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This paper aims to develop new, resilience type metrics for long-term water resources management under uncertain climate change and population growth. Resilience is defined here as the ability of a water resources management system to ‘bounce back’, i.e. absorb and then recover from a water deficit event, restoring the normal system operation. Ten alternative metrics are proposed and analysed addressing a range of different resilience aspects including duration, magnitude, frequency and volume of related water deficit events. The metrics were analysed on a real-world case study of the Bristol Water supply system in the UK and compared with current practice. The analyses included an examination of metrics’ sensitivity and correlation, as well as a detailed examination into the behaviour of metrics during water deficit periods. The results obtained suggest that multiple metrics which cover different aspects of resilience should be used simultaneously when assessing the resilience of a water resources management system, leading to a more complete understanding of resilience compared with current practice approaches. It was also observed that calculating the total duration of a water deficit period provided a clearer and more consistent indication of system performance compared to splitting the deficit periods into the time to reach and time to recover from the worst deficit events.
Ten different metrics that could be potentially used to characterise the resilience of a water resources management system to uncertain future changes (i.e. a given scenario of supply and demand) were investigated. An in-depth analysis of the metrics was carried out on a real-world case study of Bristol Water’s supply system, including an examination of metric sensitivity and correlation, and a detailed examination of the behaviour of water deficit periods. The results obtained lead to the following key recommendations for the selection of an appropriate resiliencebased performance metric:
1. Simultaneous use of multiple metrics covering different aspects of resilience and related water deficit events is recommended. This does not seem to be the case currently in the literature (where typically a single metric is used to assess water system’s resilience) nor in engineering practice (where quantitative resilience metrics are not used much, if at all).
2. Metric M2 (“the duration of longest water deficit period” metric) was observed to be one of the more informative and comprehensive performance metrics followed closely by metric M7 (“the water deficit of greatest magnitude recorded” metric). However, the analysis demonstrated a relatively high correlation between the two metrics and therefore considering just a single metric may prove sufficient to evaluate the resilience of a water resource system. A duration based metric would be a more logical assessment metric to use of the two types, as it is the duration of temporary water restrictions that most impact on customers and supply, whereas the magnitude of water deficit events is of less direct concern to customers and water companies so long as the magnitude is maintained within acceptable threshold levels. All this seems consistent with the resilience metric suggested by Hashimoto et al. (1982) now supported with actual evidence.