The California Aqueduct transports water 700 kilometers from northern California to southern California, including Los Angeles, which ranks first in the world in cross-basin water imports, according to a new study. Photo: Kluft/Wikimedia Commons
As cities grow in population and economic activity, they reach further and further out to find water to meet their needs.
These transfers often cause ecological harm in the source watersheds, but they can relieve urban water stress by making more water available to cities.
McDonald and his team estimate that 25% of cities are in conditions of water-stress, compared with estimates from earlier studies of 39%. (For these figures the range is +/- 4%.)