Modeling Upland and Channel Sources of Sediment in the Le Sueur River Watershed, Minnesota

Transcription

1 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, th International SWAT Conference Boulder, Colorado

2 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

3 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

4 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

5 LRW Cont d Three major subwatersheds 302,000 tons annual sed. yield Major source of sediment that polluted the MRB and Lake Pepin

6 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

7 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

8 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 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

9 84 sub-watersheds 4818 HRUs USGS 30 m DEM Minnesota DNR watershed subdivisions Model Setup

10 Model Calibration and Validation Calibration sub-watershed Beauford Sub-watershed Has no bluffs or ravines Only upland sediment sources Calibration year 2000 Validation Model simulation efficiency: Nash Sutcliffe coefficient (NSE) Calibrated model input parameters transferred to the entire LRW

11 Flow Calibration and Validation in Beauford NSE = 0.77 NSE = 0.89

12 SYLD Calibration and Validation in Beauford NSE = 0.67 NSE = 0.76

13 SWAT model was well parameterized Calibrated model parameters can satisfactorily be applied to the entire LRW

14 Application of SWAT Model to LRW Big Cobb Maple Upper Le Sueur Entire Le Sueur

15 Validation of Flow in LRW Model predicted discharge was accurate NSE value of 0.73

16 Validation of Sediment Yield in LRW

17 Channel Sources of Sediment in LRW

18 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 LRW = 230,502 tons Model predicted = 32,893 tons Upland contribution = 14% Channel Sources = 86%

19 LRW Upland Sediment Source Areas SYLD, ton/ha Area, ha Area, % < to to > Total

20 LRW Upland Sediment Source Areas---Cont d

21 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 Big Cobb Maple Upper Le Sueur LRW

22 Channel Vs Upland Sources Percent SYLD Upland Sources Channel Sources Percent SYLD Percent Contributing Area

23 LRW Upland Sediment Source Areas---cont d Subwatershed Percent SYLD Contribution Upland Channel Total Percent Area Big Cobb Maple Upper Le Sueur LRW

24 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

25 Acknowledgments Minnesota Department of Agriculture (MDA)

26 Questions?