Physiographic controls on pre-event hydrological states and hydrological response to extreme precipitation in the Alzette River Basin, Luxembourg

The hydrological regime of rivers in Luxembourg (Central Western Europe) is characterised by summer low flows and winter high flows. In winter, large-scale floods are typically triggered by long-lasting sequences of precipitation events, related to westerly atmospheric fluxes that carry wet and temperate air masses from the Atlantic Ocean. In recent years, several flash flood events have been observed in Luxembourg. While being a common feature of Mediterranean river basins, this type of flooding events is uncommon at higher latitudes. The design of the hydro-meteorological monitoring and forecasting systems operated in Luxembourg is not adapted to this type of extreme events and there is a pressing need for a better mechanistic understanding of flash flood triggering mechanisms.

Here, we explore two lines of research – focusing on (i) the spatio-temporal variability of flash flood generation across a set of 41 nested catchments covering a wide range of physiographic settings (with mixed land use, soil types and bedrock geology) and (ii) the responsivity (resistance) and elasticity (resilience) of these catchments to global change.

Our area of interest is the Sûre River basin (4,240 km²), characterised by a homogenous climate (temperate oceanic), as well as various bedrock (e.g. sandstone, marls, shale) and land use (e.g. forests, grassland, crops, urban areas) types. Based on 8 years’ worth of daily hydrological data (precipitation, discharge and potential evapotranspiration) we computed the increments of the water balance to determine the maximum storage capacity and pre-event wetness state (expressed as storage deficit). Based on the relationship between storage deficit and discharge we first estimated total storage at nearly zero flow conditions. Second, we compared event runoff ratios (Q/P) to pre-hydrological states (as expressed to storage deficit prior to a rainfall-runoff event) in order to assess each catchment’s sensitivity to antecedent wetness conditions. Third, we assessed the responsivity (resistance) and elasticity (resilience) to climate variations – as expressed through the PET/P and AET/P deviations from the Budyko curve – for each individual catchment. Finally, we investigated potential physiographic controls on catchment responsivity and elasticity across our set of 41 nested catchments.