Age-based constraints as a foundation for hydrological modelling of volcanic aquifers under global change

Understanding the spatio-temporal dynamics of groundwater resources is essential to anticipate future water availability under climate change and support sustainable management under competing uses. In this study, we develop a hydrological model of the Volvic volcanic aquifer system (France) using GARDENIA^®, based on more than 30 years of hydroclimatic and abstraction data. This dataset allows the reconstruction of the catchment water budget and its temporal evolution. Builded on previously established groundwater residence times (Nevers et al., 2025), we integrate here age-based constraints into hydrological modelling. We first demonstrate that this coupling links groundwater ages to aquifer responses and provides a more physically consistent representation of system behaviour under future climate scenarios. Incorporating residence time information led to the distinction of two hydrodynamic units, capturing contrasted flow dynamics within the aquifer. Results of the modelling show that both climate variability and water withdrawals impact aquifer water budgets and discharges at the system outlet. Increasing frequency and duration of heatwaves and droughts enhance water demand for drinking water supply, bottling, and agriculture, thereby reducing water availability for environmental needs. Using DRIAS climate projections (Météo-France), we simulate future changes in water availability and resource allocation. Although focused on the Volvic basin, this approach is transferable to other groundwater-dependent regions and supports integrated water resource management under climate uncertainty.