We estimate that annual mean discharge has been decreasing by 9.3% per degree Celsius of warming because of increased evapotranspiration, mainly driven by snow loss and a consequent decrease in reflection of solar radiation. Projected precipitation increases likely will not suffice to fully counter the robust, thermodynamically induced drying.
That’s from a new study by Paul Milly and Krista Dunne of the USGS.
Here’s my breakdown of the analysis. If you look at the data, discharge appears to decrease at a rate of about 14% per degree C of warming. But our hydrological models tell us that it should only be about 5% decrease per degree of warming (and of course there’s uncertainty around those numbers). So what does increased temperature do that we’re not capturing? Milly and Dunne say it’s the effects of snow loss, which reduces albedo and therefore reflection of solar radiation. If their model is correct, the drying trend to be expected with future warning over the Colorado could spell sharp reductions in water supply.
The study is published in Science.
There’s an accompanying perspective article (same issue), which makes a very interesting and timely point:
The year 2020 is a momentous one for CR water policy. The interim interstate agreement on sharing water-shortage impacts will be renegotiated this year. The new, more stringent Lower Basin Drought Contingency Plan will mandate that water deliveries to states in the lower basin be reduced—a first, and unthinkable a generation ago.
One wonders how big the new reductions will be and whether they will go far enough.