A Method for integrating snow survey and precipitation data

The analysis presented in this paper indicates that, in the Pacific Northwest, a combination of snow water equivalent and winter precipitation, coupled with fall and spring precipitation indexes, appears to yield better forecasts than with either index alone. The proportion of snow water equivalent and winter precipitation in each forecast relation varies with the basin involved. The proportion appears too range form 70-90 percent snow water equivalent plus 10 to 25 percent winter precipitation in practically pure snow-fed streams to 25 percent snow water equivalent plus 75 percent winter precipitation in predominantly rain-fed streams.The weighting methods retain the simplicity of the ‘index’ method of water supply forecasting while permitting easy comparison and combination of the two winter indexes. Each stream must be studied in detail in order to determine the proportion of snow water equivalent and winter precipitation. ‘Locking in’ all variables except the one being tested aids in this determination, particularly when analysis time is reduced to a minimum by computer techniques.