SNOWPACK TRENDS IN THE CENTRAL SIERRA NEVADA AFFECTING WATER SUPPLY FORECASTS IN THE EAST SLOPE SIERRA BASINS

Water supply forecasts in the Western US have long been based on historical statistical relations between the predominantly winter precipitation and subsequent spring and summer streamflow. The snowpack in the mountainous West, and specifically in the central Sierra Nevada has been measured since the early 1900s. Currently the USDA Natural Resources Conservation Service and the California State Cooperative Snow Survey program measure snow in the Sierra Nevada and keep long term records of the snowpack. This historic data are a good indicator of seasonal precipitation in the building of the snowpack in the region. A statistical analysis of the snowpack record at Donner Summit compared to Tahoe City precipitation reveals a shift in the relation that occurred in the late 1970s. This change is significant at the .999 level. Since the late 1970s, there has been a decrease in the amount of cumulative snowpack on April 1 in comparison to the seasonal precipitation of October through March between these two sites. This indicates that from 1980 to 2007 there has been more rain than snow compared to the previous 43 years. The change has been observed at other locations in the region. This change affects the hydrology of the region, and to adequately predict the subsequent snowmelt runoff, must be taken into account in statistical water supply forecasting models. The ability to forecast depends on the long term steady relation to continue, and forecast updating will now need to rely on the period of 1980 to present to best account for the current climatic regime.