Trends of warmer winter temperature in the Sierra Nevada are predicted to reduce local snowpack — western Nevada County’s most important source of water. In the future, precipitation increasingly will fall as rain instead of snow, scientists forecast. The impact of that shift on local water supply is dramatic. Nevada Irrigation District’s system for collecting water for local use — like other systems along the Sierra Nevada — was built on the assumption that wintertime precipitation would come as snow and stay in the mountains until the spring melt.