Initial Loss for Flood Estimation and Forecasting

Estimation of a flood from a design storm is made difficult by the need to estimate losses from the storm. The amount of initial loss is usually fixed in some arbitrary fashion. It is shown that potential initial loss may be estimated on any day from a simple antecedent precipitation index (API) model. The model provides an index of watershed wetness. The evapotranspiration of moisture from the watershed is represented by a variable API recession factor which is related to monthly evaporation, temperature or a sine curve which approximates the variation of monthly temperature. Initial loss is related to this API. Median initial loss which, it is suggested, is required for design may be estimated given daily rainfall data and the API—initial loss relationship. For eastern New South Wales, Australia, median initial loss is shown to be related to watershed area and mean annual rainfall. For ungaged watersheds sufficient information is available for an API—initial loss relationship to be estimated.