Changes in Snowmelt Timing and Peak Flow Generation in Non-Regulated Finnish Catchments

Climate change alters precipitation patterns and extends the warm season in Arctic and sub-Arctic regions, with direct consequences for river flow dynamics. Snow Water Equivalent (SWE) provides a critical link between climate forcing and streamflow response, as it represents the portion of the snowpack that is released as runoff. However, temporal analysis of SWE is challenged by the discontinuous nature of observations provided by the Finnish Environment Institute (SYKE). In this study, a degree-day model was used to generate daily SWE time series, which were subsequently corrected using observed data, for four non-regulated Finnish catchments. River flow timing was analyzed relative to snowmelt onset over the period 1982–2024. While no clear trend was identified in the calendar-day occurrence of spring peak discharge, analysis relative to snowmelt onset revealed a consistent shift toward later peak flow, indicating an increasing delay between melt initiation and maximum discharge. Temperature analysis during the snowmelt period showed a significant increasing trend, suggesting warmer melt-season conditions that promote intensified melt but also modify the timing of runoff generation. In addition, precipitation analysis indicated an increasing tendency toward rain-on-snow events, as well as a rising frequency of rainfall occurring between maximum SWE and peak discharge. These results indicate a potential shift from predominantly snowmelt-driven to increasingly rain-driven peak flow.