An intensifying but unseen force is stealing precious water from rivers in the arid West, but it’s hardly a thief in the night.

The midday sun is one of the more aggressive guzzlers of the Colorado River. Between its high Rockies headwaters and its Sonoran Desert delta, 1 to 2 million acre-feet of water evaporates each year in the Colorado River Basin. That’s a big gulp in a watershed where seven thirsty U.S. states and northern Mexico skirmish for their share of an overallocated, shrinking water supply. And the evaporation will only increase as the Southwest grows hotter and drier.