A Method for the Predicting of ϕ-Index

In Hydrologic studies it is often required to predict the runoff from a given storm precipitation over a basin. The first step in the prediction of runoff hydro-graph, is the estimation of precipitation losses or abstractions. On the basis of studies made, mostly on experimental plots, methods Imve been proposed for determining loss rate curves. However, for practical purposes and especially in cases involving large basins, one still has to resort to the use of an average abstraction rate as defined by an index like the popidarly known ϕ-index. While ϕ-index can be easily determined for storms for which runoff data are available, well defined procedures have not been developed for determining ϕ-index for a storm for which the runoff hydrograph is to be predicted. In this paper, a method for the estimation of ϕ-index for a given storm over a basin is presented. The procedure can be used where adequate rainfall-runoff data are available for past storms.