Real Time Mapping of Chlorophyll- a in the Brahmaputra River using Google Earth Engine

Ecological theory suggests that environment influences all living beings. In the case of water bodies, Chlorophyll-a (Chl-a) concentration acts as a major indicator of water quality. Historically, samples were collected from the field and analyzed in laboratory for obtaining concentration of various water quality parameters. But now a days, there has been rapid advancement in satellite based remote sensing techniques and finer spatial and temporal resolution data are also now freely available. Many studies have already been carried out in the lakes for the derivation of Chlorophyll -a concentration information from satellite remote sensing, leading to the development of various chlorophyll indices. But for rivers, there are limited studies. Also, cloud computing services such as Google Earth Engine helps in faster processing of these data without using client resources. However, these indices are site specific and hence validation needs to be carried out before applying them. Applying these indices for real time monitoring of the water bodies will help in controlling pollution levels and thus maintaining ecological balance. In this study, chlorophyll indices already developed for selected rivers in Brazil, USA and India are applied to a reach in Brahmaputra River in India, to test their applicability in this large braided river system and also to compare their results. Linear, logarithmic, exponential and quadratic relations of Blue, Green, Red and Near Infrared surface reflectance of Sentinel 2A and Landsat 8 imageries are used in this study to determine the chlorophyll indices. It is found that there is a significant variation of the values obtained by these algorithms. For algorithms using Landsat Imageries, concentrations obtained were in the range 40.727- 261.836 mg/L and for algorithms using Sentinel Imageries, concentrations obtained were in the range 1.092-25.612 mg/L in the month of February 2021. This clearly indicates that the algorithms already developed for other regions can only be applied after validation, or new site-specific algorithms need to be developed. Thus, Google Earth Engine can be effectively used for real time ecological monitoring of large water bodies (lakes) and rivers provided the algorithms used are first validated for the region of interest.