Regional analysis of PROQ transforms for flood frequency estimation based on GRADEX principles

ABSTRACT The PROQ transform which converts rainfall P to runoff RO, then factored to produce peak discharge Q, can be used for regional design flood estimation. SPEED, a French method based on GRADEX principles is a PROQ transform. 1 in 100 annual exceedance probability discharges was estimated using SPEED for 40 Australian non-urban catchments. Inputs are catchment area and areal design rainfall depth. The SPEED transform involves three constant parameters: scaling factor a, non-linearity b and retention L. SPEED was found to be simple and effective (Nash-Sutcliffe E = 0.82). A local version of SPEED produced a similar performance (E = 0.74). A general form of PROQ was developed to explain similarities with other models such as the Probabilistic Rational Method and to develop the local SPEED transform. Gauge sites can be grouped based on a and L ‘bins’, using site-based mean values of these parameters. A PROQ model using two a bins and two L bins significantly enhanced predictive accuracy (E = 0.92).