Transforms for Runoff and Sediment Transport

Transforms reduce the distributions of surface-runoff and sediment-transport variables to nearexponential distributions typical of rainfall duration and intensity. Two transform parameters are determined from field measurements using either a graphical method or the method of moments. The transforms were tested on both small experimental plots and very large watersheds for the distributions of rainfall depth, runoff volume and discharge, sediment concentration and sediment discharge, as well as concentration and transport of c!temicals from surface runoff. As expected from deterministic relationships, the inverse transform exponent b for point rainfall is close to unity, increases to about 1.5 for runoff discharge, and varies between 1.5 and 3 for sediment and chemical transport. Several examples show that the transforms are useful to determine exceedance probability, flow and sediment duration curves, expected sediment load, and sediment-rating curves for poorly correlated concentration and discharge measurements. RAINFALL CHARACTERISTICS in which the rainfall duration parameter Al = lit. is the reciprocal of the average storm duration t,. The rainstorm intensity i has also been shown to be exponentially distributed. The probability density function, p(i), is Rainfall characteristics are important because they generate surface runoff and transport of sediment and chemicals. Pointrainfall precipitation can be described as a random time series of discrete storm events, each event having finite duration and constant intensity. Several stochastic models have been develped since Todorovic (1968) and Todorovic and Woolhiser (1974). In these models, rainfall precipitation has been described as a Poisson arrival process of rainfall events of duration t, and average rainfall intensity i. The storm duration t, has been found to be exponentially distributed. Considering a period where the average rainfall duration t, remains constant, the PDF of the storm duration pet,) is given by