Every drop of the Colorado River counts. So what about evaporation?
A bathtub ring seen above the waterline around Lake Powell was created during drought that reduced the flow of the Colorado River on April 15, 2023. RJ Sangosti/MediaNews Group/The Denver Post via Getty Images For more than a hundred years, California, Arizona, and Nevada never accounted for evaporation on the lower basin of the Colorado River as they divided its water between themselves and later with Mexico. Their logic held that as long as there was more water than people used, they could ignore small losses from natural processes. More importantly, it was politically fraught—for decades, the lower basin states have been unable to reach an agreement about how evaporation should be taken into account when sharing the river’s waters. Even as a 23-year-long megadrought sucked moisture out of the already...