Troubled Waters

Preparing for Climate Threats to California’s Water System

Geeta Persad, Jose Pablo Ortiz Partida

Published Oct 7, 2020

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Cover page of Troubled Waters report

Water is core to California’s way of life.

But as climate change causes more volatile precipitation, less snowpack, more flooding, higher temperatures, and shorter wet seasons, the water system will increasingly fail to meet the needs of California’s communities, industry, and agriculture.

New analysis by the Union of Concerned Scientists shows that the data and certainty needed to quantitatively plan for climate change in California water management are available now.

This report outlines the key shifts in California’s hydroclimate; illustrates the risks for California’s residents, businesses, and agriculture if these shifts are ignored; and describes how the state can jumpstart the comprehensive climate planning that is needed.

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Troubled Waters

Water is California’s connective tissue. More than 1,300 federal, state, and local surface reservoirs across California capture precipitation, snowmelt, and runoff. Thousands of miles of canals, natural waterways, and pipes bring that water to the state’s 40 million residents, 10 million acres of irrigated farmland, and thriving industries. The state’s 515 groundwater basins supply additional water year-round, acting as a vital buffer during dry periods. However, this highly engineered and interdependent system, which has enabled so much of California’s vibrancy, is already stressed—by rising demand for water, aging infrastructure, and extreme cycles of drought and flooding. Now, climate change threatens to break California’s water system altogether, creating new vulnerabilities for which infrastructure and institutions are unprepared.

Yet California continues to make water management decisions based on the past—often because climate change is seen as too uncertain, too distant, or too difficult to integrate into decision-making. In its slowness to act, California has lost valuable time. The delay risks intensifying the existing water inequities between well-resourced communities that have the ability to pay for their own climate risk analysis and adaptation, and the already vulnerable communities that do not. Change is urgently needed.

New analysis by the Union of Concerned Scientists (UCS) and its collaborators demonstrates that climate change is transforming how, when, and where California gets water—its hydroclimate—changes that can be measured with enough certainty to enable strategic climate and water planning (Persad et al. 2020) (Figure 1). This analysis highlights several critical challenges that climate change will create for California’s water management within this century, what they mean for managing the state’s water, and how the state and water management community can respond:

*Climate change will transform key aspects of how, when, and where California gets its water. Precipitation will arrive increasingly as rain rather than snow, occur in more intense events, and be concentrated into the already wet winter months. Volatility between overall dry and wet water years will increase. Snowpack will decline dramatically

*__Climate change projections agree on a range of critical shifts that need to be accounted for in all California water decisionmaking.__Climate change datasets developed for the state climate assessment show almost universal agreement on these transformations in California’s water. But most federal, state, and local water planning does not account for this critical information.

*State agencies and water managers are underprepared for the water management challenges of California’s altered hydroclimate. The projected impacts of climate change are likely to damage the viability and sustainability of California’s surface reservoirs and increase demands on its groundwater aquifers. Water planning based on historical conditions misses these critical impacts. If water planning continues to fail to account for the full range of likely climate impacts, California risks wasted water investments, unmet sustainability goals, and increased water supply shortfalls.

*__Ensuring the resilience of communities in the face of California’s new hydroclimate will require transformational, but achievable, change in approaches to water management and decisionmaking.__The state’s altered hydroclimate will require California to become more flexible in how it uses and manages water. The climate data and water management expert communities need to become regular collaborators, working together to develop new planning protocols and operations models able to take all future climate-changed conditions into account. State and local water regulations, including the landmark 2014 Sustainable Groundwater Management Act, need to be updated to require comprehensive climate planning.



Persad, Geeta, Jose Pablo Ortiz Partida, Daniel Swain. 2020. Troubled Waters: Preparing for Climate Threats to California’s Water System. Cambridge, MA: Union of Concerned Scientists.

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