Development of a modelling framework for optimal sequencing of water supply options at the regional scale incorporating sustainability and uncertainty

As urban populations around the world continue to grow, achieving sustainable urban water supply is becoming crucial in order to meet increasing demand. Cyclical drought and climate change have placed further strain on water supplies and it is therefore necessary to plan to ensure the sustainability of water supply systems at a regional scale. Water supply sustainability planning (WSSP) necessitates the use of a long-term planning horizon and multiple criteria for assessment, both of which have inherent uncertainties and difficulties. The perceived inability to account for great uncertainties in long-term future demand, economic stability, climate and technological change when it comes to the supply and usage of water has resulted in planning dominated by short-term and politically motivated decision-making. In response, this paper outlines a proposed conceptual framework to optimally sequence water supply projects, incorporating sustainability and uncertainty, at the regional scale. Sequencing of water supply projects involves choosing which options to implement at which stages over a planning horizon. In the past, the sequencing of water supply projects was straightforward, as there were relatively few options (e.g. when to build the next reservoir) and the only criteria that had to be considered were water supply security and cost. However, as a result of climate variability, climate change and the increased adoption of sustainability principles, the problem of sequencing water supply projects has become much more complex. With the increase in urbanization, urban infill and diminishing natural water sources, there is an increase in the need to consider recycled water and desalinisation as additional or alternative water supply options. Therefore, where 30 years ago only reservoirs might have been considered when planning a secure water supply for a city, in the 21 century a large number of alternative water sources are able to be considered in the sequencing approach presented in this paper. These include desalination, stormwater re-use and rainwater tanks. The proposed model framework is based on multi-objective optimisation, so that a number of competing objectives (e.g. cost, greenhouse gas emissions) can be taken into account. The aim is to find optimal mix of water supply options, and when these projects should be implemented, can be identified from among the large number of options (e.g. rainwater tanks, different stormwater schemes, desalination etc.). Long-term planning is an important aspect of sustainable water supply systems. When planning for water supply systems for the long term future in climate and demand, as well as economic and technological conditions, are highly variable and hard to predict. These factors affect the performance of water supply systems, and thus uncertainty is accounted for in the proposed model framework. The optimisation model takes the robustness of solutions into account, so that the decisions made now will be as insensitive to future changes (e.g. population growth, climate change, new technologies) as possible. Uncertainty is the key feature of most water resource system planning related to temporal variability. To ensure, therefore, that the water resource system operates in a satisfactory manner over time, continual reassessment of the system is essential in order to identify and reduce the risk of system failure. Hence, the model framework allows long term plans to be reviewed and re-optimised at regular intervals to take account of changes in external factors.