Validation of Surface Water and Ocean Topography (SWOT) river Water Surface Elevation using in-situ gauges in Finnish rivers

Rivers in cold-climate regions are fundamental components of coupled hydrological, cryospheric, and ecological systems by governing water availability, ecosystem functioning, and human activities under strong seasonal constraints. Snow accumulation, river ice processes, and freeze-thaw cycles are major factors for tightly controlling river dynamics, making them highly sensitive to climate variability and change. Consequently, to capture changes in flow, ice conditions, and water levels, systematic river monitoring is essential which enables reliable assessment of hydrological extremes, ecosystem responses, and infrastructure risks. However, river monitoring in cold climate regions remains challenging due to prolonged ice cover, limited accessibility during winter, sensor malfunction under freezing temperatures, harsh environmental conditions, and the complex interaction between hydrological and cryospheric processes that are difficult to observe continuously and at high resolution. Therefore, increasing attention have been gained for novel monitoring approaches that minimize direct physical contact, particularly those based on remote sensing techniques, because they enable spatially extensive, non-intrusive, and continuous observation of river dynamics in cold regions and hard to access environments. In this context, by representing a major milestone as the first wide-swath altimetry mission specifically designed to observe surface water dynamics, the Surface Water and Ocean Topography (SWOT) mission was first launched in December 2022. It provides high-resolution, near-global measurements of water surface elevation (WSE), river width, and surface slope using Ka-band Radar Interferometry (KaRIn), and SWOT significantly advances satellite-based monitoring of rivers, lakes, and reservoirs. In this paper, we assess the validity of WSE derived from RiverSP data of the SWOT mission over major Finnish rivers. WSE is extracted for 14 rivers which are available in SWOT mission and representing diverse hydrological settings, including 08 regulated and 06 unregulated, and is evaluated against observed daily water levels from the nearest in-situ gauging stations. This analysis enhances understanding of the influence of river ice cover on SWOT observations and enables evaluation of the associated quality flags. Temporal in-situ satellite derived water temperature observations before and after the ice season are examined to support interpretation of detected ice-cover and open water period with SWOT observation.