Low-flow patterns and hydrological regionalisation across the Baltic Sea basin

Study region The Baltic Sea basin covers approximately 1.7 million km² across northern Europe, extending from sub-Arctic to temperate climates with pronounced gradients in snow cover, precipitation, temperature, and catchment storage. As one of the largest brackish sea basins globally, its hydrology regulates freshwater and nutrient inputs that critically influence coastal water quality and marine ecosystem functioning. Despite its ecological and hydrological importance, a consistent basin-scale assessment of low-flow regimes across national boundaries has been lacking, with existing studies limited to individual countries using different methods and indicators. Study focus The study analyses daily discharge records from 134 water gauging stations across six countries (Sweden, Finland, Estonia, Latvia, Lithuania, and Poland) for the period 1961–2020. Low-flow conditions were characterised using the annual minimum 30-day mean discharge (Q30) and its catchment-area-normalised form of specific discharge (q30), enabling comparison of low-flow magnitude, seasonality, and variability, across contrasting hydroclimatic regions. We examined spatial patterns, temporal changes between two climate normal periods (1961–1990 and 1991–2020), and identified hydrologically coherent regions through K-means cluster analysis based on q30, coefficient of variation, proportion of warm-season low-flow, and mean catchment elevation. New hydrological insights Low-flow magnitude shows a pronounced spatial dipole: q30 increased by 20–65% in high-latitude and mountainous catchments, while mid-latitude lowlands experienced declines of –10 to –30%, particularly in Poland and the Baltic States. A well-defined latitudinal transition around 61.8°N separates cold-season (snow-dominated) from warm-season (rainfall-dominated) low-flow regimes. Between the two periods, mid-latitude basins underwent a near-complete shift towards warm-season low-flow dominance, consistent with earlier snowmelt and increased evapotranspiration. Cluster analysis identified six coherent low-flow regime types that align with major climatic and physiographic gradients. While the regional structure remained stable, 40 stations (29.9%) changed cluster membership, concentrated in transitional mid-latitude zones. These findings provide the first basin-scale regionalisation of low-flow regimes in the Baltic Sea basin and offer a quantitative framework for transboundary drought screening and climate adaptation planning.