Design flood estimation in ungauged catchments : quantile regression technique and probabilistic rational method compared

Estimation of design floods in ungauged catchments is frequently required in hydrological practice and is of great economic significance. The most commonly adopted methods for this task include the Probabilistic Rational Method, the U.S. Soil Conservation Service Method, the Index Flood Method and the U. S. Geological Survey Quantile Regression Technique. The Probabilistic Rational Method has been recommended in the Australian Rainfall and Runoff for general use in south-east Australia (I. E. Aust., 1997). The central component of this technique is a dimensionless runoff coefficient which in the ARR is assumed to vary smoothly over geographical space, an assumption that may not be satisfied in many cases because two nearby catchments though are likely to share similar climatic characteristics but may exhibit quite different physical characteristics.