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

Abstract. Long-term observations of spring discharge provide an alternative to estimate the evolution of groundwater resources based on observational data at 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 karstic environments. Due to relatively short response times, karst systems are focus points for enhancing the understanding of the impact of climate change on groundwater resources. This work analyses observational data (precipitation, temperature and discharge) of more than 50 springs spatially distributed over Europe to give a continental overview of changes in groundwater resources in karst areas. The work focuses on two different periods of 20 and 40 years, to identify any possible acceleration or moderation in changes. For both periods a trend analysis of the observational data, using Mann-Kendall and Sen’s slope, was performed on the entire time series and per season. Possible process changes were considered by analysing also trends in high and low flow values. Structural differences of the systems were considered by using two indices related to the storage and inertia of the system. In combination, these indices were able to i) highlight structural differences and ii) characterize 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 calculated in this study follow the general pattern of river discharge found in literature, but the last 20 years deviate from this behaviour. During this period, increasing temperature plays a more important role in the evolution of spring discharge than changes in precipitation. These results are discussed in relation to the indirect influence of other drivers such as changes in land use or land cover, specific regional conditions but also changes in processes related to groundwater recharge and storage, providing insights for assessing groundwater recharge in the past and in the future.