SIMULATION OF GROUNDWATER FLOW AND LONG-TERM DRAWDOWN IN THE MISSISSIPPI DELTA REGION

The Mississippi Delta is a productive agricultural region encompassing ~7,000 mi² of alluvial floodplain between the Mississippi and Yazoo Rivers in northwest Mississippi. Important crops include soybeans, corn, cotton, rice and aquaculture (catfish), with a total economic impact of ~$3 billion. Irrigation to offset high rates of evapotranspiration has resulted in groundwater declines of 50 feet or more over the last century. Recent wetter than average conditions have stabilized groundwater levels somewhat, but future trends are unclear. Using MODFLOW 6, PEST++, and several open-source python packages developed in part for this project, the USGS Mississippi Alluvial Plain Study has created a groundwater model that incorporates information from a Soil Water Balance Code simulation of the terrestrial water balance, metering-based estimates of water use from thousands of wells, measured and estimated stream flows and stages, and the largest airborne electromagnetic survey flown to-date in the US. Results indicate a complex water balance that varies in time and space in terms of the terrestrial recharge, stream leakage and regional groundwater flow components, which are influenced by seasonal forcings, human activity and alluvial geomorphology. The model construction workflow is designed to facilitate regular updating, making this a “living” framework that the Mississippi Dept. of Environmental Quality and other stakeholders can use for adaptive management going forward.