Spatio-Temporal Dynamics of Surface Water Quality in Cascaded Lake Systems Due to Urbanisation: An Integrated Lake Series Approach for Bengaluru, India

Urban water bodies, such as lakes in the rapidly growing cities of the Global South, are being severely impacted by unsustainable urbanisation. This has resulted in tremendous stress on the interconnected system of lakes in Bengaluru, India. Existing studies related to degradation of lakes considers individual lakes as the unit of analysis, thus  failing to address the issues due to the interconnected or cascaded nature of lakes in the city. To address these gaps, this study adopts lake series scale as the unit of analysis, to analyse the cascading impacts of urbanisation, focusing on the severely degraded Yele Mallappa Shetty Lake Series (YMSLS) in Bengaluru.

The study uses SRTM DEM(30m) for delineation of individual lake catchment areas and identify stream orders for spatial analysis. Further, extent of urbanisation in the catchment is quantified by the Land use land cover (LULC) change analysis(1993-2023) using supervised classification techniques with Landsat 5 (TM, 30m) and Landsat 8 (OLI+TIRS, 30m) satellite images. Spatio-temporal variation of surface water quality of lakes in the catchment is analysed using the Weighted Arithmetic Water Quality Index (WAWQI) and the Overall Index of Pollution (OIP) derived from monthly water quality data (2023-2024). 

LULC change analysis revealed that in 1993, the YMSLS catchment area comprised open spaces (53.4%) and agricultural land with vegetation (35%), while built-up areas were limited to 7.2%. However, by 2023, the built-up area expanded to 34.6% of the 285 sq. km catchment, becoming the dominant land use. Rapid urbanisation has led to improper disposal of wastewater and caused water quality degradation and increase in aquatic vegetation growth in the lakes. Temporal analysis of surface water quality showed seasonal variations, wherein the WAWQI and OIP values were lowest during the post-monsoon season (Mean ± SD; 107.9 ± 43.5, 4.4 ± 1.3), followed by the monsoon season (109.3 ± 44; 4.6 ± 1.87), and peaked in the summer season (193.5 ± 62.1; 4.8 ± 1.3). Spatial analysis showed that lakes receiving inflows from higher-order streams located at downstream areas exhibited higher WQI values, indicating greater pollution levels compared to lakes associated with first order streams located in upstream areas. Additionally, the individual catchment area of lakes demonstrated a strong positive correlation with WAWQI (r = 0.71, p < 0.05) and a moderate positive correlation with OIP (r = 0.6, p < 0.05). The spatio-temporal analysis demonstrates the flushing of pollution loads, including aquatic vegetation, from upstream to downstream lakes, with the reduction in pollution levels in upstream lakes facilitated by interconnectivity of lakes.

The study highlights the urgent need for an integrated approach following hydrological units, rather than the currently adopted administrative or lake-centric units, to effectively manage interconnected lakes and their catchments. A lake series approach addresses spatial interdependencies and cascading impacts, essential for sustainable lake management and water security in urban, water-stressed regions like Bengaluru.