Monitoring Monthly Snowmelt Runoff in the Sierra Nevada to Detect Climate Change

Substantial changes in temperate zone mountain snowmelt water supply are projected to occur as a result of global warming. Water resources managers already collect extensive data to use in forecasting and in operations of water projects. If they are interested in climate change, they should look carefully at their long term records of operating projects to see if signs of change are apparent and what the rates of change are. The author will present a couple of samples from northern California of the long term history of the fraction of mountain water year runoff occurring during the April through July period of snowmelt. The charts show a small declining trend during the past 50 years, but a decrease in the rate of change during the last 15 years. Reconstruction of natural flows will be briefly discussed as well as why the change in runoff patterns is so important to the large water projects in California. A related potential effect is an increase in the size of flood events because of more watershed area contributing rain runoff during winter storms, possibly augmented by higher storm rainfall intensity. A 100-year sample of the flood record on a major Sierra river will be used to illustrate this problem.