The document discusses the Water Erosion Prediction Project (WEPP) model, which is a process-based erosion prediction tool that simulates soil erosion on hillslopes and in small watersheds. It describes WEPP's capabilities like continuous daily simulation of hydrology, erosion, and sediment delivery processes. Recent updates to WEPP include additions like automated yield calibration, multiple residue management operations, and improved contour simulation. Current development focuses on code testing, interface enhancement, and expanded databases.

1. Dennis C. Flanagan
Research Agricultural Engineer
USDA-Agricultural Research Service
National Soil Erosion Research Laboratory
West Lafayette, Indiana, USA

3.  Background
 The WEPP model
 WEPP science
 Scales of interest
 Daily simulation
 Important processes simulated
 Recent updates
 Summary

4.  Soil erosion by water is a serious problem, both within the
U.S. and throughout the world.
 Erosion prediction technologies are often used to assess
soil loss rates under current land management practices,
and effects of changes to that management.
 Existing erosion prediction methods such as the USLE
(Universal Soil Loss Equation) and RUSLE (Revised
Universal Soil Loss Equation) provide only a long-term
average annual soil loss rate.
 User agencies are looking for more information in
predictions, including sediment delivery, annual variability,
probability of occurrence, & risk analysis.

5.  WEPP is a process-based, continuous simulation, distributed
parameter soil erosion prediction model.
 For application to hillslope profiles (<100 m), and to small
agricultural watersheds (<260 ha).
 Developed by USDA since 1985, first delivered in 1995 for action
agency and public use.
 WEPP is used extensively by the USDA Forest Service for
assessment of the effects of disturbance (roads, timber-harvesting,
fires, etc.) in forested regions, especially wildfires.
 NRCS has been working with ARS since 2013 on implementation of
WEPP within its field offices, with efforts including interface and
database development.

6. Scales of interest
0.01 to 1 ha – Hillslope scale
Hillslope profiles in
agricultural fields, forested
areas, rangeland parcels,
landfills, mines, highways,
construction sites, etc.
1 to 100 ha – Field, farm scale
Small watersheds in agricultural
fields, on farms, in forested
catchments, construction sites, etc.

7. Important Processes at these Scales
 Precipitation (and weather in general) – rainfall occurrence,
volume, storm duration, intensity
 Surface hydrology – infiltration, pondage, ET, runoff
 Subsurface hydrology – percolation, seepage, lateral flow
 Hillslope erosion processes – detachment by rainfall, shallow
flow transport, rill detachment by flow shear stress, sediment
transport, sediment deposition
 Channel erosion processes – detachment by flow shear
stress, sediment transport, downcutting to a nonerodible
layer, sediment deposition.

8. Hillslope region from a small watershed

12. • Predict interrill & rill
erosion, sediment
deposition, and sediment
delivery from hillslopes.
• Estimate erosion &
deposition in channels
such as ephemeral gullies
and grass waterways,
effects of impoundments,
and sediment delivery to a
watershed outlet.

13. Major Model Components
 Climate Simulation
 Surface & Subsurface Hydrology
 Water Balance & Percolation
 Soil Component (Tillage impacts)
 Plant Growth & Residue Decomposition
 Overland Flow Hydraulics
 Hillslope Erosion Component
 Channel Hydrology & Hydraulics
 Channel Erosion
 Surface Impoundment Element

14. WEPP science
 Stochastic weather generator (CLIGEN)
 Daily updating of soil, plant, residue params.
 Infiltration predicted using a Green-Ampt
equation modified for unsteady rainfall.
 Runoff volume is predicted from rainfall
excess adjusted for depressional storage.
 Peak runoff rates using kinematic wave eqn.
 Steady-state sediment continuity equation.
 Detachment function of rain intensity, excess
flow shear stress, adj. erodibilities, crit shr.
 Modified Yalin equation for sed. trans. cap.

15. How WEPP simulation works:
 Every day – compute status of the soil, live biomass, and
dead residue cover.
 If there is rainfall, snowmelt, or irrigation, determine if
runoff occurs.
 If runoff predicted to occur, determine amount and rates,
determine soil detachment, sediment transport, and any
sediment deposition.
 Store event runoff, soil loss, deposition, & sediment yield
values.
 At end of simulation, compute simulation result statistics
and report all in output.

16. INPUT RAINFALL VARIABLES
 Option #1 - Standard CLIGEN format
 Total precipitation - rain or snow (p)
 Duration of precipitation (d)
 One storm a day
 Time to peak intensity (Tp)
 Fraction of duration where peak precipitation
intensity occurs
 Ratio peak intensity to average intensity (Ip)
 Option #2 - Breakpoint cumulative
precipitation storm depths & times

17. TIME
RATE
Ip = Peak intensity/average intensity
Average rainfall intensity =
Total precipitation/total duration
Time to peak intensity (dimensionless)
Tp = (time to peak) / (total duration)
Time varying intensity - exponentially increases
from time zero to peak, and decreases from peak
to end of storm duration
Duration
Tp

18. RATE
TIME
Infiltration rate
Average rainfall intensity
Instantaneous rainfall intensity
Tp
Time varying infiltration: Green-Ampt, based on
capillary potential, soil moisture deficit, and effective
hydraulic conductivity.

19. TIME
RATE
Runoff
rate
Tp
Instantaneous rainfall intensity
Infiltration rate
Runoff rate = difference between
rainfall intensity and infiltration rate

20. Runoff rate
Erosion and sediment transport at this
peak runoff rate and over this time

21. Rill & Interrill Erosion
Interrill
Interrill
Interrill
Rill
Rill Rill

22. Rill Detachment

23. τcrit
Rill Detachment Rates

24. Interrill Detachment

25. Deposition

26. Rills – Channels
(e.g. Furrow irrigation).

27.  Automatic yield calibration
 Irrigation specification and utilization
 Multiple tillage operations on a day.
 Contouring simulation
 Residue management operations

28.  For most crops, NRCS wish was for the model to produce
biomass and crop yield that approximates average
observed yield, from user input.
 An automatic yield calibration routine was incorporated
into the interface and model, that takes an input target
yield, and runs multiple model simulations to adjust the
crop Biomass Energy Ratio (BEINP) parameter.
 When the difference between the predicted average annual
yield and the target yield is 10% or less, the calibrated
BEINP value is used in all subsequent model simulations
for that crop.

29.  WEPP can simulate both sprinkler and furrow
irrigation, for either fixed dates or depletion
level scheduling.
 Many users may not know the amount or rates
of irrigation that are applied.
 New web interface will usually assume depletion
level irrigation scheduling with default irrigation
parameters, and very low intensity water
application.

30.  WEPP as released in 1995 simulated contours by using user
inputs of contour row length, row slope, ridge spacing, and
contour ridge height.
 These inputs applied for a cropping period specified as
having contours active.
 The runoff depth and rates, along with the geometry of the
hillslope and contour rows were used to determine if contour
failure occurred during a storm event.
 If contours failed, the user was warned, and model output
summarized the number of failures. However soil loss was
still computed using the user contour inputs.

31.  NRCS set the contour row length to a default of 50 feet.
 Contour ridge height is now equal to the calculated soil ridge height after
tillage, which then decays with time (and also sometimes is decreased after
subsequent tillage). Row spacing is set equal to the last tillage implement
ridge spacing.
 The user provides the uniform contour row slope from a picklist.
 The runoff depth and rates, along with the geometry of the hillslope and
contour rows are still used to determine contour failure during a storm.
 If contours fail, soil loss in subsequent events is computed using the up-and-
down hill slope topography, not the contour slope and row length.
 If a tillage operation occurs after failure, contours are reset.
 A threshold ridge height of 2” was set by NRCS, and hard-coded into the
WEPP model. If the ridge height decays below 2”, then soil loss is computed
up-and-down the hill instead of with the contour settings.

32.  Existing WEPP residue management options were quite limiting,
as only a single one could be specified for an entire crop period.
 WEPP model code and new interface were revised to allow
multiple residue operations within a yearly crop period.
 For annual crops these include shredding/cutting, burning,
removal & addition of residue, silage harvesting, herbicide (kill)
application, and cutting with or without removal of above ground
live biomass.
 A “release cover crop” option was added, so that an inter-
seeded cover crop such as rye could have its growth better
match field conditions the day after harvest of the preceding
crop.

33.  “Kill crop” effects have also now been associated with
certain tillage operations, so if that tillage is specified by a
user it will also kill any current live crop.
 For perennial crops, the ability to cut the crop without
removing the cut biomass has been added. This is an
important improvement for hay crops, where several days
may pass between mowing and baling.
 Grazing operations that remove a fixed percentage of live
biomass on a day have been added. (WEPP already had
grazing operations that use grazing time periods, number
of animals, etc.).

34. Current Model Development
Testing/enhancement of WEPP science model
 Code added & under evaluation to allow simulation of changes in
atmospheric CO2 levels (climate change).
 Adding code to allow simulation of chemical transport (nutrients
and pesticides) for water quality assessments.
 Testing & enhancement of tile drainage component.
Enhancement/expansion of WEPP interfaces & databases
 Development of new default cropping/management inputs
 NRCS creation of CRLMOD database for multiple models
 Testing and modification of WEPP parameters
 Updated climate database (1974-2013 data for ~2700 stations)
 Development of internet-based GIS interfaces for NRCS

35.  WEPP is a powerful process-based soil erosion prediction
model for application to hillslopes and small watersheds.
 The model simulates individual storm events through a long
period of simulation, predicts any runoff and soil loss,
tabulates values, then computes summary statistics at end.
 NRCS and ARS have been working on a cooperative project
since 2013 to develop the necessary databases and
interfaces for use by NRCS staff and technical service
providers.
 The WEPP model has been updated to meet specific NRCS
needs.
 WEPP model development continues.