Projection of 21st century irrigation water requirement across the Lower Mississippi Alluvial Valley

Lower Mississippi Alluvial Valley (LMAV) is the largest floodplain and one of the most productive agricultural regions in the United States. Irrigation is widely used in this region to improve crop production and resource use efficiency due to a mismatch between crop water requirements and precipitation timing and quantity during the growing season. In the recent decades, aquifer decline caused by groundwater withdrawals for irrigation has been recognized as a critical environmental issue threatening water security and agricultural sustainability in the LMAV. To improve agricultural water use efficiency and reduce groundwater withdrawals, it is pivotal to understand the spatiotemporal patterns of crop irrigation water requirements (IWR). In this study, we analyzed future climate changes over the LMAV cropland areas and estimated future IWR changes for major crops in the 21 ^st century using two climate scenarios (i.e. RCP45 and RCP85) and two crop growth duration length (GDL) scenarios [i.e. Fixed GDL (GDL does not change with time) and Varied GDL (GDL changes with time)]. Results show that croplands in the LMAV would experience continuous warming, and either no significant change or a decreasing level of precipitation under the RCP45 and the RCP85. If keeping current cropland areas and cropping systems unchanged, average crop IWR by the end of the 21 ^st century would increase by 4.2% under the RCP45 + Varied GDL scenario, 14.5% under the RCP45 + Fixed GDL scenario, 9.2% under the RCP85 + Varied GDL scenario, and 29.4% under the RCP85 + Fixed GDL scenario. The greatest increases would occur in the summer months. Aquifer levels in the LMAV, therefore, are expected to decline at an accelerated pace if no effective mitigation strategies are implemented. This study made the first attempt to reveal the spatially-explicit crop IWR and its future changes in the LMAV, which provides a scientific basis for developing management strategies that can enhance water use efficiency and improve agriculture sustainability.