Effects of multiscale rainfall variability on flood frequency: Comparative multisite analysis of dominant runoff processes

Abstract

We present results of a comparative modeling analysis of the effects of multiscale rainfall variability (within‐event, between‐event, seasonal, interannual, and interdecadal) on estimated flood frequency curves for three catchments located in Perth, Newcastle, and Darwin, Australia. The analysis is performed using the derived distribution approach by combining long‐term rainfall time series generated by a stochastic rainfall model with a continuous rainfall‐runoff flood model that is able to generate runoff variability over a multiplicity of timescales. Similarities and differences of the flood frequency curves (FFCs) in these rather diverse catchments are then interpreted on the basis of differences in the dominant runoff generation processes. In Newcastle, annual maximum flood peaks are caused by saturation excess overland flow over the entire range of annual exceedance probabilities (AEPs) or return periods. On the other hand, in Darwin, the shape of the FFC is determined strongly by seasonal climatic variability, which, in combination with deep soils, leads to a switch of dominant runoff mechanisms contributing to annual maximum flood peaks, from subsurface stormflow at high AEPs (low return periods) to saturation excess overland flow at low AEPs (high return periods). This leads to FFCs exhibiting a consistent break in slope in the Darwin catchment but not so in Newcastle. On the other hand, the FFCs in Perth are affected by both seasonality and long‐term climate variability and produce a variety of shapes depending on the relative strengths of these climatic controls. Because of the fact that in Perth and Darwin the shapes of the flood frequency curves depend on a possible switch of the dominant runoff generation mechanisms with increasing return period, uncertainty in hydrological model parameters relating to landscape properties contributes significantly to the uncertainty in the flood frequency curves. This uncertainty is much less pronounced in Newcastle because of the absence of such a switch of runoff generation mechanisms.