Signs of drought proliferate across the American West. California is rationing water for farmers in the state’s Central Valley. Salmon are dying en masse in the Pacific Northwest as river temperatures climb. Lake Mead, on the border of Nevada and Arizona, is drying up. … Groundwater is the source of drinking water for half of Americans, and nearly all of the country’s rural communities. … But groundwater has become dangerously depleted in places where pumping has exceeded the rate at which aquifers are naturally replenished.