A model for updating streamflow forecasts based on aeral snow cover and a precipitation index

A simple graphical method for updating streamflow forecasts during the melt season is described. The procedure, developed from data on three small forested watersheds in the Fraser Experimental Forest, is based on: (1) aerial photographs of snow cover depletion acquired during 1964-68, and (2) a precipitation index derived from April 30 storage gage measurements adjusted for subsequent precipitation input during the snowmelt runoff season. Trial forecasts with 1969 data indicate the method gives acceptable results.The 1969 melt season was a good year to test the model, since precipitation during May and June was abnormally high. Successive adjustments of the precipitation indices reduced initial forecast errors from an average 20 percent to approximately 10 percent when the flow remaining was still about 80 percent of the seasonal total.