“Fresno County is the No. 1 agricultural county in the nation, but we also happen to be situated climatically in the middle of a desert,” he said. “It really is the Sierra Nevada snowpack that makes this desert bloom.”

It was in 1908 that James E. Church, a classics professor at the University of Nevada, Reno, developed the existing method of forecasting water runoff from the depth and water content of snow. To settle a dispute over water rights in nearby Lake Tahoe, he was able to predict the water from the snowpack on Mount Rose by using a hollow tube that he called the Mount Rose snow sampler. Government agencies adopted his technique and, in 1929, California passed a law mandating that state officials measure the Sierra Nevada snowpack and issue forecasts of the water supply.

“You can almost say it made the development of the West possible,” Mr. Gehrke said. “Prior to that, they really didn’t even have tools to use to hope to predict how much runoff they would have.”

A civil engineer, Mr. Gehrke, 65, grew up in Missouri, where snow was considered more of “a nuisance” than anything else. “We would get occasional snowstorms, and our big hope was that the schools would close, though they rarely did,” he recalled.

He joined the Department of Water Resources in 1987, which coincided with the start of a six-year drought. The drought brought more resources and public attention to the snow surveys. The monthly surveys, which run from January through April, became media events starring Mr. Gehrke.

“He’s sort of iconic,” said John Laird, secretary for the state’s Natural Resources Agency, who had come to participate in the snow survey. “He’s the face of snow measurement.”

The water resources department issues forecasts after studying the data collected by Mr. Gehrke as well as 40 cooperative agencies at some 250 locations. His team includes 10 snow surveyors, some of whom travel with their measuring tubes to remote spots by hiking, snowmobile or even helicopter. Sensors located throughout the Sierra Nevada provide real-time data but are not considered as accurate as the manual surveys.