Hydrological and hydrochemical response to climate signals in high-retention river–lake catchment

• A high-retention postglacial catchment maintains a stable hydrological regime under climatic stress. • Groundwater contributes up to 95% of streamflow; BFI remained high, despite warming trends. • Stable isotope and hydrochemical data confirm significant contributions from deep aquifers to streams and lakes. • Water-quality changes were limited, with only minor seasonal and spatial ion variability. • Long-term monitoring demonstrates the buffering capacity of forested lake systems in temperate regions. Small headwater catchments with high retention capacity are important indicators of hydrological responses to climate change. This study evaluates the hydrological and hydrochemical response of a high-retention river–lake catchment (Ruda catchment, central Poland) to changing climatic conditions. Long-term meteorological and hydrological data (1952–2019), combined with recent hydrochemical and stable isotope monitoring (2017–19), were analyzed to assess the system’s sensitivity to climate signals. The results indicate a significant increase in air temperature (+0.19 °C per decade), rising evapotranspiration, and a progressively negative climatic water balance over the past two decades. Despite these changes, the Ruda system demonstrates high hydrological resilience, with stable, groundwater-fed baseflow (Base Flow Index: 89–99%) and only minor seasonal fluctuations in lake and groundwater levels. However, upper subcatchments exhibited reduced flow and greater seasonal variability. Multivariate analyses revealed spatial hydrochemical heterogeneity, especially in nitrate and sulfate concentrations. Stable isotope data (δ2H, δ1 8 O) confirmed spatially variable recharge patterns and source water contributions across the catchment. Comparisons against the 1990–94 reference period show that, despite increased temperature and water deficits in 2017–19, key flow and chemical characteristics have remained largely stable. The study highlights the role of catchment retention capacity and subsurface connectivity in buffering climatic extremes and maintaining ecological stability. These findings support the interpretation that young-glacial river–lake catchments with strong groundwater connectivity may show substantial resilience to recent climate warming. They also indicate that the Ruda catchment can serve as a useful reference example for assessing hydrological change in comparable systems of the Central European lowlands.