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Home > Natural Resources > Water Strategies to Preserve Natural Resource Supplies and Quality

Aerosol pollution from urban areas is reducing precipatation in California

Aerosol pollution from urban areas is reducing precipatation in California

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APRIL – WATER for California Water is a vital part of California’s “greenscape”, be it landscaping, agriculture or natural playgrounds. Even industries rely on water for processing. And of course, people, wildlife and plants all require water. That’s the “need”. The situation is that our fresh water supply is being endangered from a number of directions.
  • Population explosion requires more fresh water
  • Industrial applications are using more water
  • Agriculture relies on irrigation
  • Landscaping uses water
    And now…
  • Climate changes are expected to decrease the amount of water currently provided by natural systems.

And now we have another cause for concern: urban and industrial air pollution in California is impacting precipitation and stream flows.

The March 2007 report by The California Energy Commission's Public Interest Energy Research (PIER) Program is now available online to show the research results.

"Physical / Statistical and Modeling Documentation of the Effects of Urban and Industrial Air Pollution in California on Precipitation and Stream Flows" (Download Report in PDF, 167 pages, 19.7 meg)

Executive Summary

Most climate change investigations focus on the role of greenhouse gases in global warming and on the role of minute particles in the atmosphere (atmospheric aerosols) that cool the atmosphere by reflecting some of the incoming solar radiation back to space. Atmospheric aerosols were thought to counterbalance global warming; however, this is not their only role.

Recent research, funded by the California Energy Commission with Dr. Daniel Rosenfeld and Dr. William Woodley along with their research team, focused on California-specific analysis that indicates atmospheric aerosols can also impede rain and snow formation in the Sierra Nevada Mountains.

This study showed that high concentrations of extremely small (submicron) cloud condensation nuclei (CCN) aerosols (on which atmospheric water can form) slow the formation of raindrops and ice, which delays the conversion of cloud water into precipitation. This effect is exhibited in different ways in clouds – either strengthening deep convective clouds or suppressing the amount of precipitation from shallow clouds around mountains (orographic clouds). These short-lived orographic clouds are the most common in the winter on the west coast and are formed as air is forced upward when passing over barriers such as mountains. California’s annual precipitation losses over the mountains are projected at 10 to 25 percent, presumably because of the pollution aerosols that are transported from urban and industrial areas.

Project Purpose

The increasing loss of Sierra precipitation over the last 50 years was documented earlier in published studies by the research team.

According to the California Energy Commission, California depends on in-state hydroelectric power generation for approximately 15 percent of its total electricity supplies, and a loss or decline of this generation would be significant. The original research also suggests that as rainfall declines, California’s water supply, especially for agriculture, could be impacted. These losses have not noticeably affected California’s water supply because until now they have been masked in many places by an increasing trend of statewide precipitation.

This project builds on the previous work done by the research team and focuses on whether 1) the obvious precipitation losses results in reduced mountain streams flows and 2) modeling the aerosol effects on clouds and precipitation could identify the processes that produce mountain precipitation losses.

Project Results

For the first time, researchers quantified the losses of stream flows at approximately 15 to 35 percent of the annual stream flow volumes in major Sierra Nevada rivers that are downwind from urban areas. The percentage of reduced precipitation on the western or upside of a mountain correlates to similar precipitation increases on the much drier or eastern down-slope side of the mountain crest. The evidence shows that over the last 50 years, the runoff is declining in the more polluted central and southern Sierra mountain areas; however, there is no similar evidence for runoff losses in the relatively pristine rivers in northern Sierra Nevada. These results agree with an analysis of rain gauge records and suggest that human-caused air pollution has a major impact on orographic precipitation and on stream flow volumes, which are responsible for most of California’s water supplies. The research also found that no alternative explanations, such as long-term trends in cross-mountain moisture changes or climatic fluctuations were probable.

The research team found strong evidence that cloud microstructures had been altered, making them unable to form precipitation-sized drops and ice particles, which reduced precipitation and stream flow in the central and southern Sierra but not in the northern Sierra.

They also found the clouds in those areas downwind of the urban zones had increased concentrations of pollutants that led to more and smaller droplets that move to higher levels in the cloud-tops without producing large raindrops. The clouds moved over the central and southern Sierra depositing less precipitation on the western or California side.


This cutting edge research effort showed that increased aerosol pollution from urban areas over the last 50 years has reduced rain and snow formation in the central and southern Sierra Nevada Mountains. This decreasing water runoff directly impacts in-state hydroelectric generation and that California’s water supplies, especially for agriculture, could be jeopardized.

Edited by Carolyn Allen, owner/editor of California Green Solutions
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