How Climate Change Dries Up Mountain Streams

The western United States relies on mountain snow for its water supply. Water stored as snow in the mountains during winter replenishes groundwater and drives river runoff in spring, filling reservoirs for use later in summer. But how could a warming globe and a changing climate interrupt this process?

In a new study in Environmental Research Letters, a team of hydrologists that includes University of Utah professor Paul Brooks answers that question by simulating isolated climate change effects on Rocky Mountain stream systems, varying the type of precipitation (rain vs. snow) and the amount of energy (temperature) in the system. The answer, they found, depends less on how water enters the stream watershed, and more on how it leaves.

Hydrologists often construct water budgets to account for all the ways water enters and leaves a system. In the case of a mountain stream, water enters as precipitation but only a portion of this water leaves as streamflow. Much of this melt water enters soils. Here it can be used by plants or evaporate directly, with water loss from both processes combined called evapotranspiration. The water can also recharge groundwater and enter the stream later in the year. And it matters whether the precipitation falls as snow or as rain.

Climate change can affect mountain streams in two major ways: By raising the overall temperature, increasing evapotranspiration, and by shifting the precipitation from snow to rain. Both impacts could significantly alter the amount of water in a stream watershed and the amount that reaches cities downstream.

So why try to separate the influence of the two factors? "As the climate becomes increasingly more variable, we need to provide water resource managers with specific guidance on how individual warm or wet years, which may not coincide, will influence water supply," said Brooks.

Jerry and Pat Donaho