Winter precipitation versus snow survey for forecasting on Skagit River, Washington

A discussion of a paper on the monthly flow forecast of the Skagit River at Diablo Dam is presented. The monthly precipitation on 1100 sq. mi. of drainage area and the monthly mean inflow into the reservoir over a 25 year period were compared. It is shown that 68.7 percent of the rainfall enters the reservoir in an average year. Also half of the rainfall occurs from October thru December; the runoff is less than 18% during the same period. The difference forms a surplus to be drawn upon during the remaining 9 months. Each year the percentage of normal precipitation for Oct.-March is near to the percentage of normal inflow from April-Sept. The flow of the river for each particular period can be predicted within a mean average of 5% from the 25 year record by summing up the maximum and minimum figures for each month. The value derived by using the snow survey seems questionable since the prediction of increase runoff from snow-reservoir conditions is affected by temperature. Discussion by H.H. Boardman.