Detecting Cryosphere Signals in Runoff: Trends in Genetic Components in Norwegian Reference Streamflow and its Implication for Norwegian Hydropower Regulation

Deglaciation, declining seasonal snow cover, and permafrost thaw are among the most visible consequences of climate change. Yet, the implications of these cryosphere changes for river runoff—and for practical water management—remain insufficiently quantified. This study contributes to a broader initiative aimed at comprehensive assessment of past and future climate-driven changes in snow- and ice-dominated regions.We applied graphical–analytical hydrograph separation into genetic runoff components (baseflow, spring snowmelt, rainfall floods, thaw floods) to 94 gauging stations from the Norwegian Streamflow Reference Dataset for 1960–2024, using the grwat R package. Daily discharge was combined with watershed-averaged daily precipitation and air temperature from the ERA5 reanalysis as input. We identify two key hydrograph transformations associated with cryosphere change: (1) a reduction in the volume of the spring snowmelt flood (vårflom) and (2) an intensification of winter thawing processes. The latter manifests as either higher runoff volumes during episodic winter thaw and rain-on-snow floods, or an increase in the frequency and duration of thaw-flood events, particularly since the 1990s–2000s.As many Norwegian hydropower reservoirs were designed to capture spring snowmelt and release water during winter, these shifts imply increasing mismatch between inflow seasonality and existing regulation strategies. Possible operational tensions include reduced spring refill reliability, higher winter spill risk, or changing flood-control constraints. The observed trends therefore highlight a need to adapt reservoir operations to the likely continued redistribution of runoff from spring toward winter.The study was funded by the European Union’s Horizon Europe research and innovation programme through the project LIQUIDICE (grant number: 101184962).