Seasonal Water Supply Forecasting

Water supply forecasts predate flood forecasting efforts in most parts of the Western U.S. These forecasts are extremely important to water managers interested in providing water for agricultural, municipal, environmental, and commercial purposes. Winter precipitation stored in the mountain snowpack makes seasonal water supply forecasting possible. In many basins of the western U.S., as much as 70 percent of the annual streamflow arises from the melting of the seasonal snowpack each spring. The CNRFC uses two modeling approaches to prepare its seasonal runoff estimates.

Statistical models have been used to forecast spring runoff for many years and they remain the tool of choice for those involved in this process. Statistical models are simple to use and make direct use of monthly snow course, snow pillow, and precipitation data. Because they are optimized to predict the seasonal volume, they are among the most accurate and dependable. The statistical model resembles a multiple linear regression where the independent variables include monthly snow course, snow pillow, precipitation, and occasionally streamflow observations. The models are calibrated in advance using an average of 30 years of observed data. Principal components analysis is used to deal with correlation among the input variables. Cross validation errors are computed to find equations that forecast as well as fit the observed data.

The same process (ESP) used to generate the weekly snowmelt forecasts can be extended to estimate seasonal volumes. ESP forecasts for each watershed are developed and compared with the results from the statistical models. Often the approaches complement each other lending insight to the process. Small adjustments are typically made to ensure that the ESP (both weekly snowmelt and seasonal) and statistical models are consistent.

Seasonal volume forecasts are coordinated with other agencies, such as the NRCS and the CADWR. The exchange of information in this process improves the quality of forecasts and minimizes conflicting forecast information. Water supply forecasts are generated and published by the CNRFC on a monthly basis, January through May. In some years, a June forecast is provided. Water supply forecasts are placed on the CNRFC Home Page as well as printed and mailed out to non-online customers.