Groundwater–surface water interactions in an anthropized Mediterranean braided river: insights from combined UAV-based thermal imagery and electrical resistivity tomography

Although the interaction between groundwater (GW) and surface water (SW) has been the subject of numerous studies, few of them have focused on braided river environments. Yet these environments form essential freshwater resources and unique ecosystems. This work analyses GW-SW interactions in the Var River alluvial plain (30 km²), which is located in south-east France. The alluvial aquifer of the Var River is a crucial resource for the nearby, densely populated coastal area of the French Riviera. The objective is therefore twofold: i) to improve the conceptual understanding of GW-SW exchanges in an anthropized Mediterranean braided river, and ii) to support the operational management of the water resource. In order to achieve a multi-scale understanding of the different processes involved in GW-SW interactions, a multidisciplinary approach is being implemented in the Var alluvial plain. It incorporates methods from hydrometry, hydrogeophysics, geochemistry, aerial and satellite imaging, hydrosedimentology and numerical modelling. Here, we show results obtained by combining two methods: unmanned aerial vehicle (UAV)-based infrared thermal imagery and electrical resistivity tomography (ERT). Thermal infrared imaging of the Var riverbed was carried out in July 2025 (when piezometric levels began to decrease) using a light-weight UAV, and ERT was performed at four study sites along the river between September and November 2025 (when piezometric levels were at their lowest). The results obtained led to a new conceptualization of the connectivity and interactions between the river and the aquifer. This conceptualization is intended to be further enriched by combining more results from the multidisciplinary approach, particularly from a fully integrated modelling of surface and subsurface flows.