Why the Sierra Nevada Mountains Are Drying Up

Researchers estimate a 26 percent drop in runoff from one of California's key freshwater sources.

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Sep 2 2014, 9:30pm

Image: approaching Mount Whitney/Cullen328

California's Kings River provides water to an area populated by some 750,000 people, generates $3 billion annually in revenues from hydroelectric power sales, and supplies much of the water needed to irrigate the state's Central Valley agricultural region. And if current climate trends continue, the river is going to keep drying up.

The river's source lies well about 12,000 feet in Kings Canyon National Park, making it something of a snow runoff pipeline. The higher one travels up the river's course, the higher the precipitation levels found. Water levels found in the river are a direct function of what's known as evapotranspiration (ET), which is the sum of water lost through regular evaporation and water lost via plants sucking it up for their own purposes.

So, actual water runoff has a lot to do with vegetation. More plants means less water for the river. This works out for rivers like the Kings because as they climb upwards in elevation toward their sources and temperatures grow colder, vegetation tends to decrease, and we're left with a river supplying adequate water. 

Climate change has a say in this distribution of plant-life, however, and new models from researchers at UC Irvine, published this week in the Proceedings of the National Academy of Sciences, suggest that as a warming climate continues to foster increasing densities of biomass at high elevations, we can expect dramatic decreases in river flows. This is a process already well underway.

The Kings River has a kind of sweet spot around 2,300 meters (7,545 feet), above which is a "cold-limited zone" responsible for much of the river's runoff potential. "Climate projections for 2085 – 2100 indicate as much as 4.1 °C warming in California's Sierra Nevada," the paper notes, "which would expand high rates of ET 700 meters upslope if vegetation maintains its current correlation with temperature."

If the mountains lose 700 meters of this crucial cold-limited zone, we've then lost 26 percent of desperately needed water, the result of a 28 percent rise in overall ET effects.

This implies a potential widespread reduction in water supply with warming.

The UC Irvine researchers, led by engineering professor Roger C Bales, gathered their data with help from a set of four atmospheric monitoring towers (eddy covariance towers, properly) along with remote sensing data. From there it was possible to model the correlations between climate and ET at different altitudes. 

Clearly, ET effects decreased as the monitoring towers increased in elevation and vegetation decreased as a function of colder high-altitude temps; the authors concluded that the Kings River flow is thus "highly sensitive" to temperature variation and the corresponding vegetation expansion.

This isn't so much a prediction of things to come as it is a description of what's already happening. "Widespread increases in subalpine tree growth, tree-line altitude, and species distribution with elevation have been reported with recent climate trends in California and elsewhere," Bales and team write, "implying that rapid vegetation shifts are possible."

Finally, it should be noted that this particular 26 percent loss is merely an example rather than a summation of climate runoff effects for the state or even the Sierra. "We found a consistent relationship between watershed ET and temperature across the Sierra Nevada," Bales and team writes. "This consistency implies a potential widespread reduction in water supply with warming, with important implications for California's economy and environment."

California is, as the current study notes, a state with 20 million people largely dependent on runoff for their basic water supplies, to say little of agriculture and hydropower. Given that California is already writing water checks it can't cash, while its general supply situation has farmers looking to pseudoscience and "water witches" for help, it's all the more clear that even the smallest decline in river flows will mean very large-scale effects.