Analysing the change in water quality parameters along river Ganga at Varanasi, Mirzapur and Ghazipur using Sentinel-2 and Landsat-8 satellite data during pre-lockdown, lockdown and post-lockdown associated with COVID-19
Nilendu Das, Rajarshi Bhattacharjee, Abhinandan Choubey, Ashwani Kumar Agnihotri, Anurag Ohri, Shishir Gaur
Abstract
The mapping of the river or the inland water quality using remote-sensing technology was not new for the scientific community, and this activity has been practiced since the 1970s with the launch of the Landsat series of satellites (Klemas et al. 1971 ; Ritchie et al. 1976 ). The conventional procedures for testing the water quality parameters are expensive as well as time-consuming, and it provides information only for the point(s) under measurement. The remote-sensing technology can be utilised as an advantageous alternative option for the measurement of the water quality parameters. The remote-sensing-based satellite images can provide systematic and periodic coverage of an area. More information can be deduced for a place because of the periodic nature of the satellite revisit (de Moraes Novo et al. 2006 ; Lamaro et al. 2013 ). Every feature of the earth's surface behaves uniquely while interacting with electromagnetic (EM) radiation. This distinctiveness generates in the form of the spectral signature from each surface feature, and this signature has been utilised to identify the surface feature through satellite images (Elachi 1987 ; Joseph 1996 ). When a slight change in the composition of a feature occurs, it simultaneously changes the spectral signature (Moore 1980 ). Several factors accompany the change of the spectral signature for a feature, and the same can be valid for the water feature. Some of the factors responsible for generating the different spectral signatures of water are the incidence angle of the sun, the season of the year, water surface roughness, turbidity, depth of the water and vegetation growth on the water surface (Garg et al. 2020 ). By studying these different spectral signatures, the qualitative and sometimes the quantitative assessment of a particular component can be accomplished (Chander et al. 2019 ; Luis et al. 2019 ). Several water-quality parameters such as temperature, turbidity, dissolved oxygen (DO), pH, total dissolved solids and total suspended solids (TSSs) can be studied with the help of remote-sensing technology (Lamaro et al. 2013 ; Khattab and Merkel 2014 ; El Din and Zhang 2017 ; Garg et al. 2020 ; Patel et al. 2020 ).