• CCHE-1D (must be coupled with a physically-based upland erosion model)
• WEPP (Water Erosion Prediction Project)
KEY COMPONENTSKEY 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 WEPP: State-of-the science, Physically-science, Physically-based, Water Erosionbased, 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 Using WEPP for TMDL DevelopmentDevelopment
• Multiple model runs• Varied by slope• Varied by cover
percent• Varied by buffer
width• Varied by hillslope
length• Varied by manage-
ment intensity• Under random
climate conditions
Using WEPP for SedimentUsing WEPP for SedimentTMDL BMP EvaluationTMDL BMP Evaluation
• Multiple manage-ment scenarios
• Time-sequence analysis
• Varied buffer widths
• Varied manage-ment intensity
• Varied climatic conditions
Some Current WEPP Some Current WEPP Components Are Limited Components Are Limited For Watershed ApplicationsFor 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 WEPPCurrent Work to Improve WEPP
• Incorporate channel routing routines for large watersheds (50 mi2)
• Incorporate improved snow distribution routines
WEPP’s Current Channel WEPP’s Current Channel Routing ApproachesRouting 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 Limitations Of Current WEPP Channel Routines Channel Routines
• Adequate for small (640 ac), agricultural watersheds
– with moderate topography– with only a few small
channels– without gullies– without permanent streams– dominated by hillslope
hydrology
Improved Channel RoutingImproved Channel RoutingMethods For WEPPMethods 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
SummarySummary• 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 Summary contcont’’dd
• 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!THANK YOU!• Questions?Questions?
• Comments?Comments?