Evaluating the impacts of crop rotations on groundwater storage and recharge in the Mississippi Delta

Author(s): Dakhlalla, A.; Parajuli, P.

The Mississippi River Valley Alluvial Aquifer, which underlies the Big Sunflower River Watershed (BSRW), is the most heavily used aquifer in the state of Mississippi. Because the aquifer is primarily used for irrigating crops such as corn, cotton, soybean, and rice, the water levels have been declining rapidly over the past few decades. The objectives of this study are to (1) develop a calibrated and validated model using SWAT for streamflow and water table depths in the BSRW, (2) analyze the relationship and trends between evapotranspiration and groundwater recharge rates within the model, and (3) to model the effects of various crop rotation strategies on groundwater storage and recharge. The model performed well during the calibration periodR2= 0.53 to 0.68 and NSE = 0.49 to 0.66) and validation perio(R2= 0.55 to 0.75 and NSE = 0.49 to 0.72) for daily streamflow, which was achieved by the SUFI-2 auto-calibration algorithm in the SWAT-CUP package. The model also performed well in simulating seasonal water table depth fluctuations at the calibration sub-basin (R2 = 0.58 and NSE = 0.56) and at the validation sub-basin (R2 = 0.72 and NSE = 0.63). The crop rotation scenarios with rice planting resulted in the lowest groundwater storage (-8.3% to -9.6%) 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 (+19.4% to +59.5%), likely because of the response to the deficiency of groundwater needed for irrigation as well as the limited water uptake by the shallow rice plant roots. The crop rotations with corn and cotton resulted in the largest increases in groundwater storage (+9.6% to +26.7%), which is the result of the low irrigation rates as well as the short time period between planting and harvesting. The results of this study is expected to aid farmers and watershed managers to conserve groundwater resources, but still maintain crop production.

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