Every year for almost half a century, California snow surveyor Pat Armstrong has trekked the rugged Sierra Nevada with three simple tools: a snow core tube, a scale and a notebook. For as long as he can remember, state water officials have relied on the accuracy of those tools to deliver crucial data on the size of the Sierra snowpack and its ability to sustain a growing population. “It hasn’t changed in a hundred years,” Armstrong said of the survey. But there is a growing belief that this low-tech process alone is becoming too unreliable to accurately manage California’s water needs.