Annual heat budget and seasonal variations in a northern river with ice processes

Understanding the heat budget dynamics of high-latitude rivers is crucial for predicting thermal regimes, assessing the impacts of climate change on aquatic ecosystems, and informing effective water resource management. Despite their importance, detailed studies on the seasonal and transitional heat exchange processes in cold region environments remain limited. This study investigates the annual heat budget and seasonal variations of the Vantaanjoki River in southern Finland, with a particular focus on the influence of ice cover processes. Using meteorological, remote sensing, and in-situ measurements data collected from November 2022 to October 2023, a water/ice-air heat exchange analysis was performed to analyze four primary heat components: shortwave radiation, longwave radiation, sensible heat, and latent heat. The results show pronounced seasonal variations, with summer months dominated by shortwave radiation as the main heat gain source, while winter periods are characterized by significant heat losses due to longwave radiation. The annual net heat flux averaged −11.6 W/m2, indicating a near-balanced energy exchange over the year. Ice dynamics were monitored via image-based estimation of ice cover fraction, allowing classification into three surface conditions: open channel, partially ice-covered, and fully ice-covered. During freezing periods, longwave radiation accounted for over 60% of total heat loss, while shortwave radiation contributed marginally. In contrast, during melting periods, shortwave radiation became a more prominent heat gain component. The results provide critical insights into river heat budgets in high-latitude environments and contribute to improved understanding of river-atmosphere interactions under changing climatic conditions.