PHYSICAL INTERPRETATIONS OF REGIONAL VARIATIONS IN THE SCALING EXPONENTS OF FLOOD QUANTILES

The concepts of simple scaling and multiscaling provide a new theoreti cal framework for the study of spatial or regional flood frequency rel ations and their underlying physical generating mechanisms. In particu lar, the scaling exponents in the power law relationship between flood quantiles and drainage areas contain a 'basic signature of invariance ' regarding the spatial variability of floods, and therefore suggest d ifferent hypotheses regarding their physical generating mechanisms. If regional floods obey simple scaling, then the slopes do not vary with return periods. On the other hand, if regional floods obey multiscali ng, then the slopes vary with return periods in a systematic manner. T his premise is expanded here by investigating the empirical variations in the scaling exponents in three states of the USA: New York, New Me xico and Utah. Distinct variations are observed in the exponents among several regions within each state. These variations provide clear emp irical evidence for the presence of both simple scaling and multiscali ng in regional floods. They suggest that snowmelt-generated floods exh ibit simple scaling, whereas rainfall-generated floods exhibit multisc aling. Results from a simple rainfall-runoff experiment, along with th e current research on the spatial scaling structure of mesoscale rainf all, are used to give additional support to these physical hypotheses underlying two different scaling structures observed in floods. In add ition, the rainfall-runoff experiment suggests that the behaviour of t he flood exponents in small basins is determined by basin response rat her than precipitation input. This finding supports the existence of a critical drainage area, as has been reported for the Appalachia flood data in the USA, such that the spatial variability in floods in basin s larger than the critical size is determined by the precipitation inp ut, and in basins smaller than the critical size is determined by the basin response.