From The Los Angeles Times (William Yardley):

…there is one place where the precipitation has been particularly welcome and could be transformative: the Colorado River basin, which provides water to nearly 40 million people across seven states.

“We’re in a really good spot as far as snow accumulations,” said Malcolm Wilson, who leads the Bureau of Reclamation’s water resources group in the upper Colorado River basin.

Under federal guidelines that kick in when water flows reach certain volumes, the Bureau of Reclamation, which oversees the river basin’s largest reservoirs, Lake Powell and Lake Mead, could release enough water from the former to raise the elevation of the latter by 20 feet or more — providing a remarkable shot in the arm for a lake that has been declining steadily during a devastating drought that started in 2000.

The process — lowering one reservoir to lift another — is called equalization, and a few weeks ago, it was not even viewed as a viable option. Now, Wilson said, “It’s in the realm of possibility.”

Even if that optimistic scenario does not play out — the region would need several more weeks of strong precipitation without a substantial warmup — there is still reason to savor a moment of relief on the Colorado…

As of last month, the bureau was forecasting about a 50% chance that, for the first time, the river and its reservoirs would not be able to fulfill the water demands of states that rely on it, beginning in 2018.

But this week, the bureau quietly updated that forecast, saying the chance was only about 34%. By the end of this year, it expects Lake Mead to be at least 3 feet above the threshold at which an official “shortage” would be declared.

Not only that, the bureau said the likelihood of a shortage through 2021 is no greater than 33%. Just a few weeks ago, the chances of shortages in that time frame were about 60%.

Click here to buy the report. Here’s the abstract:

Between 2000 and 2014, annual Colorado River flows averaged 19% below the 1906-1999 average, the worst 15-year drought on record. At least one-sixth to one-half (average at one-third) of this loss is due to unprecedented temperatures (0.9°C above the 1906-99 average), confirming model-based analysis that continued warming will likely further reduce flows. Whereas it is virtually certain that warming will continue with additional emissions of greenhouse gases to the atmosphere, there has been no observed trend towards greater precipitation in the Colorado Basin, nor are climate models in agreement that there should be a trend. Moreover, there is a significant risk of decadal and multidecadal drought in the coming century, indicating that any increase in mean precipitation will likely be offset during periods of prolonged drought. Recently published estimates of Colorado River flow sensitivity to temperature combined with a large number of recent climate model-based temperature projections indicate that continued business-as-usual warming will drive temperature-induced declines in river flow, conservatively -20% by mid-century and -35% by end–century, with support for losses exceeding -30% at mid-century and -55% at end-century. Precipitation increases may moderate these declines somewhat, but to date no such increases are evident and there is no model agreement on future precipitation changes. These results, combined with the increasing likelihood of prolonged drought in the river basin, suggest that future climate change impacts on the Colorado River flows will be much more serious than currently assumed, especially if substantial reductions in greenhouse gas emissions do not occur.

Here’s a report from John Fleck writing for InkStain. Here’s an excerpt:

A warming climate is already reducing the flow in the Colorado River, and the future risk is large, with a worst case of the river’s flow being cut in half by the end of the century, according to a new study from a pair of the region’s leading researchers. While precipitation declines since the turn of the century have been modest, Brad Udall and Jonathan Overpeck found, a little less rain and snow have translated to a lot less water in the river.