Spatiotemporal dynamics of riparian vegetation NDVI as indicators of bio-hydromorphological interactions

Abstract. The Normalized Difference Vegetation Index (NDVI) can be effectively used for monitoring the spatial and temporal dynamics of riparian vegetation. However, quantitative and efficient evaluations of the relationship between NDVI and bio-hydromorphological processes remain limited, particularly in the context of riverine floodplain management, where dense in-channel vegetation can obstruct flow and reduce conveyance capacity. Using 200 cloud-free Sentinel-2 images (2015–2024) for a 20-km reach of the Chikuma River (Japan), we evaluated the utility of NDVI (extracted from Sentinel-2 images) and the greenness index (defined as NDVI > 0.2) as quantitative indicators of bio-hydromorphological interactions, focusing on: (1) the relationship between NDVI dynamics, flood magnitude, and lateral channel morphology of relative elevation, and (2) the seasonal dynamics of riparian vegetation within frequently disturbed channels. Results indicated that NDVI fluctuations strongly corresponded to flood disturbances at lower elevations, while vegetation at higher elevations remained relatively stable. Along cross-channel transects, the maximum greenness ratio was well represented by a logistic model, with parameters varying according to flood magnitude from the previous year. Annual vegetation greenness additionally exhibited clear seasonal cycles, showing a late-summer greenness peak (August–September). The spatial and seasonal characteristics of NDVI displayed its potential as an indicator for operationalizing the “where” (priority bands by relative elevation) and “when” (phenological window) of vegetation control, and offered a transferable, remotely sensed basis for flood-risk mitigation and ecohydraulic planning.