What do water supply forecasts represent?
Water in the West almost never flows unobstructed from where it fell and melted (since it is often snow)
to where it will rest in an ocean or lake. Rather it is regulated by being rerouted through tunnels
and ditches, diverted onto croplands and into treatment plants, and temporarily impounded in reservoirs
many times along the way.
Water supply forecasts issued by the River Forecast Center are primarily unregluated forecasts, meaning
they do not account for the water that may be diverted into or out of the stream, or stored in reservoirs
upstream during the runoff season. Unregulated forecasts do not represent what will be observed in
the stream unless no upstream regulation takes place.
The relationship between observable hydrologic parameters (e.g., precipitation, snowpack, terrain, etc.)
and unregluated (or unimpared) runoff is predictable and well-defined; the same cannot be said for regulated
(or managed) runoff.
Exceptions: Where upstream water management displays a consistent behavior from year
to year, or where upstream reservoir future operations are known, regulated forecasts can be generated. However,
these forecasts are currenlty generated for only a couple of sites in the Great Basin and they are noted
in the water supply forecast publication.