Robust optimisation of urban drought security for an uncertain climate
This study presents a methodology to find solutions for managing urban bulk water systems in the presence of deep uncertainty about future climate. Using a case study, the method shows that (possibly large) uncertainty about future climate may not necessarily produce significantly different performance trajectories. Sensitivity to climate change is determined not only by differences between climate scenarios but also by other external stresses imposed on the system, such as population growth, and by constraints on the available options to secure the system against drought. The methodology uses multi-objective optimisation to identify a set of “good” solutions that optimally trade-off expected performance against robustness or sensitivity of performance over a plausible range of future climates. A case study based on the Lower Hunter in New South Wales demonstrated the methodology, using optimisation to consider a range of options, including: desalination, new surface water sources, demand substitution using rainwater tanks, drought contingency measures and operating rules. The robustness (or conversely the sensitivity) of solutions to uncertainty about future climate varied considerably between the two demand scenarios modelled. For the “1.28 x demand” scenario (representing expected consumption in 2060) there was limited sensitivity to the “2070 “dry” and “wet” climate scenarios – in the worst case, the difference in present worth cost between the climate scenarios was less than 4% of the expected present worth cost of $360m. For the “2 x demand” scenario (representing a highly stressed system), the trade-off was considerably greater – in the worst case, the difference in present worth cost between the climate scenarios was over 40% of the expected present worth cost of $1092m.
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