Investigating the spatial-temporal pattern of the trophic state index in Hanoi’s largest urban lake (Vietnam) using Sentinel-2 images

ABSTRACT

Urban lakes provide aesthetic, recreational and ecological benefits, making understanding their trophic status vital for effective management. Utilizing Sentinel-2 imagery, we investigated the spatial-temporal patterns of the Trophic State Index (TSI) in Hanoi’s West Lake over a 2-year period. Our study proposed a robust TSI estimation model, incorporating three band ratios: ${\mathrm{R}}_{\mathrm{rs}}\left(655\right)/{\mathrm{R}}_{\mathrm{rs}}\left(490\right)$, ${\mathrm{R}}_{\mathrm{rs}}\left(705\right)/{\mathrm{R}}_{\mathrm{rs}}\left(655\right)$, ${\mathrm{R}}_{\mathrm{rs}}\left(655\right)/{\mathrm{R}}_{\mathrm{rs}}\left(443\right)$, along with 124 in situ point data. The model exhibited high accuracy (the determination coefficient, R² = 0.63) and minimal errors (the root mean square errors, RMSE = 1.1 and 0.9). Consequently, West Lake consistently registered as hypereutrophic (TSI > 70), with seasonal variations linked to climate factors, leading to algal blooms and reduced water clarity. Spatial analysis highlighted the influence of sewage drains on TSI, indicating an urgent need for wastewater control. Our study demonstrates the cost-effective utility of Sentinel-2 imagery for monitoring urban lake eutrophication and emphasizes the urgency of managing eutrophication in these vital ecosystems.

KEYWORDS:

• Eutrophication
• urban lakes
• multivariate band-ratio model
• TSI
• Sen2Cor
• Sentinel 2

Acknowledgements

This research is funded by the Taiwan–Vietnam bilateral collaborative project with the code NĐT/TW/21/16. We appreciate ESA for providing the Sentinel-2 images. Additionally, we extend our gratitude to three anonymous reviewers for their valuable comments and suggestions.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

The work was supported by the Taiwan – Vietnam bilateral collaborative project with code NĐT/TW/21/16 [NĐT/TW/21/16].