Cal Water Climate Change Risk Assessment

California Water Service (Cal Water) is the largest regulated water utility in the western United States, serving over 2 million people in 24 different districts throughout California. To assess how vulnerable water supply sources for Cal Water’s 24 districts could be to impacts from climate change, Cal Water engaged with Carollo to develop a Climate Change Risk Assessment and Adaptation Framework. The Framework included a review of existing vulnerability assessments and adaptation planning efforts, a vulnerability assessment of Cal Water’s infrastructure, operations, and services, and an assessment of climate change impacts to water supply and demand. This risk assessment incorporated new and existing projections for climate change impacts to supply for surface water, groundwater, and imported water sources.

Carollo evaluated climate change impacts to supply for three different time horizons (early-century, mid-century, and late-century) using up to 10 global climate models (GCMs) and two emission scenarios (RCPs) for each supply source. This allowed for the inclusion of a range of possible future outcomes for the risk assessment. The risk assessment was based on supply vulnerability metrics that were developed for average annual change in supply, change in driest year supply, maximum drought duration, and dry year frequency across all districts and all water supplies.

The resulting data created a decision support framework that highlighted which Cal Water supplies were most vulnerable to climate change, empowering the utility to prioritize mitigation actions for its most vulnerable supplies.