In the Mississippi alluvial plain (MAP) area, the demand for groundwater resources from the alluvial aquifer for agricultural irrigation has led to significant reductions in groundwater-level elevation over time. In this study, we use the hydrologic model SWAT + to quantify long-term changes in groundwater storage within the MAP in United States, wherein groundwater is used extensively for irrigation. We apply a linear quantile regression method to perform trend analysis for wet, dry, and average conditions for the 1982-2020 period. The SWAT + model uses the gwflow module to simulate groundwater storage and groundwater-surface water interactions in a physically based spatially distributed manner, with groundwater pumping linked to field-based irrigation demand. Results indicate significant trends in storage and groundwater fluxes. In wet conditions, significant decline trends are noted in groundwater head (-18.0 mm/yr.) and groundwater evapotranspiration (-0.7 mm/yr.). Under dry conditions, trends are in groundwater head (-28.0 mm/yr.), recharge (-5.5 mm/yr.), and groundwater discharge (-5.5 mm/yr.). For average conditions, decreases include groundwater head (-20.6 mm/yr.), recharge (-6 mm/yr.), and groundwater discharge (-9.3 mm/yr.). This underscores the significance of local management solutions.