The upper pannonian thermal aquifer: Cross border cooperation as an essential step to transboundary groundwater management

•Transboundary thermal aquifer shared by four countries is delineated.•Aquifer characterised using harmonised geological-hydrogeological data and models.•Cross border areas with significant transboundary flow rates were identified.•Thermal water should be exploited using doublets.•An indicator system is defined for thermal groundwater resource management.