Climate change uncertainty challenges water supply planning (Gleick, 1989(Gleick, , 2000. Infrastructure systems, which are designed to last for many decades, must perform well under highly uncertain conditions (Cosgrove & Loucks, 2015;IPCC, 2022). Traditional water supply planning approaches usually oversize water infrastructure using a safety factor to account for uncertainty and reduce the chance of system failure (Stakhiv, 2011). However, overbuilding adds costs and environmental impacts, which are especially detrimental in more resource-scarce regions. One strategy to plan under uncertainty is flexible planning. Flexible, or adaptive, planning approaches can help mitigate infrastructure over-or underbuilding by changing the design or operations of infrastructure to respond to evolving conditions over time (Bertoni et al., 2021;Culley et al., 2016;Hui et al., 2018). For example, flexible plans may respond to hydrologic shifts, like a climate trending drier (Kumar et al., 2013), or changing societal values, such as a focus toward sustainability and ecological benefits (Kermisch & Taebi, 2017). Flexible plans also have downsides. They require identifying signs of system vulnerability (Dewar et al., 1994;Haasnoot et al., 2013) to trigger adaptations (Walker et al., 2001), which assumes that we can identify a reliable signpost for when to adapt. This may not be the case in many systems (Raso et al., 2019). Flexible plans also are expensive and may require additional costs for monitoring systems or repetitive structural elements to allow for future adaptations (
