Trends in hydroclimate extremes: how changes in winter affect water storage and baseflow

Abstract. Northern ecosystems are undergoing accelerated climate warming, with average temperature increases exceeding the global mean. In snow-dominated catchments, where cold-season conditions are essential for sustaining streamflow across subsequent seasons, substantial uncertainty persists regarding the impacts of future warming on catchment water storage and runoff dynamics. Here, we utilized 40 years of hydrological data from the boreal Krycklan Catchment, set within a 130-year climate record from a nearby station, to evaluate how 27 extreme climate indices can capture changes and trends in water storage and stream low flow during winter and summer. Our results show that annual temperatures have risen by 2.2 °C over the past 4 decades, with even more pronounced seasonal impacts. Notably, six winter extreme indices and two summer indices revealed distinct trends. We found that warm winters have led to increased winter stream runoff but reduced summer runoff. Predictive modelling indicated that the accumulated freezing degree days (AFDD<0) were the strongest predictor of minimum winter flow, while a combination of AFDD<0 and maximum summer temperature (MaxTmax) best explained variations in minimum summer flow. Furthermore, analysis of streamflow partitioning using water isotopes and the seasonal origin index (SOI) over the past 22 years revealed an increasing winter precipitation signal in winter runoff, accompanied by a declining contribution to summer streamflow. Together, these findings demonstrate that warm winters are fundamentally altering catchment-scale water storage and flow partitioning, with important implications for water availability and ecosystem functioning during the growing season in boreal landscapes.