As Colorado River shrinks, water evaporation becomes critical to California’s future supplies

LOS ANGELES — Much of the Colorado River’s water is diverted from reservoirs and transported in canals to the farmlands and cities of the desert Southwest. But some of the water also ends up going elsewhere — vanishing into thin air.

Water lost to evaporation has become a central point of contention in the disagreement between California and six other states over how to divide reductions in water use.

A proposal submitted by Arizona, Colorado, Nevada, New Mexico, Utah and Wyoming calls for relying heavily on counting evaporation and other water losses from reservoirs and along the river in the Lower Basin — the portion of the watershed that begins near the Grand Canyon and stretches to northern Mexico.

Counting those losses would mean immediate reductions of more than 1.5 million acre-feet across the region. It would also translate into especially large water cutbacks for California, which uses the single largest share of the river.

Under the proposal by the six states, Southern California water agencies would be required to absorb the largest share of the cuts, including