Flood probability analysis for un-gauged watersheds by means of a simple distributed hydrologic model

Abstract A methodology is proposed for the inference, at the regional and local scales, of flood magnitude and associated probability. Once properly set-up, this methodology is able to provide flood frequencies distributions at gauged and un-gauged river sections pertaining to the same homogeneous region, using information extracted from rainfall observations. A proper flood frequency distribution can be therefore predicted even in un-gauged watersheds, for which no discharge time series is available. In regions where objective considerations allow the assumption of probability distribution homogeneity, regional approaches are increasingly adopted as they present a higher reliability. The so-called “third level” in regional frequency analysis, that is the derivation of the local dimensional probability distribution from its regional non-dimensional counterpart is often a critical issue because of the high spatial variability of the position parameter, usually called “index flood”. While in gauged sites the time series average is often a good estimator for the index flood, in un-gauged sites as much information as possible about the site concerned should be taken into account. To solve this issue, the present work builds from the experience developed for regional rainfall and flood frequency analyses, and a hydrologic model, driven by a specific hyetograph, is used to bridge the gap between rainfall and flood frequencies distributions, identifying flood discharge magnitudes associated with given frequencies. Results obtained from the application in the Liguria region, Northern Italy, are reported, and validation is proposed in gauged sites against local flood frequency distributions, obtained either from local records or from the regional frequency distribution of non-dimensional annual discharge maxima, made dimensional with the local discharge record.