Assessment of agricultural groundwater-irrigation demand in the Mississippi Delta

Study region The Mississippi Delta, a lenticular alluvial plain of approximately 19,000 km² in northwestern Mississippi, USA, encompassing 19 counties wholly or in part and underlain by the Mississippi River Valley Alluvial Aquifer (MRVAA), the principal source of irrigation water in the region. Study focus Groundwater management in the Mississippi Delta is constrained by the scarcity of metered groundwater withdrawal records, because flowmeter installation is voluntary and reporting is aggregated at coarse temporal scales. This study (i) characterized daily growing-season groundwater-level (GWL) dynamics using continuous records from 11 USGS observation wells for the 2016–2021 growing seasons, and (ii) developed a spatially distributed framework to estimate agricultural groundwater-irrigation demand — used here as a proxy for groundwater withdrawal — at seasonal, monthly, and daily time steps. The daily values represent monthly-average daily irrigation-demand rather than day-to-day pumping. Estimates were derived from annual USDA-NASS Cropland Data Layer maps of corn, cotton, rice, soybean, and aquaculture, combined with crop-specific seasonal application depths and monthly distribution coefficients, and were allocated to 1921 proxy well locations. The estimates are not independently measured withdrawals; they were evaluated against groundwater withdrawals reported for the MRVAA by the U.S. Geological Survey to assess plausibility. New hydrological insights for the region Growing-season GWL declines were spatially centered in the central Delta, where a well in Sunflower County showed an average decline of 3.15 m across the 2016–2021 growing seasons. Estimated irrigation demand averaged 21.04 million m³ per day across the growing season, peaking in July and August, with soybean and rice accounting for the majority of demand. Bolivar, Sunflower, Washington, and Leflore counties consistently showed the highest demand, reflecting crop composition and cultivated extent. After adjustment for crop-specific irrigated fractions, the estimated 2016 crop irrigation demand agreed with the independently reported value for the aquifer to within 28%. Because the framework resolves demand in both space and time, it provides transient pumping inputs for regional groundwater-flow models and a transferable approach for other irrigated alluvial aquifer systems where metered withdrawal data are sparse.