Surveyors will ski into the Sierra Nevada mountains Tuesday to perform what has become an anxiously watched rite of spring in drought-stricken California: measuring snow to determine how much water will flow to the state this year.

A critical measure of a precious resource, April's survey will influence whether the state's water officials declare that the drought is easing or that it persists. At stake is the fate of summer water deliveries to farms and cities. (Related: "Could California's Drought Last 200 Years?")

Trailed by news media, surveyors will traverse a granite ridge on Lake Tahoe's 6,800-foot-high (2,073-meter-high) Echo Summit—dense with fragrant pine, fir, and cedar—then drive about ten aluminum tubes into the snow to measure depth. They weigh the samples