CALIBRATION PROCEDURES FOR RATIONAL AND USSCS DESIGN FLOOD METHODS

Flood estimation models are often used to predict design floods of a s elected average recurrence interval (ARI) using rainfall information o f the same ARI. That is, they are used for probabilistic purposes rath er than for forecasting during actual events. Design values of model p arameters need to be derived in a manner corresponding with the way th e models are applied in design. Design values of the main parameter of two simple flood estimation models, the rational method (runoff coeff icient) and the United States Soil Conservation Service (USSCS) method (curve number), were derived for 105 small agricultural catchments in and around south east Queensland, Australia. These were obtained by r eversing the way the models are used in design flood estimation, utili zing frequency analyses of rainfall and flood data, The derived values were considerably different from conventional handbook values; the la tter gave inaccurate estimates of design runoff and flood peaks. The v alues derived from observed data were only weakly related to catchment characteristics including land use and soil type, but were regionally consistent. ARI and the method of estimating design rainfall duration had greater influences on the values derived.