Beyond drought: Denver Water prepares for long-term climate change

Modest temp increases lead to significant streamflow changes

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By BOB BERWYN
summit daily news
Summit County, CO Colorado
March 29, 2008

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SUMMIT COUNTY — Although this cold and snowy winter has spelled good short-term news for Dillon Reservoir and the rest of Denver Water’s supply, global warming is keeping the Front Range utility modest.

Climate change is real, and it’s here, so Denver Water recently tried to determine how increasing temperatures would affect stream flows and water supplies, said Marc Waage, manager of water resource planning.

Waage said Denver Water considered two different scenarios:

• 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.

Up to now, planners have based their planning on past streamflows, for example using the worst recorded droughts as a baseline.

“We need to go beyond that,” Waage said, explaining that global warming could result in more severe droughts. Denver Water’s long-range planning effort is trying to factor those unknowns into the equation.

The city utility has also joined in a regional effort with other municipalities from Fort Collins to Colorado Springs.

“We hope it will show the value of regional planning,” Waage said. The joint studies will encompass the three major watersheds — the South Platte, the Upper Colorado and the Upper Arkansas — that provide supplies to Front Range municipalities.

Bob Berwyn can be reached at (970) 331-5996, or at bberwyn@summitdaily.com.