Assessing seasonal flow characteristics of two northern latitude rivers using static thermal infrared video data and image velocimetry methodology

The changing climate and increasing anthropogenic pressures on hydrological systems emphasize the need for continuous river flow observations to support water resource management and hydrological research. Conventional monitoring methods, despite their long history, are expensive, intrusive, labor-intensive, and require significant maintenance, making them impractical for remote locations. In recent decades, non-intrusive image velocimetry techniques have emerged as an alternative, enabling surface flow velocity analysis from sequential image frames typically captured by commercial digital cameras. However, these methods have primarily been validated over short durations and have rarely been applied to northern latitude rivers, where hydrology is influenced by seasonal ice and snow cover. Furthermore, the reliance of optical imaging systems on visible wavelengths of light limits their usability in the low-light conditions typical of these regions.

This study employed hourly video data from statically installed thermal infrared cameras to analyze seasonal variations in surface flow velocities and discharges over two years of ice-free flow seasons in two hydrologically distinct northern latitude rivers in Finland. The methodology involved video frame pre-processing with photogrammetric re-projection, surface flow velocity detection using Space-Time Image Velocimetry (STIV) and Large-Scale Particle Image Velocimetry (LSPIV), and validation against in-situ Acoustic Doppler Current Profiler (ADCP) measurements. River discharges were computed using the mid-section method with bathymetry data derived from aerial laser scanning and ADCP datasets, and compared with national hydrological observations based on conventional stage-discharge relationships.

Validation of surface flow velocities obtained using the STIV technique showed strong agreement with near-surface ADCP measurements, consistent with findings from earlier studies. In contrast, results from the LSPIV technique were unreliable and insufficient for accurate discharge computations. Daily averaged discharges computed from STIV velocities effectively captured the seasonal flow dynamics at both sites and corresponded acceptable with conventional stage-discharge observations. These findings demonstrate that image velocimetry techniques, particularly STIV, can be used for near-continuous flow observation over extended periods, even in challenging northern latitude conditions. With further refinement to address existing uncertainties, remote sensing observation systems could offer a viable alternative to traditional hydrological monitoring.