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

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

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

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

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

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

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

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

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

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

Preliminary results

Preliminary results Complex data extraction and processing steps Soil loss (kg/m2) Sediment Yield (t/ha) WEPP model 0.14 1.481 Coupled model 0.054 0.576 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

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

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

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

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

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

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 (1980-1990)

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

Thank you Questions? Suggestions?