Trends in hydroclimate extremes: How changes in winter conditions affect seasonal baseflow and storage

Abstract. Northern ecosystems experience rapid climatic change at a rate where average temperatures are increasing above global averages. Yet, for the boreal snow-dominated catchments that rely on the winter snow accumulation and spring melt for sustained stream flow across preceding seasons, much remains unknown about how catchment water storage and baseflow are affected. Here we used 40 years of data from the boreal Krycklan catchment, placed into a 130 climate record from a nearby location, to test how 27 extreme climate change indices have been affected, and how these, in turn, can explain seasonal low flows during the winter and summer. Our results show that while annual temperatures have increased by 2.2 °C over the last four decades, even more distinct seasonal impacts were detected as exemplified by eight extreme indices demonstrating that winters have become warmer with less precipitation. The analysis also showed that summers have become warmer shown by four significant increases in climate indices. Using the significant winter indices to predict winter baseflow and winter/summer indices to predict summer baseflow we found that the accumulated degree day below zero (AFDD<0) was the best predictor of winter minimum flow and AFDD<0 and Summer maximum temperature (MaxTmax) were the best predictor of summer minimum flow. Additional isotopic analysis of stream flow partitioning found an increasing contribution of winter rain/snow in stream runoff during winter over the last 22 years, as well as a decreased contribution to the preceding summer stream flow. These findings imply that warmer winters have affected water storage and runoff patterns in the boreal catchment which can have important feedback on terrestrial ecosystems, particularly on water availability in later parts of the growing season.