Most notable in the protracted drought conditions that have worried resource managers in the American Southwest is the decreasing snowpack in the Rocky Mountains. In the region surrounding Denver, Colorado this has prompted Denver Water to explore two scenarios of rising average temperatures, and the impacts those scenarios would have on regional water supplies. The following is from the story in Summit Daily News, beginning with the two temperature scenarios as described by Marc Waage, manager of water resource planning:

• With a temperature increase of two degrees over a 50-year stretch — assuming no change in precipitation — streamflows and water supplies would decrease by 7 percent.

• The second scenario plugged a 5 degree temperature increase into the model. Streamflows would drop by 19 percent, with a 14 percent impact to Denver Water’s supply.

Both temperature scenarios are “modest” compared to what many climate change models are predicting, he added. Most of the decrease in stream flows and supplies is due to increased evaporation and sublimation. The bottom line is that Denver Water’s system is very sensitive to warming temperatures, Waage said.

“It would cost a bundle of money to replace 14 percent of our water supply,” he said.
It would also take a significant increase in precipitation to make up for the losses.

Some recent climate change models are actually predicting an increase in winter precipitation in Colorado. Under those scenarios, the northern Rockies could see a snowfall regime similar to the Sierra Nevada, with more wet and heavy snow.

Future snowpack seasons will be shorter and runoff will occur earlier. The point of the studies is to develop a long-term water plan that will work in different climate change scenarios, Waage said.

“We know the climate is variable, but within a certain range. Water planners have always dealt with uncertainties, like population change. Now, we also have to deal with hydrological uncertainty,” he said.

Resource manager around the world have similar worries about water supply and how factors including rising temperatures, rising populations and changing precipitation patterns will stress existing systems.