Abstract. Flash floods have dramatic economic and social consequences, and efficient adaptation policies are required to reduce their impacts, especially in a context of global change. Developing more efficient flash flood forecasting systems can largely contribute to these adaptation requirements. The aim of this study was to assess the ability of a new seamless short range (0–6 h) ensemble quantitative precipitation forecast (QPF) product, called PIAF-EPS and recently developed by Météo-France, to predict flash floods when used as input of an operational hydrological forecasting chain. For this purpose, eight flash flood events that occurred in the French Mediterranean region between 2019 and 2021 were reanalysed, using a similar hydrological modeling chain to the one implemented in the French “Vigicrues-Flash” operational flash flood monitoring system. The hydrological forecasts obtained from PIAF-EPS were compared to the forecasts obtained with different deterministic QPFs from which PIAF-EPS is directly derived (i.e. the AROME-NWC numerical weather prediction model, and the deterministic PIAF product). The verification method applied in this work uses scores calculated on contingency tables, and combines the forecasts issued on each 1 km2 pixel of the territory. This offers a detailed view of the forecast performances, covering the whole river network and including the small ungauged rivers. The results confirm the added value of the ensemble PIAF-EPS approach for flash flood forecasting, in comparison to the different deterministic scenarios considered.
<p>Flash floods have dramatic economic, natural and social consequences, and efficient adaptation policies are required to reduce these impacts, especially in a context of global warming. This is why it remains essential to develop more efficient flash flood forecasting systems. This study was carried out in order to assess the ability of a new seamless short range ensemble rainfall forecast product, called PIAF-EPS and recently developed by Meteo France, to predict flash floods when it is used as input in an operational hydrological forecasting chain.</p> <p>For this purpose, eight flash flood events that occurred in the French Mediterranean region between 2019 and 2021 were reproduced, using a similar forecasting chain as the one implemented in the French &#8220;Vigicrues-Flash&#8221; operational flash flood monitoring system. The hydrological forecasts obtained from PIAF-EPS were compared to the hydrological simulations obtained from the radar observations, and to three deterministic forecasts using varied scenarios (future constant rain, deterministic PIAF, and a numerical nowcasting system called AROME-NWC).</p> <p>The verification method applied in this work uses rank diagrams and scores calculated on contingency tables, in an original way. The verification process has been conducted on each 1km&#178; pixel of the territory.</p> <p>The results illustrate the added value of the ensemble approach for flash flood forecasting, and the benefits of the use of a &#8220;seamless&#8221; product combining radar observations and numerical nowcasting. &#160;&#160;</p>
Abstract. The allocation of points in a river network to pixels of a coarse-resolution hydrological modelling grid is a wellknown issue, especially for hydrologists who use measurements at gauging stations to calibrate and validate distributed hydrological models. To address this issue, the traditional approach involves examining grid cells surrounding the considered river point and selecting the best candidate, based on distance and upstream drainage area as decision criteria. However, recent studies have suggested that focusing on basin boundaries rather than basin areas could prevent many allocation errors, even though the performance gain is rarely assessed. This paper compares different allocation methods and examines their relative performance. Three methods representing various families of methods have been designed: area-based, topology-based and contour-based methods. These methods are implemented to allocate 2580 river points to a 1 km hydrological modelling grid. These points are distributed along the entire hydrographic network of the French southeastern Mediterranean region, covering upstream drainage areas ranging from 5 km2 to 3000 km2. The results indicate that the differences between the methods can be significant, especially for small upstream catchments areas.
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