Surface–groundwater interactions across scales: implications for integrated water resources management in Estonian catchments, northeastern Europe

Surface water and groundwater form a single, dynamically connected system. Yet water resources management often treats them as separate components. This separation becomes particularly problematic in geomorphologically flat, groundwater-dominated landscapes, where local subsurface processes exert strong control over catchment-scale hydrological response and water availability. These challenges are further amplified by the presence of shared aquifer systems and cross-border flow paths, which link local hydrological processes directly to transboundary water management decisions. In this study, we combine process-based modelling, hydrological indicators, and conceptual analysis to examine how surface water and groundwater interact across different spatial and temporal scales. We focus on the active water zone that is most relevant for water resources management. We combine results from local catchment-scale studies, drinking-water abstraction areas, and a regional transboundary groundwater flow model to examine how groundwater recharge, storage, and release shape river discharge and baseflow dominance. Emphasis is placed on distinguishing between water participating in the contemporary hydrological cycle and older, weakly connected groundwater, and on identifying the scales at which these components interact. Local headwater catchments and springs exhibit long memory effects and delayed responses to recharge, whereas borehole capture zones reveal how pumping alters natural flow paths and redistributes surface–groundwater exchange.Using integrated modelling tools, including PRMS and MODFLOW-based regional models, we demonstrate that surface water, shallow groundwater, and deeper aquifers cannot be managed independently without risking serious misinterpretation of water availability and vulnerability. Our results indicate that surface–groundwater interactions and transboundary groundwater flows are primarily influenced by shallow, actively circulating aquifer systems. This directly links local water use decisions to regional and cross-border impacts. Our results underscore the need for an integrated assessment of surface water and groundwater as a joint resource, explicitly accounting for scale-dependent flow processes, recharge pathways, and the impacts of abstraction. This approach provides a more realistic basis for sustainable water resources management, drinking-water protection, and transboundary water governance in northern European lowlands and similar hydrogeological settings.