Contents volume 84, 1986

Abstract

The well-known rational method, where the subject criterion variable of peak flow rate is evaluated by using a synthetic storm pattern composed of identical return frequencies of storm pattern input, is shown to be an effective approximation to a considerably more complex probabilistic model. The rational method is shown to be of equivalence to a model of a highly discretized catchment with linear routing for channel flow approximation, and effective rainfalls in subareas which are linear with respect to a selected “loss” function's effective rainfall output. The use of a simple “loss” function which directly equates to the distribution of rainfall depth-duration statistics (such as a constant fraction of rainfall) is shown to provide a higher level of statistical significance in estimating T-year outputs for a hydrologic criterion variable than use of an arbitrary “loss” function. When peak flow rates are the criterion variable of interest, the unit hydrograph method can be simplified to the well-known rational method. This simpler procedure is used to develop computerized master plans of drainage for urbanized regions.