Regional flood–rainfall duration-frequency modeling at small ungaged sites

SummaryFlood frequency data for different durations of floods are required in many practical hydrologic applications. The estimation of flood frequency as an integrated function of return period and flood duration can be accomplished by flood-duration-frequency modeling. This study introduces a new approach to regional flood-duration-frequency modeling that is based on statistical properties of combined flood–rainfall events. The approach integrates flood-duration-frequency (QDF) and rainfall depth-duration-frequency (DDF) models into one regional flood–rainfall duration-frequency model (QDDF). The proposed model has only one local parameter, which accounts for site-specific physiographic characteristics. Regional parameters of the model are determined from statistical properties of regional rainfall depth-duration-frequency curves. The main advantage of the proposed approach is that it relies on rainfall data, which are spatially and temporally more abundant than streamflow data, and usually also available in hydrologically ungaged areas. The regional QDDF model was applied to a set of small catchments from a hydro-climatologically homogeneous region in south-western Ontario, Canada. The performance of the model was compared to the performance of the conventional regional converging QDF model by means of a jack-knife procedure. The results showed that the proposed approach significantly outperformed the converging QDF model, leading to quantile estimates with three-times lower average BIAS and RMSE. The proposed QDDF model seems to be a promising alternative for the regional QDF modeling of floods in the study area.