Characterizing groundwater and surface-water interaction throughout the Mississippi Delta using hydrograph-separation techniques combined with near-stream geophysical and groundwater-level data

Author(s): Killian, C.; Barlow, J.; Barlow, P.; Kress, W.; Schmitz, D.

The Delta, an area dense in agriculture, is situated between the Mississippi and Yazoo Rivers in northwest Mississippi. Stream and groundwater levels in the Delta have shown declines with the increase in irrigation to support agricultural production. In 2016, the U.S. Geological Survey (USGS) began a study to better understand the effects of pumping on groundwater and its availability in the Mississippi River Valley alluvial (MRVA) aquifer. The alluvial aquifer is the uppermost hydrologic unit in the Delta and supplies most of the groundwater used for agricultural irrigation. Understanding the relation between withdrawals and groundwater response in the alluvial aquifer could allow for the estimation of changes in groundwater availability over time and can help to determine the best water-resource-management practices for the study area. A spatially-distributed network of paired groundwater and surface-water streamgage sites provided hydrologic data to characterize groundwater/surface-water interaction throughout the Delta. Baseflow, the amount of groundwater that contributes to streamflow, was estimated for each site using hydrograph-separation methods. The USGS Groundwater Toolbox open-source software provides several techniques for hydrograph separation and was used for this study. Recently collected geophysical data along selected streams in the Delta provided insight to the hydraulic conductivity, or ease with which water moves through the soils and unconsolidated sediments, was coupled with the hydrograph-separation results. This combination of techniques allowed for better characterization of groundwater/surface-water interaction at the selected sites. Characterizing and defining these types of hydrologic relations will help USGS scientists refine a regional model of the Delta that will be used to aid water-resource managers in future decisions pertaining to the alluvial aquifer.

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