Coupling geomorphology and hydrological modelling to characterize the spatio-temporal variability of groundwater-surface water interactions in karst rivers

Flood hazard and groundwater resource management in karst catchments require a better understanding of the groundwater/surface water interactions. Due to their high infiltration capacity, karst outcrops limit runoff production on hillslopes, but promote in the same time lateral gains to rivers due to fast transfers underground. This topic is a great challenge, as 30% of Europe is cover by such karst areas.

The aim of this communication is to present recent developments on the characterization of karst-river interactions through two main approaches: a spatial approach aiming at localizing karst areas promoting surface flows, and a temporal approach aiming at modelling lateral flows from karst units in rivers.

The first spatial approach is based on the GIS index IDPR (Index of Development and Persistency of River networks, developed by BRGM©), quantifying the hydrological connectivity to the hydrographic network. From the standard version of the IDPR over France (25 m resolution), we have compare IDPR calculations differentiating intermittent and perennial reaches of rivers in order to detect infiltrations zones that contributive temporary to rivers, as many karst units. Results show that the presence of large karst units promote infiltration, but also temporary runoff that illustrate fast groundwater flows to rivers during floods.

The second approach is a modelling framework based on the inverse problem for the diffusive wave model, to simulate lateral flow during floods on a river reach between two stations. Knowing the upstream and downstream hydrographs, the lateral one is simulating, given informations on the hydrological processes involved in the intermediate catchment (losses, gains, or both processes during flood events). A new development has been tested, by adding continuous solute data (electrical conductivity) in order to track the origin of the water during floods. Applying such approach on river reaches crossing karst areas is a new way to quantify river losses, river gains from surface runoff or groundwaters, characterizing localized recharge and aquifer drainage to rivers, respectively.

We propose to illustrate these two approaches through several case studies in France, where a better characterization of groundwater-surface water interactions and flood risk management are critical issues.