Remote sensing of tropical riverine water quality using sentinel-2 MSI and field observations

Abstract

Monitoring the water quality as well as assessing its ecological status, both in coastal and inland environments, are essential tasks and pose serious environmental challenges. Although both field measurements and observations from permanent measuring stations may be available, the advent of modern remote sensors, such as MSI (MultiSpectral Instrument), on board the Sentinel-2 satellite, has made it possible to obtain inland biogeochemical products at high spatial (10 m) and temporal (5-days) resolutions. However, validating Sentinel-2 derived water quality estimations in tropical riverine and costal water is an ongoing effort.This study presents one of the first attempts to locally adapt and validate the C2RCC atmospheric processor for the inversion of IOPs to water quality parameters of productive tropical riverine waters in South East Asia using a time series of 65 Sentinel-2 MSI images from 2018 to 2021. We developed locally adapted relationships between C2X-Nets-derived inherent optical properties (IOPs) and surface reflectances with field observations of chl-a concentration, colored dissolved organic matter (CDOM), turbidity, transparency, and in situ reflectance spectra. Results have been validated against 3-years long time series of observations collected at six monitoring stations of the Metropolitan Waterworks Authority (MWA) of Thailand covering approximately 130 km-long stretch of the Chao Phraya river.Locally adapted C2X-Net processor showed good performances in predicting the chl-a concentrations, turbidity and Secchi disk depth. However, relatively low performance was noted for C2X-Nets processor in predicting CDOM absorption. During the period 2018–2021, the temporal trends of chl-a concentration and turbidity showed a clear seasonal pattern with peak turbidity values in the wet seasons. Spatial pattern of retrieved water quality products showed a noticeable change in correspondence with the built-up area of Bangkok. Even if elevated chl-a concentrations have been generally observed all year around, highest peaks have been observed in the dry and at beginning of the wet seasons.To the best knowledge of the authors, our findings firstly demonstrated the integration potential of C2RCC and satellite-retrieved water quality estimations into existing national water quality monitoring network in the context of water resources management and decision-making in the tropical Southeast Asian countries.