Water quality monitoring with Sentinel 2 in small watercourses : Investigating the measurability of phosphorus using proxy data

Abstract: Inland water has for a long time showed vast stress due to eutrophication, mainly caused by increased levels of phosphorus. Applying remote sensing as a tool for monitoring water parameters has long been used. In the past, inland watercourses measurements have proven to be challenging, often due to the limitations of satellite missions' spectral resolution or difficulties in implementing the appropriate methodology. This project investigates the potential to use a high-resolution satellite mission, Sentinel 2, to monitor phosphorus with the proxies total suspended matter (TSM) and turbidity in two smaller watercourses, Fyrisån and Sävjaån, in Uppsala, Sweden. From April to November, a period spanning three years (2018, 2019, and 2021), empirical modeling was employed to conduct investigations. The three years all represent different weather patterns and discharge velocities. The bands 2 to 8 were investigated individually and together to see if there was a potential using a single band correlation or multiple to correlate with turbidity or TSM. The two optically active water parameters are known to have a high correlation with the non-optically active phosphorus. There was no correlation found between the proxies and each band individually for any of the years investigated. Using a multi regression analysis both 2018 and 2019 showed high correlation for TSM, and 2019 for turbidity. While the results for 2021 were not significant for any of the proxies. The conclusion indicates that with right surrounding factors it’s possible to use TSM and turbidity as a proxy for phosphorus when using Sentinel 2 in these smaller watercourses. Nevertheless, further studies are needed to investigate how the proxy and the nutrient acts together with satellite data for peaks etc. before using Sentinel 2 results as a direct interpretation.