Global warming: How rising alpine vegetation could hit California water supply

A warming-climate-induced march of alpine vegetation up a large river basin on the western slope of the Sierra Nevada could slash by one-fourth the annual flow of water the basin delivers to California’s thirsty Central Valley by the last two decades of this century, a new study says.

The results of the Kings River Basin study imply that the same type of risk holds for another 10 major river basins along the western Sierra, although to varying degrees, say the researchers involved in the study. It also could hold similar implications for other regions around the world that rely on alpine snows for much of their fresh water.

The results also imply that in a warming world, some forests may have to be managed as much to ensure adequate water supplies downstream as they will be to reduce the hazards of large wildfires.

Managing vegetation for stream flows already has been undertaken to some extent along rivers and streams in the Southwest, where invasive plants such as tamarisk took root along the banks, siphoning far more water than native species such as cottonwoods. Groups have been removing the tamarisk and other water-binging invasives in hopes of maintaining or restoring normal seasonal stream flows.

Based on the new Sierra study, such approaches may have to be applied on a much larger scale to alpine vegetation migrating up mountainsides, where it can intercept, take up, then release as water vapor runoff from melting snow, suggests Roger Bales, a researcher with the University of California at Merced's Sierra Nevada Research Institute.

The results highlight "the need to do more scientifically based vegetation management and link that to water," says Dr. Bales, one of two scientists involved in the study.

At the heart of the study rests a process known as evapotranspiration. Its the total amount of water plants extract from the ground and eventually release to the atmosphere from leaves or needles as water vapor, plus the amount of water that evaporates from soils and waterways. This combined loss of ground and surface water to the atmosphere in a drainage basin is compared with precipitation to derive stream flows.

Previous studies have largely relied on computer models to estimate the effect of changing vegetation on stream flows in the Sierra Nevada. One study, published in 2010, suggested that the average annual flow of water down the Kings River would decline by between 1 and 4 percent by the end of the century.

In the latest study, the researchers gathered data from instrument towers at different elevations in the upper Kings River Basin, as well as satellite data, which allowed them to estimate the rate of evapotranspiration at various altitudes up to about 12,000 feet. The goal was to measure the relationship between evapotranspiration, altitude, and temperature.

The team found that annual evapotranspiration was most intense at altitudes ranging from about 3,800 feet to 6,600 feet – implying that this zone accounts for the largest losses in water working its way from the snow pack on the summits. Evapotranspiration dropped off significantly above and below that band.

Evapotranspiration drops off at higher altitudes because vegetation becomes increasingly less dense, notes Michael Goulden, an ecologist at the University of California at Irvine whose research focuses on biological, chemical, and energy cycles in terrestrial ecosystems. Indeed, the much lower rate of evapotranspiration between 7,900 and 11,800 feet in the upper Kings River Basin – a band that accounts for about 50 percent of the watershed's area, is responsible for some 68 percent of the river's annual flow, the team estimates.

The researchers also found that above 7,900 feet, evapotranspiration was limited more by temperature than by the availability of surface water. One reason: Plants at those altitudes send their roots deep into crevices in bedrock, giving them a more constant source of water than surface water alone.

The team then used a climate model and the greenhouse-gas emissions scenarios from the latest set of climate reports from the Intergovernmental Panel on Climate Change to gauge the effects of warming in the lowest 13,000 feet of atmosphere over central California on plant migration up slope.

Assuming constant precipitation throughout the 21st century, the duo found that by 2085-2100, vegetation currently at 6,500 feet migrated another 650 feet up in altitude for the smallest projected temperature increase, about 1.3 degrees Celsius (about 2 degrees Fahrenheit). This led to a 10 percent increase in the basin's evapotranspiration. Under the highest projected temperature increase, 4.1 degrees C., plants hiked nearly 2,300 feet up, boosting evapotranspiration in the basin by 28 percent.

"The magnitude of the effect was a surprise," says Dr. Goulden, the lead author of a formal report of the results appearing in the current issue of the Proceedings of the National Academy of Sciences.

"I expected we'd see some effect," he says, but he didn't expect anything close to 28 percent.

Still, he cautions, that is likely to be a worst-case scenario. The Kings River Basin is atypical, because it reaches to higher altitudes and so is colder than other basins along the Sierra Nevada. The study also assumed that vegetation moves up slope rapidly – a process that is poorly understood today.

The results represent an initial attempt to use measurements to tease out the effects climate change could have on alpine vegetation's influence on water availability, Goulden says.

"The issue had kind of fallen through the cracks before, since it bridges hydrology and ecology," he writes in an e-mail. "Neither field had done much on it, at least from a quantitative angle.

He adds that he hopes the results "will increase the science interest on this possibility."