Computing Run-Off from Rainfall and Other Physical Data

The paper points out the necessity of basing conclusions with respect to stream flow on extended physical data. As most stream-flow observations available for the engineer's use extend over a comparatively few years, there is need for a method of computing run-off from other physical data for the purpose of extending and supplementing short-term stream-flow records. Inasmuch as run-off or stream flow consists of the residual rainfall after all losses have been deducted, the paper treats first of the method for computing these losses. The loss of rainfall through evaporation from water surfaces and from snow and ice is first discussed. The factors modifying the rate of evaporation and their relative importance are considered, and curves are presented, based on observations and checked by an evaporation formula, for the computation of these losses for any given water-shed, from temperature records, and the area and character of the bodies of water found on the water-shed.