Using damage reports to assess different versions of a hydrological early warning system

Abstract. In Europe, flash floods affect mainly the Mediterranean and mountainous regions, even if other regions also occasionally suffer from them. The catchments involved are usually small and ungauged, with short time of concentration. Forecasting this type of event remains difficult using hydrological models, and assessing the models is even more problematic. Typically, assessment is limited to gauged catchments that have relatively different geomorphological characteristics. The aim of this article is to present a method for assessing the models on real ungauged catchments through the use of damage reports and a multi-threshold approach, with assessment criteria that are based on a contingency table of the Critical Success Index type. The main conclusion, as demonstrated by Irstea's "Adaptation d'Information géographique pour l'Alerte en crue" for "Geographic information adaptation for flood warning" (AIGA) flood forecasting system and by the new version of AIGA for high-altitude catchments, is that while assessing hydrological models on gauged catchments is necessary, it is never sufficient and must be supplemented by assessments on ungauged catchments. This underlines the utility of building flood damage databases that are as exhaustive as possible. Such databases can be a valuable addition to more standard, often limited sources of data, especially for mountainous regions.