Brad Udall, a senior scientist at Colorado State University and an expert on water supplies in the West, told a congressional panel last month that the lower basin uses about 10.2 million acre-feet of water from the river each year, while upstream flows provide just nine million. (An acre-foot is the volume of one foot of water over one acre, about 325,000 gallons.)

Beyond that drain, climate change is bringing on a long-term crisis. “The Colorado River, and the entire Southwest, has shifted to a new hotter and drier climate, and, equally important, will continue to shift to a hotter and drier climate for several decades after we stop emitting greenhouse gases,” he said in his testimony.

In an interview, Mr. Udall said the influence of climate change was already apparent in the West. “Climate change is not some distant process,” he said. “It’s here, it’s now, it’s in our faces. It’s creating messes we have to deal with.”

Jonathan T. Overpeck, a climate scientist at the University of Michigan, said that politicians and policymakers needed to factor climate change into their plans. Lack of river water will lead people to pump more groundwater, which was deposited in the ice ages. “We’re using this fossil groundwater in unsustainable ways,” he said.

In a warming world, Dr. Overpeck said, less water in rivers and lakes is inevitable, whatever relief a wet season might bring. But for the most part, Western political leaders “don’t want to talk about it,” he said. “It