Precipitation-runoff relationships as a basis for water-supply forecasting

The early history of snow surveying is reviewed. The surface runoff from a watershed equals the total precipitation minus losses through evaporation, transpiration, leakage, and ground water storage. Factors affecting runoff yield per precipitation unit are time of occurrence and form of precipitation, and temperature. Runoff can be measured by standard methods within 5 to 10% accuracy. Precipitation measurement, however, is difficult and expensive. Water losses, depending on many factors, cannot be quantitatively measured. The best basis yet developed for forecasting is by measuring the accumulated snow cover at representative points (always the same points) and use these data as an index to seasonal runoff. Results can be adjusted for abnormal spring precipitation or temperatures. Procedures for these relationships are shown graphically. Examples of data are tabulated and discussed.Discussion by Ray K. Linsley, Merrill Bernard are included.