Modeling Upland and Channel Sources of Sediment in the Le Sueur River Watershed, Minnesota Solomon Folle and David Mulla Department of Soil, Water and Climate University of Minnesota August 6, 2009 5 th International SWAT Conference Boulder, Colorado
Introduction and Background Minnesota River Basin (MRB) Contains headwaters of the Mississippi River basin Drains 4.4 million ha in Minnesota Impaired for sediment 12 major watersheds Le Sueur River Watershed (LRW) is one LRW 7% of the MRB area 53% of the MRB TSS Lake Pepin Downstream of the MRB Impaired for sediment 85% from the MRB 45% from LRW
Objective of the study To quantify sediment loads from upland sources of the LRW using physically based spatially distributed model, SWAT To investigate sediment source areas in the LRW To estimate the relative proportions of upland versus channel sources of sediment
The Le Sueur River Watershed (LRW) Le Sueur River August 29/2007 2,850 sq. km 87% Agric. Land: Corn-Soybean Slope 82% with 0-2% 14% has 2-6% Soils Very deep & poorly drained Fine textured Mollisols Extensive tile drainage
LRW Cont d Three major subwatersheds 302,000 tons annual sed. yield Major source of sediment that polluted the MRB and Lake Pepin
Where does the LRW sediment comes from? 3 major sources i. Soil erosion from uplands ii. Head-cut and knick point migration of ravines iii. Stream bank erosion and river bluff slumping
Modeling Sediment losses of the LRW SWAT model: to estimate sediment losses from the uplands only SWAT model routines to estimate the channel sources are not good enough Estimating sediment load from channel sources Channel degradation was set zero Indirect methods Channel SYLD = Measured SYLD - Predicted Upland SYLD
SWAT Model Input Data Data Type 30m DEM SSURGO soil Land use/ USDA NASS CLD Stream network Weather Point Sources Flow and Water Quality Source http://seamless.usgs.gov http://www.ftw.nrcs.usda.gov http://datagateway.nrcs.usda.gov http://mrbdc.mnsu.edu/gis/lesueur http://climate.umn.edu USGS, MN Metropolitan Council, MPCA, DNR and MDA USGS USDA USDA MRBDC U of M MPCA LOADEST program was used to estimate missing water quality data
84 sub-watersheds 4818 HRUs USGS 30 m DEM Minnesota DNR watershed subdivisions Model Setup
Model Calibration and Validation Calibration sub-watershed Beauford Sub-watershed Has no bluffs or ravines Only upland sediment sources Calibration year 2000 Validation 2001-2006 Model simulation efficiency: Nash Sutcliffe coefficient (NSE) Calibrated model input parameters transferred to the entire LRW
Flow Calibration and Validation in Beauford NSE = 0.77 NSE = 0.89
SYLD Calibration and Validation in Beauford NSE = 0.67 NSE = 0.76
SWAT model was well parameterized Calibrated model parameters can satisfactorily be applied to the entire LRW
Application of SWAT Model to LRW Big Cobb Maple Upper Le Sueur Entire Le Sueur
Validation of Flow in LRW Model predicted discharge was accurate NSE value of 0.73
Validation of Sediment Yield in LRW
Channel Sources of Sediment in LRW
Model predicted SYLD was much smaller than measured This is expected result Only upland sources The difference was contributed by channel sources River bluffs, stream banks and ravines Example: Year 2006 measured SYLD@ LRW = 230,502 tons Model predicted = 32,893 tons Upland contribution = 14% Channel Sources = 86%
LRW Upland Sediment Source Areas SYLD, ton/ha Area, ha Area, % < 5 258692 90.6 5 to 10 19001 6.7 10 to 20 7513 2.6 > 20 471 0.2 Total 285676 100
LRW Upland Sediment Source Areas---Cont d
LRW Upland Sediment Source Areas---Cont d Sub-watershed Predicted Upland Sediment Load Channel Erosion (estimated by difference) Measured Total Sediment Load Average Sed. Yield tons % tons % tons tons/ha Beauford 178 83 36 17 214 0.10 Big Cobb 8998 38 14785 62 23783 0.30 Maple 11047 22 39290 78 50337 0.57 Upper 20019 12 146462 88 166481 1.44 Le Sueur LRW 32893 14 197609 86 230502 0.81
Channel Vs Upland Sources 90 100 80 90 Percent SYLD 70 60 50 40 30 20 Upland Sources Channel Sources 80 70 60 50 40 30 20 Percent SYLD 10 10 0 0 0 20 40 60 80 100 Percent Contributing Area
LRW Upland Sediment Source Areas---cont d Subwatershed Percent SYLD Contribution Upland Channel Total Percent Area Big Cobb 3 6 9 28 Maple 4 17 21 31 Upper Le Sueur 7 63 69 40 LRW 14 86 100 100
Conclusions SWAT model was found to be very accurate at predicting discharge and sediment losses from upland agricultural regions of LRW A small proportion of the LRW uplands (55%) contributes disproportionally high sediment yields (80%) Upland sediment sources are a small fraction of the total sediment losses Only 14% arises from upland agricultural areas 86% from river bluffs, ravines and eroding stream channels 69% of the sediment comes from the Upper Le Sueur Has 40% the LRW area
Acknowledgments Minnesota Department of Agriculture (MDA)
Questions?