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?