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.
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