Evolving efficiency of inter-basin water transfers in regional water stress alleviation

Inter-basin water transfer (IBT) is widely used to mitigate water stress by diverting water from a relatively water -rich region. However, it is unclear how the IBTs' role evolves with the dynamic hydroclimatic and socioeconomic circumstances. Here, we propose a new Stress Relief Index and an integrated water stress simulation framework to quantify the efficiency of 50 active IBTs across the conterminous United States in alleviating regional water stress. The IBTs vastly reduced the coverage of highly stressed area (8.7 x 104 km2) and population (4.3 million people) during 1986-2015. The widespread (74%-80% of the IBTs) increasing IBT efficiency suggest that IBTs have been increasingly important in securing water supply in a drying climate. However, the complex combi-nations of changes in climate, water use behaviors in energy and food production, population, and transfer magnitudes caused divergent changes in IBT efficiency, revealing the challenges posed by streamflow depletion and spatial migration of water demand.