What winter brings to the Sierra Nevada determines, more than anything, how much water can be tapped in summer by California’s communities and businesses, including the state’s $35 billion agricultural industry. About 60 percent of the water used in California comes from melting Sierra snow.

Water managers have always faced a difficult task trying to allocate the state’s most precious resource. They need to know how much water to store in reservoirs and how much to send downstream, and when. But estimates of the amount of Sierra water the reservoirs will receive sometimes are off by as much as 20 to 30 percent.  At rates of $100-$600 per acre-foot delivered to metropolitan areas, a 20 percent error could translate into a more than $150 million shortage or excess of water needed downstream.

New models of climate change predict an increase in the proportion of water that will fall on the Sierra as rain instead of snow, further limiting the ability to rely on the snowpack as a kind of cold-season water storage facility. This could place new pressures not only on human use but also on wildlife and overall ecological stability.  

Historically, the amount of water held in the Sierra winter snowpack has been estimated by intrepid snow surveyors who ski and helicopter in to selected mountain sites to measure snow depth monthly with steel tubes shoved into the snow.

The obvious solution seems to be wiring up the Sierra with sensors, but this has its own built-in limits: The physical constraints of laying wires throughout remote regions would still limit coverage, not to mention the unavoidable hazards of bears plowing through the wires and rodents chewing them up.

The solution lies in matching broad-coverage satellite data with more detailed measurements from an extensive wireless monitoring grid on the ground. Specially designed wireless sensors developed at UC Berkeley now are being deployed and tested in an ambitiouspilot project directed by researchers at the UC Merced Sierra Nevada Research Institute.

The Berkeley-Merced collaboration uses matchbook-sized sensors with wireless communicating micro-processors known as motes to measure soil moisture, snow depth and other features critical to predicting spring and summer water availability. The system is designed to transmit the real-time measurements to researchers and water managers.

The prototype grid is being established on a 2.5-square kilometer site in the Sierra Nevada, which itself is a portion of an even more ambitious ecological research project called the Southern Sierra Critical Zone Observatory, or CZO, an National Science Foundation-supported mountain research site located about an hour east of Fresno. (See video on home page.)

Such a grid of independent but integrated sensors not only could improve estimates of water availability, but also help scientists understand large-scale processes that otherwise would be intractable. Incoming solar radiation, snow depth, soil temperature, atmospheric gases, plant respiration and moisture at different depths — the wireless grid can measure all these ecological features.  

With this mother lode of data, researchers hope to dissect such complex processes as how landscape variations affect the way soil moisture, evapotranspiration and stream flow respond to snowmelt and rainfall. The research is led by hydrologist Roger Bales, professor of engineering at UC Mercedanddirector of the Sierra Nevada Research Institute. Bales (in photo above) is also the principal investigator on the CZO, which involves collaborators from five UC campuses.

“What we are deploying right now in this small watershed lays the foundation for new information systems to greatly improve ecological measurement and hydrologic forecasting,” Bales says. “At the same time, it is advancing sensor technology and provides insights about basic Earth-atmosphere interactions.”