WATER BUDGET ACCOUNTING IN THE MISSISSIPPI EMBAYMENT REGION AND MISSISSIPPI ALLUVIAL PLAIN, 2000-2020, WITH UNCERTAINTY ANALYSIS, SEASONAL IMPACT OF IRRIGATION ON THE WATER BUDGET, AND ALTERNATE MODEL COMPARISON

In support of a broad series of investigations into the hydrology of the Mississippi Alluvial Plain aquifer, a USGS Soil-Water-Balance model was constructed and calibrated to better understand the distribution and timing of water budget components from 2000-2020 in the study area. The calibration included history matching with observations of direct runoff, base flow, actual evapotranspiration (AET), and irrigation application. Uncertainty analysis using a Monte Carlo approach showed that the AET had the lowest uncertainty of all the components and the net infiltration had the highest uncertainty. The model analysis illustrated the effect of summer irrigation on the overall water budget in the alluvial plain, which increased the summer AET by 22 percent and total combined runoff and baseflow by 62 percent. The soil-water-balance model uses a rooting zone accounting method, which produces net infiltration estimates higher than groundwater recharge values calculated by means of a MODFLOW model of the study area, likely because of the presence of horizontal clay lenses that block much of the infiltration from reaching the water table. Challenges with determining true base flow from groundwater discharge in such an altered hydrologic system were partly responsible for an over-estimation of potential recharge relative to values estimated through MODFLOW history matching.