Comment on hess-2022-26

Abstract

With flash flood events having been repeatedly observed in Central and Western Europe in recent years, there is a growing interest in how catchment physiographic properties and hydrological conditions are eventually controlling rapid and concentrated hydrological responses. Here, we focus on a set of two nested catchments in Luxembourg (Europe) that have been exposed in 2016 and 2018 to flash flood events. While being of similar size (~30 km2) and having analogous hydrological distance distributions, their geological bedrock and landscape features are notably different. The upper catchment (KOE) is dominated by marly bedrock (Km3) and moderately steep Luxembourg sandstone outcrops (Li2). The lower catchment has its drainage network deeply cut into the Luxembourg sandstone, with half of it being covered by marly plateaus (Li3) featuring heavy clay soil. Based on data generated from a dedicated hydro-meteorological monitoring network, we calculated for 23 rainfall-runoff events observed between August 2019 and July 2020 the corresponding net rainfall transfer time distributions (TTDs) from the hillslopes to the catchment outlet. We then compared the TTD properties and related them to the catchment’s hydrological state and rainfall properties. We observed a seasonality in TTDs for both catchments, albeit controlled by different factors. In the KOE catchment, we found the water transfer time to be essentially driven by onset and cessation of hydrological connectivity on the flat marly terrain – the latter operating like a variable contributing area in terms of deep soil storage dynamics (except for one summer event). The HM section exhibits contrasted TTDs throughout the year, suggesting threshold dependent hydrological processes. More specifically, particularly quick runoff transfers seem to dominate under dry conditions. Correlation analyses compared to the literature on runoff generation on the one hand and our descriptive knowledge of the catchments on the other hand suggest multiple causes for the triggering of these rapid flows. The fractured marly plateaus, but also the hydrophobic forest litter forming during dry conditions, stand as our main hypotheses in this respect. Moreover, the absence of a riparian zone, preventing any dampening of (observed) abrupt and massive flows during extreme precipitation events, seems as well to be a key feature of the rapid runoff transfer. For improving our understanding and forecasting capabilities in Luxembourg (and more broadly in the nearby regions of Germany, Belgium and France with similar physiographic and climate conditions), we recommend further studies focusing on catchments with fractured bedrock and limited riparian zones. Special attention may equally be given to the hypothesized responses of forest litter and marly soils to heavy precipitation events occurring after extended dry spells.