Evaluation of Agricultural Managements on Groundwater Storage and Recharge.

Abstract The Mississippi River Valley Alluvial Aquifer, which underlies the Big Sunflower River Watershed (BSRW), is declining over the past few decades. The objectives of this study were to (1) develop a calibrated and validated model using the Soil and Water Assessment Tool (SWAT) for stream flow and water table depths in the BSRW, (2) analyze the relationship and trends between evapotranspiration and groundwater recharge rates, and (3) assess the effects of various crop rotation practices on groundwater storage and recharge. The model performed well during the calibration period (R2= 0.53 to 0.68 and NSE = 0.49 to 0.66) and validation period (R2= 0.55 to 0.75 and NSE = 0.49 to 0.72) for daily streamflow. The model also performed well in simulating seasonal water table depth fluctuations at the calibration sub-basin (R2= 0.76 and NSE = 0.71) and at the validation sub-basin (R2= 0.86 and NSE = 0.79). This study demonstrated a seasonal relationship between evapotranspiration and groundwater storage and recharge in the BSRW. In general, groundwater storage decreased during the summer months while ET rates were high, and increased during the winter and spring months when ET rates were low. The crop rotation scenarios that include rice planting resulted in the lowest groundwater storage (down to -10.7%) compared to the baseline crop scenario, which is due to the high irrigation rates of the rice crop. However, the rice crop rotations resulted in the highest increases of groundwater recharge rates (up to +60.1%), likely because of the response to the deficiency of groundwater needed for irrigation. The crop rotations with corn and cotton resulted in the largest increases in groundwater storage (up to +27.2%), which is the result of the low irrigation rates. The results of this study are expected to aid farmers and watershed managers to conserve groundwater resources, but still maintain crop production.