| 17 Oct 2025
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.
Review of: Spatiotemporal dynamics of riparian vegetation NDVI as indicators of bio-hydromorphological interactions
Authors: Zhou, Toda & Zhu
Reviewer: Maarten G Kleinhans, d.d. 5 Nov 2025
The manuscript reports on combination of bed elevation data, a proxy for vegetation (greenness from NDVI) and water level measurements used to conduct flow modelling for a steep Japanese (gravel) river. The data are promising, the manuscript is reasonably well written and the analyses look repeatable and valid. However, the scope of the paper is unclear and if the objective is indeed to study relations between river channel change and vegetation, then review of relevant literature and a specific hypothesis is needed. In view of the concrete analyses possible with the data to address such a concrete topic, this suggests a rejection.
MAIN POINTS
The scope of the paper is not so clear. The 3 research questions are somewhat clear but the earlier stated objectives are too vague. Furthermore, there is no clear answer to the first research question, even though these data clearly allow answering it. The mixture of results and discussion is part of this problem: separation of these sections is needed precisely to make clear what the result of this work are and how this is contextualized by literature.
The literature on channel pattern and channel mobility in interaction with vegetation is ignored. I am thinking of the classic experimental work by Tal and Paola on braided river experiments with vegetation, conceptual work by Gurnell and Corenblit, the modelling by us (van Oorschot et al) and many other works. For review, see papers by Corenblit and by me (e.g. https://doi.org/10.1144/SP540-2022-138, also available as open access through utrecht university on my profile page).
Alternatively, one could think of analysis of tolerance limits of the various species to inundation depth, and erosion, and ripping out by high flow velocity. Also in this case that needs comparison to literature, plus analysis of morphological change and possible effects of seasonality and the timing of floods (there is a figure suggesting that bed elevation maps of the river are available for three years).
The figures need improvement and clarification and some map figures would be much appreciated.
Making data available upon request is not acceptable in this day and age anymore. One can easily provide the depth and greenness maps and the timeseries through an online repository. This would be valuable for future analyses of vegetation-river interactions.
DETAILED POINTS (referring to figures and line numbers)
Fig 1 there is not much to see of the river and its morphology. Please enhance image resolution and consider including another figure with a small section shown much larger as image and as (detrended) bed elevation
45-56 the objective is rather vague. The first primary research question is concrete, but this requires review before on the possible effects of riparian vegetation on expected effects on the interactions between vegetation, inundation duration (hydroperiod) and channel morphodynamics. Line 138 is too late and merely a beginning.
Fig 3 and the text suggest you have three bed elevation maps and the greenness maps but these are shown nowhere. I strongly suggest showing these for a short stretch of the river as to illustrate the patterns and the dynamics, so that the reader knows what lateral channel dynamics to expect in the data analysis.
Fig 4 would be more understandable if zoomed out a bit, or if showing more upstream and downstream of the river (make the panel higher)
Fig 6 says "10 hours' acuumulated water level" but what is accumulated water level? time-averaged?
Two or three panels will do to show the general pattern and then we know what the data underlying the data reduction in later figures is.
Fig 9 help the reader and state again what h0 is. Combine with Fig 7 (put the panels together in one multipanel figure at the same scale)
Fig 10 explain RE again in the caption. I suggest that this figure is much more readable if it is turned into a line plot with lines for all the years and RE on the horizontal axis. These will then be much easier compared.
Fig 12 and Fig 13 can also be combined in a multipanel figure with exactly the same x-axis for better analysis by the reader