Climate change challenges efficiency of inter-basin water transfers in alleviating water stress

Inter-basin water transfer (IBT) is widely used to mitigate water shortage at the cost of compromising water availability in water-exporting regions. Yet, we do not know how efficient are the IBTs in alleviating inter-regional water stress in a changing climate and water supply-demand context. From a socio-hydrological perspective, we here quantify the efficiency of more than 200 IBTs across the United States by a Stress Relief Index that measures the impact of water redistribution on the overall water stress level. Based on the assumption that an IBT-induced increase and reduction in water availability would respectively constitute a positive and negative impact on regional water security, we show that 29% of the IBTs could be considered socially inefficient by 2010 as they shift water stress from water-receiving to water-exporting and downstream regions. Future stress escalations induced by growing population, declining runoff, and increasing demands for energy production and irrigation will alter IBT efficiency disproportionately. The inefficient IBTs would amount to 32% and 35% by the end of the 21st century under the scenarios of representative concentration pathway (RCP) 4.5 and RCP8.5, with 7 similar to 16 IBTs reaching a tipping point that their role in the water system could switch from alleviating to aggravating the overall water stress. Our results indicate that the evolving climatic and socioeconomic status can largely affect transfer efficiency, highlighting the need of basin-level adaptation strategies for sustainable use of the IBTs.