Impact of karst areas on runoff generation, lateral flow and interbasin groundwater flow at the storm-event timescale

Abstract. Karst development influences the hydrological response of catchments. However, such impact is poorly documented and even less quantified, especially over short space and time scales. The aim of this article is thus to define karst influence on the different hydrological processes driving runoff generation, including interbasin groundwater flow (IGF) for elementary catchments at the storm-event time scale. Three types of storm-event descriptors (characterising water balance, hydrograph shape and lateral exchanges) were calculated for the 20 most important storm events of 108 stations in three French regions (Cevennes Mountains, Jura Mountains and Normandy), representative of different karst settings. These descriptors were compared and analysed according to catchment geology (karst, non-karst, or mixed) in order to explore the specific impact of karst areas on water balance, hydrograph shape and lateral exchanges. A statistical approach showed that, despite the variations with study areas, karst promotes: i) Higher water infiltration from rivers during storm events; ii) Increased characteristic flood times and peak-flow attenuation; and iii) Lateral outflow. These influences are interpreted as mainly due to IGF loss that can be significant at the storm-event scale, representing around 50 % of discharge and 20 % of rainfall in the intermediate catchment. The spatial variability of such effects is also linked to contrasting lithology and karstification degree. Our work thus provides a generic framework for assessing karst impact on the hydrological response of catchments to storm events; moreover, it can analyse flood-event characteristics in various hydro-climatic settings, and can help testing the influence of other physiographic parameters on runoff generation.