California’s water quantity, quality, and infrastructure are already experiencing climate impacts. The warming climate reduces winter snowpack, intensifies drought, and drives more intense storms that worsen flooding. Each region of California will be affected differently, but the changing climate risks make it increasingly important to update water infrastructure and management, restore and reconnect aquatic habitat, manage groundwater sustainably, and build the resilience of the most vulnerable communities. Disadvantaged communities have challenges securing safe water and sanitation and often lack the capacity to deal with additional declines in water quality and availability in the face of climate change. Investment and planning to address water access and quality for these communities is critical for public health and the overall well-being of California’s communities. For additional background information and discussion of climate impacts and vulnerabilities visit the Background & Climate Impacts information excerpted from the Adaptation Planning Guide and explore the Topic search below.

Building resilience for California’s water systems requires comprehensive action across all components of the water cycle, from restoring upper watersheds to recycling wastewater. Examples of adaptation strategies are provided on the Adaptation Strategies tab and can be found by search below.”

The following is excerpted from the 2020 Adaptation Planning Guide, Appendix A.

Defining the sector

The water sector covers fresh water in all its forms. Fresh water is the lifeblood of the state, affecting every activity, person, and ecosystem. This sector includes the timing, form (snow or rain), magnitude or absence of precipitation events, and the quality of the water available to ecosystems and for use by people. California has always had a variable hydrological system with periods of heavy rain, snow, and drought. The timing of these events and our ability to harness at least some of the resources or rein in some of the impacts influences the availability and quality of drinking water, viability of agriculture, health of ecosystems, and safety of communities.

Heavy rainfall and rapid snow melt result in flooding. Floods pose a direct public safety threat to all homes, businesses, utilities, and communities in or near a floodplain. They also result in erosion events and altered water quality. Floods occur based on regional weather patterns, existing river conditions, geology, and the patterns of human development.1 Every county in the state has been declared a federal disaster area due to flooding in the last 30 years.2 Flooding threatens lives and assets totaling billions of dollars. In the future, climate change may result in large atmospheric river events that deliver substantial precipitation in very short periods of time, further impacting California communities3. Human development has worsened flood risk due to activities such as encroachment on floodplains and introduction of impervious surfaces such as roofs, roads, and parking lots, which increase runoff. In coastal areas, ocean waters can cause inundation. This impact is covered in the coastal sector.

California has a long history of flooding, but it also has a legacy of recurring droughts. Droughts are prolonged periods of little or no precipitation, whose extreme conditions impact the state. Besides threatening drinking water supplies and water available for natural systems, droughts particularly distress the agricultural sector, which must rely more heavily on a dwindling groundwater supply during drought events. This combination of drought and greater reliance on irrigation stresses groundwater supplies. Groundwater levels have been dropping in California for decades, and the rate increases with each dry period. The Sustainable Groundwater Management Act was passed in 2014 to limit unrestricted groundwater withdrawal and establish a long-term, sustainable balance between groundwater withdrawal and recharge. Further complicating the groundwater supply is a history of industrial and agricultural water pollutants entering this supply, making groundwater use more challenging without treatment.

The quantity of available water is only part of the challenge in many parts of California. Water quality also inhibits use by communities, industry, and ecosystems. One often-overlooked water quality impact is temperature—increasing temperature in waterways is associated with lower dissolved oxygen and other negative impacts to species such as salmon. Other water quality challenges are due to past land uses that left a legacy of water pollutants in both surface and groundwater such as industrial and agricultural uses. Currently, pollutants of concern are nonpoint sources that include fertilizers and pesticides from agricultural and residential areas; oil, grease and toxic chemicals from urban runoff; and sediment from improperly managed construction site, and crop and forest lands. Drought and increased rates of evaporation caused by warmer temperatures serve to condense pollutants in water, making the resulting water quality more detrimental to its eventual users.

The rise (rainfall and snowmelt) and fall (drought) of water availability in California means water providers in the state should identify ways to ensure a consistent, ongoing supply of clean water regardless of rainfall or snowmelt. The snow that accumulates in the Sierra is a primary source of the state’s water, both for drinking and for irrigation.4 How long the snowpack stays in the mountains before it melts in spring, when rainfall rates begin to drop, is critical to balancing water supplies and assuring that groundwater use does not surpass recharge rates.

Aquatic ecosystems rely on the same water that serves communities, agriculture, and industry. The impacts on these ecosystems is covered in the biodiversity sector.

Major Vulnerabilities

Climate change causes the weather to become more variable with longer and more frequent droughts, as well as increased periods of heavy rain and snow events. This results in an intensification of impacts on all water-related or dependent activities and systems. Addressing the threat of climate change to California aquatic and water systems is a state priority, as noted in declarations such as Executive Order N-10-19.5

Climate change slowly alters seasonal patterns of rainfall and temperature and makes extreme events such as drought and heavy rainfall more severe and frequent. Less snowpack that melts earlier, and more precipitation falling as rain rather than snow threatens state water supplies. Changes in temperature and water availability also alter water quality by increasing evaporation rates and decreasing total precipitation levels, concentrating water contaminants in the remaining water. All of these effects can affect water supplies, groundwater levels and associated land subsidence, water quality, and erosion. They also interact with other climate effects to affect the water system. With higher temperatures, people and plants dry out faster, resulting in higher water demand from both households and agriculture. Higher temperatures also change the rate of chemical reactions in water and reduces water’s ability to hold oxygen. These factors reduce the viability of water to support wildlife and ecosystems. Communities that rely on smaller water treatment systems may be especially affected, as these systems may not be able to effectively treat heavily contaminated water.

In coastal areas, sea level rise combined with groundwater withdrawal results in sea water intrusion, making coastal aquifers less suitable for human uses, including agriculture. Sea level rise also increases flood risk for coastal areas by expanding the width of floodplains at the outlet of rivers and streams and backing up stormwater systems that drain to the ocean. Together these changes increase flood risk and drought impacts and impair ecosystem health.

Flooding—from extreme rainfall events and/or rapid snowmelt and changed land use patterns—already places billions of dollars in assets and the lives of millions of Californians at risk.6 Climate change will increase the severity and frequency of flooding. Locally, this means that communities will face rising flood risk with expanding floodplains and increased frequency. The growing risk demands action at all levels of government. Local governments should adjust their disaster management and local hazards plans to prepare for worsening risk to both structures and community members. Floods also impact aquatic ecosystems through erosion and water quality shifts. Increased intensity and frequency of extreme rainfall also means that local stormwater systems should be evaluated for their ability to accommodate larger events due to climate change as well as changing land use patterns. Without adjustment of stormwater systems, localized flooding will occur with greater frequency. In coastal areas, rising sea levels further exacerbate the challenges for local stormwater systems because higher sea levels have the potential to back up stormwater into city streets.

Climate change is projected to increase the frequency and severity of drought events. This places more pressure on groundwater, which already supplies 40 percent of California’s water.7 The past reliance on groundwater during times of drought is not sustainable—groundwater levels have been dropping for decades and now have state protection.8 Further complicating the challenge of water supply is the projection of reduced snowpack. Less precipitation in the Sierra Nevada, or more rain than snow, diminishes the spring snowmelt, which is a primary source of the state’s water supply. Reduced snowpack, reduced precipitation, and increased drought threaten water supply for drinking, agriculture, and ecosystems. To shore up supply for drinking and agriculture, communities should work with water providers and rural landowners to develop strategies for drought years.

Aquatic ecosystems such as streams, rivers, and lakes will also be affected by climate change, as will any ecosystem that relies on water. Water levels in streams and rivers will change with altered seasonal precipitation patterns. High flows will be higher and low flows may be lower. This results in alteration of habitats, water temperature, and other water quality indicators. These changes affect all species residing in these aquatic ecosystems. Industries from hydropower to fisheries to recreation will also be affected by the increasing variability introduced by climate change. Vegetation is responsive to altered precipitation rates, which has tremendous impact on habitats and terrestrial ecosystems (covered in the Biodiversity sector).

The Adaptation Strategies tab of this topic, excerpted from Appendix D of the 2020 APG, provides examples of ways that communities can support a more resilient water sector as part of adaptation planning effort. These strategies are generalized approaches that can be refined for the specific activities in a community.


1. California Department of Water Resources, California’s Flood Future: Recommendations for Managing the State’s Flood Risk, 2013, California_Flood_Future.pdf.

2. California Water Boards, Groundwater Basics, 2019, water_issues/programs/groundwater/gw_basics.html.

3. E. Smith, “Climate Change May Lead to Bigger Atmospheric Rivers,” 2018, NASA Global Climate Change,

4. California Water Boards, Groundwater Basics, 2019, water_issues/programs/groundwater/gw_basics.html.

5. State of California, Executive Order N-10-19, 2019, uploads/2019/04/4.29.19-EO-N-10-19-Attested.pdf.

6. California Department of Water Resources, California’s Flood Future: Recommendations for Managing the State’s Flood Risk, 2013. California_Flood_Future.pdf

7. California Water Boards, Groundwater Basics, 2019, water_issues/programs/groundwater/gw_basics.html.

8. Tara Moran and Dan Wendell, “The Sustainable Groundwater Management Act of 2014: Challenges and Opportunities for Implementation,” 2014, http://waterinthewest.stanford. edu/sites/default/files/WitW_SGMA_Report_08242015_0.pdf.

Climate hazards
Adaptation Strategy
Factors to Consider
Sector overlap
Responsible Agencies
Examples & Sources
Flooding, drought

Strategy W-1: Reduce flood and drought risk through integrated watershed management.

This is a high level, integrated approach to balancing water availability (both surface and groundwater), demand, and an ecosystem’s minimum water needs to ensure a balanced consideration of economic, social, and ecosystem welfare. Healthy watersheds where development is planned to minimize impact and use and water sources are carefully managed to assure long-term, sustainable supply are more resilient to both variable, intense rainfall and drought. A healthy watershed maintains wetland areas as flood mitigation and maintains undeveloped natural areas, promoting soil health to blunt flood impacts and to assure greater resilience to drought.

Each city should assess its context in a watershed and, if needed, engage in a collaborative relationship with other entities in the local watershed. This should include collaboration with various departments in a community from planning, to natural resources, to water providers, and more. Conservation measures should be pursued to preserve wetlands, particularly upstream of a community, and manage undeveloped areas. Planning should work to ensure that floodplains remain intact with minimal development and have safety margins that expect expanded flood extents. Cities can collaborate with water providers to encourage increased water efficiency (e.g. graywater systems, reduced outdoor use, low flow fixtures) and to diversify water sources. Operational Programmatic Agriculture Biodiversity Land Use and Community Development Cities and counties (planning, hazards, & natural resource departments) Water providers General Fund DWR Integrated Regional Water Management Grant Programs DWR. “Integrated Regional Water Management” The Water Institute. “Focus on Integrated Watershed Management”
Flooding, drought

Strategy W-2: Reduce local flooding through implementation of low impact development.

This strategy is specific to the stormwater management system in a community. The approach through installation of bioretention elements in parking lots and on the street margin. It can be implemented through green infrastructure plans, landscape codes, green street standards, and off-site standards. The approach seeks to mitigate the local flooding resulting from the effects of climate change and impervious surfaces on runoff events. These strategies also increase groundwater recharge and moderate increased streamflow in the receiving waterways.

This requires cities to assess the underlying soils, geology, and water table to assure infiltration is possible. Strategies should be developed appropriate for the local conditions. It is best to combine these approaches with road paving schedules and implementation of road diet approaches as the reduction of lane frees space for bioretention along the road margin in combination with other amenities such as sidewalks, bike lanes, and bus stops. Plans, Regulations, and Policy Development Capital Improvement/Infrastructure Project Biodiversity Land Use and Community Development Cities and counties (utility dept.) General fund Capital improvement fund State Water Resources Control Board: Stormwater Grant Program Central Coast Low Impact Development Initiative (CCLIDI). “Municipal Codes” from State Water Resources Control Board: Stormwater Strategy

Strategy W-3: Increase water efficiency and conservation.

Cities, on their own or through collaboration with water providers, encourage increased water conservation and efficiency (e.g. graywater systems, reduced outdoor use, low flow fixtures). This is a key strategy to facing increasing frequency of drought conditions as it allows existing water supplies to stretch a further. Such efforts can be implemented through a variety of tools from turf replacement programs to building codes.

As assessment primary water uses in a community can help narrow and focus the measure chosen such as communities with high levels of outdoor use can pursue strategies to limit the amount used and type of water used (e.g. gray or recycled). This can be promoted through adjustments to a tiered fee system, outreach, turf replacement programs, or building requirements for low-flow fixtures. The fee-based options require careful assessment of impacts on lower income community members. Plans, Regulations, and Policy Development Education/ Outreach/ Coordination Land Use and Community Development Cities and counties General fund DWR Water Use Efficiency Grants DWR. “Water Use & Efficiency” from
Flooding, drought

Strategy W-4: Upgrade water and wastewater systems to accommodate projected changes in water quality and availability.

Under future conditions, existing water and wastewater systems may not be able to meet community demand. For example, wells and intake systems may be too shallow to effectively pull enough water supplies from groundwater aquifers and surface water bodies, higher levels of water contaminants may exceed the capacity of water treatment systems, and water storage tanks may not be able to hold enough water to meet demand if there is a supply interruption. This can be a particular challenge for smaller systems. Making retrofits to these pieces of infrastructure in advance of future conditions can help communities be better prepared and ensure that there are fewer disruptions to water and wastewater services.

When upgrading existing water and wastewater systems, it is critical to identify the specific needs of the systems under future conditions, and what upgrades are required to ensure these needs are met. As with any engineering project, retrofits should be designed to err on the side of caution in case future conditions are more extreme than projected, without overengineering retrofits to the point of wastefulness. Retrofits are also a prime opportunity to consider other upgrades not directly related to water availability and quality, but which provide other adaptation benefits, such as energy efficiency and renewable energy. Capital Improvement/ Infrastructure Project Public Health Water and wastewater service providers General fund Capital improvement fund Water and wastewater infrastructure improvement grants State Water Resources Control Board: Clean Water State Revolving Fund Water and Wastewater Infrastructure Improvement Grants  

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