Increased plant growth projected to use more water
FRISCO — Forests and brush moving up mountainsides as the climate warms could take a big gulp from streams and rivers, potentially cutting runoff by as much as 25 percent by the end of the century. Warmer temperatures will accelerate plant growth, triggering more water absorption and evaporation, according to researchers with UC Irvine and UC Merced.
“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 UCI professor of Earth system science Michael L. Goulden. “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 losses from forest evaporation.”
According to the researchers, runoff from mountain ranges is vulnerable to temperature hikes that lengthen growing seasons and result in more vegetation growth at high elevations, according to the study, to be published this week in the Proceedings of the National Academy of Sciences.
Snow-dominated mountain forests that are currently dormant in winter with cold temperatures have lower vegetative density and less evapotranspiration than downslope forests in the snow-rain transition zone, which have year-round growing seasons. Evapotranspiration is the combination of water evaporation from land and the loss of water through plant-leaf transpiration.
Goulden and UC Merced’s Roger C. Bales investigated the potential influence of a warming climate on evapotranspiration in the Kings River Basin in California’s Sierra Nevada by gauging water vapor emission rates and combined those measurements with remote sensing imagery to determine relationships among elevation, climate and evapotranspiration.
According to the data, freshwater mountain runoff is highly sensitive to expanded vegetation growth.
“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. 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,” Goulden said.
“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 study concluded that greater vegetation density at higher elevations in the Kings basin with the 4.1 degrees Celsius warming projected by climate models for 2100 could boost basin evapotranspiration by as much as 28 percent, with a corresponding 26 percent decrease in river flow.
Further, the relationships among evapotranspiration, temperature and vegetation density were similar across a broader area of the Sierra Nevada, suggesting that the impact of climate change on evapotranspiration and freshwater availability could be widespread.