Impacts of historical droughts on maize and soybean production in the southeastern United States

The trend of yield losses to drought in United States (U.S.) agriculture has increased over the years, in spite of financial investments and technological development to alleviate drought impacts. As climate change threatens to worsen future drought conditions, it is crucial to evaluate drought impacts on crop production for efficient yet robust decision-making at the regional level. This study evaluated the robustness of modified Standardized Precipitation Indices (SPIs) using effective precipitation (EP) and the Standardized PrecipitationEvapotranspiration Index (SPEI) in assessing drought impacts on county-level maize and soybean yields in the southeastern U.S during 1979-2019. The results showed that the modified SPI using the CROPWAT EP formula can be as effective as SPEI in assessing drought impacts on crop production. Assessment of drought impacts on crop yield year-to-year variability was then assessed by fitting linear regression through the yield-drought time series. Probability of yield loss due to drought in the region was estimated by using a copula-based probabilistic model. Results show that drought events occurring during the critical development growth stage had significantly negative impacts on year-to-year yield variability in over 50% of the counties in the study region, though the impact magnitude varied across the region due to different characteristics, historical climatic condition and irrigation infrastructure. On average, an extremely dry event occurring during the critical development growth stage resulted in a region-wide average yield reduction between - 42.7% and - 31.9% for maize and between 25.4% and - 23.4% for soybean. Moderately wet conditions may increase crop yield, but excessively wet conditions may diminish the benefits for both crops. The average probability of yield loss of the study region under the extremely dry condition is more than 80%. Irrigation may help to reduce the impacts of drought on crop production: maize-producing irrigated counties on average were found to be 50% less sensitive to drought than non-irrigated counties, though this effect is smaller for soybean. As crop production in the region is highly vulnerable to drought, adaptation interventions under are required to adapt to warming climate scenarios.