Trends in long-term hydrological data from European karst areas: insights for groundwater recharge evaluation

Long-term observations of spring discharge provide an alternative for estimating the evolution of groundwater resources based on observational data at the catchment scale. Karst springs can be found in large parts of Europe, covering all climate zones of the mid-latitudes. Continuous spring discharge measurements are holistic signals, representing both fast and slow flow components, typical of karst environments. Due to relatively short response times, karst systems are pivotal in improving our understanding of the impact of climate change on groundwater resources. This study analyses observational data (precipitation, temperature, and discharge) from over 50 springs distributed across Europe (AT, FR, GB, SI), offering a continental perspective on groundwater resource changes in karst areas. The work focuses on two periods spanning 20 and 40 years, aiming to detect possible accelerations or moderations in trends over time. For both periods, trend analyses of the observational data were conducted using the Mann-Kendall test and Sen's slope on full time series as well as seasonal data. Additionally, potential process changes were examined through trends in high and low flow values. Structural differences among karst systems were accounted for using two indices related to storage and inertia of the system, which were used to (i) highlight structural differences and (ii) classify karst systems accordingly. The results show that the sensitivity of karst aquifers to climate change is not controlled by their degree of karstification. Long-term trends in spring discharge observed in this study align with the general patterns of river discharge found in the literature. However, the behaviour during the last 20 years diverges from these historical patterns. In this most recent period, increasing temperatures play a more signifi-cant role in the evolution of spring discharge than changes in precipitation. These findings are contextualized with consideration of indirect drivers, such as changes in land use or land cover, specific regional conditions, and shifts in groundwater recharge and storage processes. Together, they offer valuable insights for assessing groundwater recharge in the past and in the future