Evaluating the impacts of crop rotations on groundwater storage and recharge in an agricultural watershed
The Mississippi River Valley Alluvial Aquifer, which underlies the Big Sunflower River Watershed (BSRW),is the most heavily used aquifer in Mississippi. Because the aquifer is primarily used for irrigating cropssuch as corn, cotton, soybean, and rice, the water levels have been declining rapidly over the past fewdecades. The objectives of this study are to analyze the relationship and interactions between evapotran-spiration and groundwater recharge rates in the BSRW, and model the effects of various crop rotationpractices on groundwater storage and recharge.The model performed well during the calibration period (R2= 0.53–0.68 and NSE = 0.49–0.66) and val-idation period (R2= 0.55–0.75 and NSE = 0.49–0.72) for daily streamflow, which was achieved by theSUFI-2 auto-calibration algorithm in the SWAT-CUP package. The model also performed well in simulat-ing seasonal water table depth fluctuations during calibration (R2= 0.76 and NSE = 0.71) and validation(R2= 0.86 and NSE = 0.79). This study demonstrated a seasonal relationship between evapotranspira-tion and groundwater storage and recharge in the BSRW SWAT model. In general, groundwater storagedecreased during the summer months while ET rates were high, and increased during the winter andspring months when ET rates were low. The crop rotation scenarios that include rice planting resultedin the lowest groundwater storage (down to -10.7%) compared to the baseline crop scenario, which isdue to the high irrigation rates of the rice crop. However, the rice crop rotations resulted in the highestincreases of groundwater recharge rates (up to +60.1%), likely because of the response to the deficiency ofgroundwater needed for irrigation as well as the limited water uptake by the rice crop. The crop rotationswith corn and cotton resulted in the largest increases in groundwater storage (up to +27.2%), which is theresult of the low irrigation rates as well as the short time period for irrigation applications. The results ofthis study are expected to aid farmers and watershed managers to conserve groundwater resources, butstill maintain crop production.