ARKANSAS GROUNDWATER INITIATIVE: CONTINUOUS ESTIMATION OF BASEFLOW IN STREAMS WITHIN THE GRAND PRAIRIE AND CACHE CRITICAL GROUNDWATER AREAS

In 2019, the Natural Resources Conservation Service (NRCS) began the Arkansas Groundwater Initiative to address critical groundwater level decline in eastern Arkansas by implementing targeted conservation practices and management plans. In 2020, the U.S. Geological Survey (USGS) and the Agricultural Research Service started a collaborative project with NRCS to monitor and analyze the implementation of conservation irrigation practices in the Grand Prairie and Cache Critical Groundwater Areas (CGA) and the impacts to groundwater levels in the Mississippi River Valley alluvial aquifer. Base flow is the part of streamflow usually attributed to groundwater discharge. Daily and annual baseflow values were quantified using a chemical hydrograph separation (CHS) approach at five streams within the Grand Prairie CGA and two streams within the Cache CGA. The watersheds assessed were medium in size (less than 350 km²) over a temporal scale of three to five years. Using existing discharge and continuously collected specific conductance data, a well-established end-member mixing analysis was adapted and applied to the Arkansas Delta. Integration of these baseflow values into groundwater flow models will enhance our understanding of the water budget in the Cache and Grand Prairie CGA. Chemical mass balance approaches, though often restricted in broad application due to limited data availability, are considered more reliable than graphical hydrograph separation (GHS) methods because more specific watershed geochemical information is incorporated instead of using stream discharge alone. The annual baseflow discharge results of the CHS and GHS method were compared, and GHS baseflow discharge values were biased high by an average of 18%. Valid baseflow estimates are important because they are used to calibrate the hydraulic properties of groundwater flow models, which in turn influence recharge rates and the overall water budget of an aquifer. Quantifying components of the water budget is crucial for overseeing current water use and fostering sustainable future usage.