In Arizona, water scarcity is like the background hum of conversation in a popular restaurant: unrelenting. But even in this desert state, the ever-present strain on water supplies could soon be felt more acutely. As soon as 2019, the water level in Lake Mead on the Colorado River could drop below an elevation of 1,075 feet. That will trigger mandatory cutbacks in water diversions from the reservoir under an agreement negotiated between the federal government and three lower-basin states that rely on the river: Arizona, California and Nevada.