The Nine Percent Headline

The headline message in the first iteration of the Bureau of Reclamation’s Colorado Basin supply-demand study is “9 percent”:

Under the Downscaled GCM Projected scenario, the mean natural flow as measured at Lees Ferry over the next 50 years is projected to decrease by approximately 9 percent, along with a projected increase in both drought frequency and duration as compared to the observed historical streamflow record. Droughts lasting 5 or more years are projected to occur 40 percent of the time over the next 50 years. Projected changes in climate and hydrologic processes include continued warming across the Basin, a trend toward drying, although precipitation patterns continue to be spatially and temporally complex, and increases in evapotranspiration and decreases in snowpack, as more precipitation falls as rain rather than snow.

But we already knew that, right? Basically the same data, with some minor tweakage, was behind the Bureau’s Climate Change Impacts in Western Basins report released in March. Its headline number was 8.5 percent for the Colorado. But don’t hang two much on the number. There’s too much uncertainty surrounding the modeling (especially the specification of an emissions scenario to drive the climate change models) to think of this as predictive.

What’s more important is the development of a range of scenarios to better understand the range of risks we face not only on the supply side (climate change, drought) but also on the demand side – as a result of population change, changed usage patterns and also changes in use in response to climate change. In the snapshot of the study released today, this work is only starting to get underway.