Northern Croatia has peculiar geological and thermal characteristics favouring the outflow of thermal water in springs with temperatures up to 65 °C at two dozen localities. These waters represent a valuable natural, social, historical, and economic resource for the region since they have been used for millennia and are currently the basis for tourism and health care centres. Thermal springs are part of local and intermediate scale hydrothermal systems which generally include: i) a recharge area in the mountainous hinterland of the spring, ii) a main geothermal aquifer represented by Mesozoic carbonate rocks, and iii) a discharge area with favourable structural characteristics that increase the fracturing of the bedrock and enhance the local permeability field. The continuous functioning of such systems depends on a delicate balance between groundwater flow velocities, precipitation/dissolution processes, heat flow from the deeper part of the crust, and an active tectonic setting. In order to maintain this balance and use thermal water resources in a sustainable manner, a system-level understanding is required. 

The HyTheC project aims to reach this goal through a multidisciplinary approach including structural geology, hydrogeology, geothermal, hydrogeochemical, geophysical, and remote sensing investigations. The methodology is tested in three pilot areas in northern Croatia (Daruvar, Hrvatsko zagorje, Topusko) where thermal water is used. The results are used to perform 3D geological modelling of the investigated areas, hydrogeological and thermal parametrization of the geological units involved in the thermal fluid flow, construct conceptual models of the systems, and conduct numerical simulations of the system functioning in undisturbed conditions and with different extraction scenarios. 

The HyTheC project includes a collaboration between the GFZ (section 4.5 Subsurface Process Modelling) and the Croatian Geological Survey (HGI-CGS) to apply the numerical modelling framework as developed at GFZ within Section 4.5 at the pilot areas. The objectives of the modelling are to corroborate the conceptual model of the hydrothermal systems, quantify the impact of geological and thermal processes on the groundwater flow and heat transport, and to conduct exploitation scenarios in order to develop sustainable utilization scenarios of the hydrothermal resources.