The role of groundwater connectivity in sustaining European lake water systems

This study investigates the potential connectivity of groundwater with lakes (groundwater-lake connectivity), using the groundwater table depth to maximum lake depth ratio (GW/L) as an indicator. Results from 189 European lakes show that those that are disconnected or have low connectivity (GW/L > 0.5), typically at higher altitudes (>1000 m a.s.l.) in catchments with less-permeable geological formations (e.g., crystalline rocks), exhibit a higher sensitivity to evaporation (evaporation to inflow rate, E/I > 0.40) compared to lakes more highly connected to groundwater (GW/L < 0.1, E/I < 0.20). Lakes with higher groundwater-lake connectivity are also more resilient to climatic changes, with groundwater contributions correlating positively with catchment characteristics (e.g., land use and precipitation). While most lakes are primarily recharged by warm precipitation (>80 % of the rainfall contribution in lake waters), particularly in disconnected lakes, cold precipitation (snow and rain from November to April) contributes <20 % to the total recharge in the warmer months (May–October). Nevertheless, cold precipitation plays a crucial role in maintaining groundwater-lake connectivity by recharging groundwater and stabilizing lake water levels. Nitrate contamination is strongly associated with urban and agricultural land use, with concentrations increasing with groundwater input. Lakes with higher groundwater-lake connectivity tend to exhibit elevated nitrate levels, contributing to eutrophication. Overall, the GW/L indicator shows strong potential for future studies and application in hydrological assessments. These findings emphasize the importance of incorporating groundwater–lake connectivity into climate change vulnerability assessments, especially in relation to water balance, nutrient cycling, and ecosystem health.