ASSESSMENT OF FUTURE GROUNDWATER AVAILABILITY UNDER CHANGING CLIMATE IN THE CACHE AND GRAND PRAIRIE REGIONS OF NORTHEASTERN ARKANSAS

The Cache and Grand Prairie regions of northeastern Arkansas reside in the Mississippi Alluvial Plain (MAP), a region that supports a multibillion-dollar agricultural industry. Agricultural production relies heavily on groundwater for irrigation and has resulted in decades of significant groundwater-level declines in the underlying Mississippi River Valley Aquifer. The region has been subject to numerous studies to improve the conceptual understanding of the hydrologic system and to assess the future sustainability of the Mississippi Embayment Regional Aquifer System (MERAS). This study, a part of the U.S. Geological Survey’s MAP Study, addressed the need to better understand the complex hydrogeology and effects of climate change on the groundwater system using the latest available techniques and methods. The study included the development of two inset numerical groundwater-flow models for the Cache and Grand Prairie regions, using the U.S. Geological Survey’s latest MODFLOW 6 groundwater-flow simulation code, with boundary conditions extracted from a regional model of the MERAS. Physical properties of the aquifers were derived from integration of data from a regional airborne electromagnetic geophysical survey. Inputs of recharge and irrigation were supplied by soil water balance and water use models. The primary history matching calibration period was discretized into monthly stress periods from April 2007 through December 2018. The forecast scenario models were discretized into monthly stress periods from January 2019 through December 2055. History matching automated calibration was achieved using the PESTPP-Iterative Ensemble Smoother via 13,420 and 26,015 adjustable parameters for the Cache and Grand Prairie models, respectively. Preliminary calibrated mean absolute groundwater-level residuals were 2.8 (good fit) and 7.4 (marginal fit) meters and calibrated average annual recharge was 2.4 and 2.9 inches for the Cache and Grand Prairie models, respectively. Ten climate scenarios simulated the effects of changing climate on recharge, irrigation, and groundwater availability. The scenarios used the Coupled Model Inter-Comparison Project 5 Representative Concentration Pathways 4.5 and 8.5 downscaled global climate forecasts.