Surveyors will crunch across the deepest mountain snowdrifts California has seen in years on Thursday to take the first snow measurement of the spring, a time when the snow begins to melt and flow downhill. The snowpack stretches along 400 miles of the Sierra Nevada, creating an icy reservoir that provides roughly one-third of irrigation and drinking water to the nation’s most populous state during hot, dry months of the year. The water content of the snowpack measured at 164 percent of normal Wednesday, according to the state’s electronic monitors throughout the mountain range.