Modeling the Impacts of Climate change on Crop Yield, Runoff, and Soil Loss Using the Water Erosion Prediction Project (WEPP) Model

Understanding changes in biomass amount with rising CO₂ levels and temperature is essential for runoff and soil erosion predictions. The Water Erosion Prediction Project (WEPP) model was modified to investigate the effects of an atmospheric CO₂ concentration of 550 ppm projected for the mid-21^st century on live biomass and crop yields. WEPP hillslope modeling was carried out for corn and soybeans for the state of Iowa and winter wheat for Oklahoma with dominant soils at the county level under both irrigated and non-irrigated conditions. A General Circulation Model (GCM), GFDL-ESG2G, from the fifth phase of the Coupled Model Intercomparison Project (CMIP5) was adopted for simulating future climate under the Representative Concentration Pathway (RCP) 8.5 scenario. WEPP model simulation scenarios included: 1) baseline historical climate (1995–2005) representative of 370 ppm CO₂ level (Scenario A), 2) baseline historical climate (1995–2005) with an elevated 550 ppm CO₂ level (Scenario B), and 3) future climate (2045–2055) with an elevated 550 ppm CO₂ level (Scenario C). For Scenario A simulations, a biomass energy ratio parameter in WEPP was calibrated to ensure that simulated mean crop yield during baseline climate years was comparable to the mean observed crop yield from the National Agricultural Statistics Service (NASS). Simulations for Scenarios B and C were carried out by extrapolating the biomass energy ratio of crops at an elevated CO₂ level of 550 ppm based on Free-Air Concentration Enrichment (FACE) experimental studies. Projected mid-21^st century climate in comparison to historical baseline from the GFDL-ESG2G model suggested increases in precipitation within Iowa in the range of 11% to 26% and across Oklahoma up to 12%. Increases in temperatures across Iowa ranged from 1.5−1.85°C, and for Oklahoma 1.6−2.1°C. Results of changes in simulated live biomass and crop yields from scenarios A and B, and A and C are reported here. The preliminary modeled results suggest that under irrigated conditions, increases in CO₂ level and temperature may decrease live biomass for corn, soybeans, and winter wheat, but result in a slight increase in crop yields for soybeans and winter wheat, and a decrease in corn yields. Under non-irrigated conditions, modeled results suggest decreases in live biomass for corn and winter wheat and a slight increase for soybeans. Crop yields under non-irrigated conditions show increasing trends for all of the crops. More investigation is needed as to how further increases in elevated CO₂ levels and temperatures may impact live biomass and yield, which are important factors for soil erosion modeling. Further work is needed to parameterize plant parameters, particularly for corn where simulated yields were lower than those reported in the literature. Future work also involves comparisons of runoff and sediment yields from baseline to future climate with the use of other CMIP5 and CMIP6 models.