Importance of baseflow contribution in mountainous Mediterranean watersheds highlighted by geochemical and isotope tracers

Most of the Mediterranean basin coasts are bordered by high mountain ranges (Atlas e.g.). As a result, most of the coastal socio-economic activities are highly dependent on the availability of water from mountainous catchment areas. However, these resources are increasingly vulnerable to climate change, population growth and agricultural development. Given the seasonality of rainfall with high water deficit during summer, groundwater covers a large part of the water supply and appears to be also essential to maintain river flows as well as their ecological continuity. However, one of the most important knowledge gap remains in the characterization and quantification of the watershed contributors supplying river flow through time and space. And this is especially the case for groundwater and delayed subsurface flow. Therefore, the aim of our research consisted in characterizing the baseflow component, as the contribution coming from groundwater and delayed subsurface flow, over two full hydrological years for selected representative mountainous watersheds: The Tavignanu and Fium’Altu basins (Corsica, France). Due to its location in the western Mediterranean basin as well as its diversity in catchment morphologies and lithologies, Corsica is an excellent observatory of any mountainous hydrological processes. In this purpose, different promising tools scarcely used in the Mediterranean context are available to perform baseflow analysis:

- On the one hand, the non-tracer-based methods, including several technics ranging from an empirical to an analytical basis

- On the other hand, the tracer-based methods including the use of water stable isotopes and hydrogeochemical tracers in a mass balance procedure

It allowed to test and highlight the high potential of hydrogeochemical tools in the Mediterranean mountainous context in many ways:

- By correlating, calibrating and validating some of the non-tracer-based methods with monthly tracer data for a Mediterranean use

- By using the validated non-tracer-based methods to perform continuous baseflow separation on a daily basis in order to assess baseflow seasonal patterns and trends over the last twenty years on both catchments

Thus, we clearly highlighted that baseflow, over the years, constitute the main contributor to river flow during dry periods (with a mean Baseflow Index up to 93% for both catchments) and still remains as an important part during high flow periods (with a mean contribution of 67% for the Tavignanu and 73% for the Fium’Altu basin). Therefore, we showed the importance of groundwater and delayed subsurface flow contributions to sustain river flow and its ecological continuity in a mountainous Mediterranean context. Geological features may explain differences in the Baseflow Index distribution between the two basins, implying that some components in the baseflow (groundwater or subsurface flow) are more or less present depending on the period considered. Our next steps consist in going further using environmental tracers to provide conceptual models describing all components of the hydrological cycle which contribute to baseflow. At the end, this will serve as indicators for stakeholders in order to perform sustainable management and to assess the resilience of water resources facing global climate change, not only in Corsica, but for any similar region.