Abstract
Sustainable management of River Water Quality (RWQ) in tropical river systems requires an approach that addresses both spatial and temporal risks associated with hydrological dynamics, anthropogenic influences, and ecological health. This study develops an uncertainty-aware spatiotemporal risk assessment approach integrating upstream hydrological dynamics (SWAT), operational variability with Environmental Flow (Eflow), water quality simulations (QUAL2K), and probabilistic Water Quality Index (WQI) calculations. Block-Bootstrap ensembles and Monte Carlo techniques are used to quantify uncertainty propagation and construct confidence intervals within the model chain. The monthly WQI estimates under the different Eflow and pollution control scenarios are used to assess the spatial risk variability of the river system. Temporal risk was assessed with WQI using various probabilistic measures, including mean, variance, loss probability, entropy, mean excess loss, and value at risk. A unified risk ranking was developed by using Borda and Copeland aggregation techniques. Categorised spatial risk maps were created using GIS by integrating Eflows and ecological index. The results revealed significant seasonal variations in water quality, with April, March, and May identified as high-risk months due to increased pollution levels. The monthly WQI-mean in the Monte Carlo analysis (1000 iterations), as per the severity of the risk, April, March, and May months are identified as the riskiest months. Spatial risk mapping revealed distinct high-risk zones and highlighted the necessity of pollution treatment level of 25-50 % to reduce ecological risk under optimal Eflow regimes. This comprehensive framework underscores the need for both flow regulation and pollution management to protect river ecosystems, providing actionable insights for effective river health protection.