Toward an Integrated Framework for Assessing Vulnerability and Risk to Compounding Global Change and Shocks: Application to San Francisco City and Bay Area Water System

Water resources managers face a need to improve or build new infrastructure to increase water system resilience to climate and demand changes. Natural hazards and other shocks put additional threats on urban water systems and could create short-term disruption of services. This paper introduces a modeling framework that combines the long-term vulnerability assessment of water resource systems to climate and demand changes, with a vulnerability assessment to short-term outages resulting from the occurrence of shocks. In addition, we propose a pragmatic approach that combines results from the long-term vulnerability assessment with imperfect probabilities of climate change to estimate the climate-informed robustness of the water system in question following implementation of adaptations, without requiring a new vulnerability assessment. The modeling framework is illustrated for the regional water system operated by the San Francisco Public Utilities Commission (California, United States). Tipping points of change in temperature, precipitation and demand under which the system fails are identified. The estimated climate-informed robustness is sensitive to the climate information used for the assessment (e.g., climate projections from general circulation models or climate expert elicitations), warranting further research to address this source of uncertainty. Results also show that the response to shocks is sensitive to current conditions and to underlying climate and demand conditions, which provides motivation to investigate further the relationship between the underlying conditions for these shocks and system performance.