About 60 percent of the water used in California comes from Sierra Nevada snowmelt. During the winter, intrepid snow surveyors ski and helicopter in to select mountain sites to measure the snowpack’s depth with steel tubes shoved into the snow. Those measurements, taken monthly, help water managers estimate the amount of water held in the snowpack and allow them to allocate the state’s most precious resource.  Now, the Sierra Nevada is going high tech. Wireless sensors developed by Steven Glaser, professor of civil and environmental engineering, are being tested in an ambitious pilot project directed by hydrologist Roger Bales at the UC Merced Sierra Nevada Research Institute.