Characterizing Groundwater and Surface-Water Interaction in the Mississippi Delta Using Hydrograph Separation

Understanding the relationship between groundwater withdrawals and aquifer response can allow for the estimation of changes in groundwater availability over time and help determine best water-resource-management practices to sustain groundwater and surface water resources for agricultural irrigation, ecological flow, and other uses. An increase in groundwater withdrawals from the Mississippi River valley alluvial (MRVA) aquifer for agricultural irrigation has resulted in stream and groundwater level declines in the Mississippi Delta region, in northwest Mississippi. In 2016, the U.S. Geological Survey (USGS) began a study to better understand the effects of pumping on groundwater availability in the alluvial aquifer. Two USGS continuous continuous-gaging stations and co-located piezometers provided hydrologic data to characterized groundwater/surface-water interaction at two sites in the Delta. The sites are located at the Sunflower River at Sunflower, Mississippi and the Tallahatchie River at Money, Mississippi. Baseflow, the amount of groundwater that contributes to streamflow, was estimated at each site using hydrograph-separation and trend-analysis techniques provided in the USGS Groundwater Toolbox open-source software. Recently collected streambed resistivity data provided insight into the variability of hydraulic connectivity along streambeds and values were compared with the hydrograph separation and trend analysis results. This combination of techniques allowed for better characterization of the hydrogeologic conditions and the groundwater/surface-water interactions at the selected site. Characterizing hydrologic relations such as this will help refine a regional groundwater model of the Delta that will aid water-resource managers in future decisions pertaining to groundwater availability of the alluvial aquifer.