1. Ag. & Biological Engineering
Development of a coupled soil erosion and large-scale hydrology modeling system
Dazhi Mao, Keith A. Cherkauer
Ag. & Biological Engineering
Purdue University
Dennis C. Flanagan
NSERL

2. Outline Introduction Concerns in coupling processes Coupling scheme
Preliminary single cell analysis
Development afterwards
Current status

3. Introduction
Soil erosion by water is a major concern for resource management
Frozen soil modifies surface runoff generation and erodibility of the soil
Large-scale hydrology model to represent the impact of frozen soil on erosion at management scale

4. Introduction (continued)
Erosion models
USLE, RUSLE, MUSLE
Based on field-scale observations
Lumped approach
The WEPP model
Continuous process-based model
Capable of estimating spatial and temporal distribution of soil loss
Macro-scale hydrologic model
The Variable Infiltration Capacity (VIC) model
Mosaic-type representation of soil and vegetation
Infiltration curve to control spatial variability in soil moisture and infiltration
Improved representation of cold season processes

5. Coupling possibilities
The VIC model improved representation of cold season processes (Cherkauer and Lettenmaier, 1999)
The stand-alone WEPP-Hillslope Erosion (WEPP-HE) code (Flanagan et al. 2005) provide basis for coupling with hydrology model
Predicts soil loss for single storm event
Needs only soil texture, slope profile, adjusted daily erodibility and friction factors, and hydrologic pass files
Use basic erosion algorithms from the WEPP model to represent soil erosion in the VIC model

6. Concerns Different scales Different parameters WEPP VIC Spatial scales
Temporal scales
Different parameters
WEPP
Soil texture, management options, slope profile, erodibility, friction factors, rainfall intensity, duration, peak runoff, etc.
VIC
Soil parameters, vegetation type, precipitation depth, runoff depth
WEPP
VIC
field scale
large scale
WEPP disaggregated
rainfall intensity
VIC sub-daily
rainfall depth

7. Approach Downscale VIC model I/O to run WEPP-HE code
Represent topographic variability
Statistical/stochastic presentation of output from WEPP-HE in a VIC grid cell

8. Conceptual coupling scheme
VIC model output
Hydrologic pass file
DEM processing
Slope profile
WEPP-HE code
VIC model soil
Soil texture
WEPP integrated
algorithms
Adjustment
parameters
Annual soil
Erosion
Soil erosion probability in VIC cell

9. Preliminary coupling scheme
- iterated process
30 arc-sec
DEM
Soil loss
- original input data
- data processing
Iterated for sampled slopes
- data for WEPP-HE
- intermediate data
Daily
climate
forcing
Hourly
precipitation
30m DEM
Slope
profile
Soil &
vegetation
Adjusted erodibility,
friction factors, &
random roughness
WEPP-HE Code
WEPP model
ArcGIS
Precipitation
& runoff
output
VIC

10. Preliminary single cell test
Minnesota
Watonwan watershed
30 arc-second DEM
Selected
VIC grid cell
at 1/8 degree
Agricultural land use (corn, no-till)
Nearest station climate file
30m DEM
Random sample 25 slopes (spatial analyst) for single cell test evaluation

11. Preliminary results

12. Preliminary results Complex data extraction and processing steps
Soil loss (kg/m2) Sediment Yield (t/ha)
WEPP model
Coupled model
Complex data extraction and processing steps
Coupled model under-predict soil loss and sediment yield due to under-estimation of hydrology parameters
Difference in rainfall disaggregation
WEPP-breakpoint
VIC-uniformly distributed by hours of duration

13. Adjustments needed
Break down precipitation (daily-subdaily) using WEPP code
Extract soil erodibility adjustment code from WEPP model to remove the intermediate process
Reinvestigate and solidify slope profile rescale method
Develop soil loss probability distribution
Application in larger study area/multiple cells

14. Adjusted coupling scheme
- iterated process
30 arc-sec
DEM
Soil loss
- original input data
- data processing
Iterated for sampled slopes
- data for WEPP-HE
- intermediate data
Hourly
precipitation
30m slope
Slope
profile
Soil &
vegetation
WEPP-HE Code
Extracted
WEPP model
Source code
Generate
adjustment
parameters
ArcGIS
Precipitation
& runoff
output
VIC
WEPP disaggregated
CLIGEN

15. Current development Rainfall disaggregation
Regrid study domain to create .PAR file for each VIC grid cell based on existing CLIGEN PAR stations (inverse distance)
Use CLIGEN to generate .CLI file for each cell
Extract disaggregation code from WEPP source code to generate daily disaggregated storm
Integrate disaggregated rainfall into hourly precipitation
Hourly precipitation and daily TMIN, TMAX, and WIND are used to create VIC climate forcing files

16. Current development Soil adjustment code
Extracted soil texture, friction, erodibility adjustment codes from WEPP source code
Compiled to form a new Fortran program that reads in VIC soil and climate input data and output adjustment variables for each day

17. Still need to work Slope profile rescale
Monofractal scaling method (Bowling et al, 2004) where fractal dimension is estimated using variogram technique (Zhang et al. 1999; Xu et al. 1993)
The VIC grid cell is divided into subregions (3x3 30arc) to maintain local fracal properties
The distribution of slope for the whole watershed fits a Laplace distribution
30arc-DEM
30m slope

18. Improved result Soil loss (kg/m2) Sediment yield (t/ha) Mean Std WEPP
0.1007
0.0424
1.0384
0.4248
Coupled model
0.0993
0.0479
0.9936
0.479
Average annual ( )

20. Still need to work Rescale issue, validation
Erosion estimation under different vegetation cover/land-use
Statistical analysis of results, distribution fit, etc.

21. Thank you Questions? Suggestions?