Numerical quantification of current status quo and future prediction of water quality in eight Asian megacities: challenges and opportunities for sustainable water management.

Finite freshwater sources are facing huge threats both for quality and quantity from uncertain global changes, namely population growth, rapid urbanization, and climate change. These threats are even more prominent in developing countries where institutional capacity of decision-makers in the field of water resources is not sufficient. Attention of scientific communities to work on adaptation barriers is increasing as the need for global change adaptation becomes apparent. This paper presents a comparative study of assessing the current water quality as well as predicting its future situation using different scenarios in eight different cities of South and Southeast Asia. The idea behind this transdisciplinary work (integrated use of hydrological science, climate science, social science, and local policies) is to provide scientific evidence to decision-makers to help them to implement right management policies at timely manner. Water Evaluation and Planning (WEAP), a numerical simulation tool, was used to model river water quality using two scenarios, namely business as usual (BAU) and scenario with measures. Water quality simulation was done along one representative river from all eight cities. Simulated results for BAU scenario shows that water quality in all the study sites will further deteriorate by year 2030 compared to the current situation and will be not suitable for fishing category as desired by the local governments. Also, simulation outcome for scenario with measures advocating improvement of water quality compared to current situation signifies the importance of existing master plans. However, different measures (suggested upgradation of wastewater handling infrastructure) and policies will not be sufficient enough to achieve desirable river water quality as evident from the gap between concentration of simulated water quality and desirable water quality concentrations. This work can prove vital as it provides timely information to the decision-makers involved in keeping inventory for attaining SDG 6.0 in their regions and it also calls for immediate and inclusive action for better water resource management.