Sierra Nevada Freshwater Runoff May Drop by 26 Percent from Climate Change

It turns out that freshwater may be in short supply for Sierra Nevada in the future. Scientists have found that freshwater runoff in the region could decrease by as much as 26 percent by 2100 due to climate warming on higher slopes.

As temperatures increase, plants are able to grow at higher elevations. This could potentially trigger more water absorption and evaporation, which would mean there would be runoff declines. This could be a huge problem for farms and residences in the region that rely on this runoff.

"Scientists have recognized for a while that something like this was possible, but no one had been able to quantify whether it could be a big effect," said Michael Goulden, one of the researchers, in a news release. "It's clear that this could be a big effect of climate warming and that water managers need to recognize and plan for the possibility of increased water loses from forest evaporation."

The researchers calculated the loss of water runoff by gauging water vapor emission rates and combining the measurements with remote sensing imagery in order to determine relationships among elevation, climate and evapotranspiration. They found that freshwater mountain runoff is highly sensitive to expanded vegetation growth. In fact, the 4.1-degree Celsius rise in temperatures projected for 2100 could boost basin evapotranspiration by as much as 28 percent, causing a 26 percent decrease in river flow.

"Most people have heard about the giant forests around Yosemite and Sequoia national parks, but these areas have not been a focus of this type of research," said Goulden. "Understanding of Sierran hydrology has improved recently with the National Science Foundation's Critical Zone Observatory, and data collected there allowed us to look at the problem from several perspectives. All of our analyses pointed in the same direction: An upslope expansion of forest with warming would cause a large increase in evaporative water loss and lead to reduced water availability."

The findings are published in the Proceedings of the National Academy of Sciences.