A reflectance-based water quality index and its application to examine degradation of river water quality in a rapidly urbanising megacity

The water quality in rivers around Dhaka is deteriorating very fast given extensive sources of pollutants associated with rapid urbanisation and industrialisation. This paper introduced a water quality index (WQI) utilising Landsat bands 1 to 3 (0.45 to 0.67 µm) against band 5 (1.55 to 1.75 µm). The value of the proposed index varies from – 1 to + 1, whereby around 0 (small positive and negative) indicated probable contamination, if the water depth is around 2 m or above. This approach was developed using Landsat data to study long-term water quality change in the rivers surrounding Dhaka megacity, Bangladesh. The values were converted to total suspended solids (TSS) by utilising observed in-situ data, which showed a clear seasonal influence of water pollution, suggesting it reduces to the background level with the onset of high monsoonal river flows. However, long-term analysis showed that the rivers remain polluted throughout the year, particularly since 2011. The main reason for increasing river water pollution may be linked to the contaminant load which has increased extensively in recent years. The approach developed in this study may be used in other urban rivers, particularly in Asia.