Quantifying the Impacts of Climate Change on Soil Erosion and Runoff in Minnesota Agricultural Watersheds using the WEPP Model

Abstract

In the past several decades, soil water erosion and runoff in agricultural regions across the Midwestern United States have contributed to significant A-horizon soil losses, declining soil productivity, and the widespread impairment of water resources. Rising temperatures and changes in precipitation trends have the potential to significantly impact future soil water erosion and runoff in the Midwest, yet relatively few studies have attempted to quantify these impacts. To help expand this area of research, the Water Erosion Prediction Project (WEPP) Model was used to quantify growing season soil water erosion and runoff for a baseline period (1965-2019) and two future periods (2020-2059 & 2060-2099) in agricultural hillslopes across three Minnesota HUC12 watersheds. Future daily weather inputs were generated for WEPP using downscaled daily climate projections from two CMIP5 climate models (HadGEM2-CC and GFDL-ESM2G) and three emissions scenarios (RCP 4.5, RCP 6.0, and RCP 8.5). The climate scenarios were combined with varying adoption rates of perennial crops, conservation tillage systems, and cover crops in order to determine the effectiveness of these mitigation strategies in reducing future soil loss and runoff. When the baseline management scenario was used, most future climate scenarios showed decreases in runoff (34-58%), soil loss (3.3-40%), and the number of hillslopes with unsustainable soil loss (1-8%). However, there were also moderate to significant increases in runoff (17-38%), soil loss (4.2-40%), and the number of hillslopes with unsustainable soil loss (+3%) in several of the moderate emissions scenarios (RCP 4.5 and RCP 6.0). Watersheds with steep slopes and widespread conventional row crop systems had a significantly greater number of hillslopes with unsustainable soil losses, even if runoff and soil loss were projected to decrease in the future. Additionally, growing season runoff and edge-of-field sediment delivery values required to meet TSS total maximum daily loads (TMDLs) were only met in the highest emissions scenarios (RCP 8.5) where future soil loss and runoff significantly decreased. In order to mitigate agricultural runoff and unsustainable soil losses in Minnesota under current and future climates, we recommend implementing a combination of perennial alfalfa cropping systems and reduced tillage systems across hillslopes in watersheds with widespread steep slopes and conventional row cropping systems. However, requirements for TSS TMDLs were still only met in the highest emissions scenarios when these systems were implemented into the majority of hillslopes. As a result, additional off-site mitigation measures will likely be required in at-risk watersheds to meet TSS TMDLs and soil conservation goals under most future climates.