By Ryan Moin

California’s ongoing struggle to provide enough water to meet increasing demand from our communities, agriculture and industry is not a secret.  But did you know that one of the greatest threats to California’s future water supply is linked to climate change?

Melting snowpack from the Sierra Nevada mountain range in the eastern part of California has historically played a vital role for California’s water supply during the summer months. 

Snowpack is unique as a water resource because the time from when the snow falls, to when it melts and runs off into streams is delayed thanks to freezing winter temperatures in the mountains.  Sierra Nevada mountain precipitation which falls above the freezing line therefore creates a frozen reservoir, which holds the water until rising temperatures in spring and summer cause the snow to melt, bringing water to our storage and conveyance systems when demand is highest. 

The scale of the water resource contribution from the Sierra Nevada to the state is also notable.   The Sierra Nevada region covers one quarter of California’s land area, and yet provides 60% of the state’s fresh water.  Half of this contribution, or 30% overall, is in the form of snowpack.  The Hetch Hetchy reservoir, which provides much of Silicon Valley’s water, is fed by the Tuolumne River, which is in turn fed by Sierra Nevada mountain runoff.    

Tuolumne River is fed by Sierra Nevada snowpack melt. Photo credit: US Geological Survey

Global warming threatens to greatly diminish the Sierra Nevada snowpack resource.  It is estimated that if the “Business as Usual” status quo with respect to greenhouse gas emissions remains, the Sierra Nevada region could see a seven to ten degree Fahrenheit increase in temperature by the end of the 21st century.  Such a scenario would lead to the loss of at least two thirds of the Sierra snowpack volume when compared with end of 20th century levels.  Snowmelt runoff would also come earlier in this scenario, to the tune of around 50 days according to estimates.  This would diminish Sierra Nevada runoff as a water resource during the warmest months of the year, when California agriculture has its greatest water demand. 

A logical question in response to these warnings could be:  Can we not capture and store this water, which will fall as rain instead of snow, as well as the earlier snowmelt runoff? 

The problem with this idea is that earlier and flashier runoff is more difficult to capture and store with our current infrastructure than a steady and dependable flow from gradual snowmelt.  Water managers would have to allow some of the heavier flow to go downstream rather than risking over-filling of reservoirs, which could present a flood-risk to nearby development.

There is however good news regarding the potential future of our Sierra Nevada water source.  Under the “Mitigation” scenario, where the world acts in the near future to reduce emissions as envisioned in the 2015 Paris Agreement, end of century negative impacts to our Sierra Nevada water supply will be cut in half across critical parameters such as temperature change, loss of snow-covered area, loss of snowpack volume, and change to runoff timing.   

It is also important to note however, that we are already committed to some climate-change induced impacts on our water supply because the climate system takes time to respond to changes in greenhouse gas emissions.  In other words the climate will warm in the coming decades regardless of corrective actions the world takes, but we can cut those impacts in half by the century’s end if we start acting now.  It is for this reason that in addition to our actions on climate, which will pay dividends in the long term, we should also be prepared to adapt for inevitable climate change impacts in the near future.

So what can California do to adapt for impacts of Sierra snowpack loss on our water supply?

More dams are probably not the answer.  It takes time, and significant capital to build new water infrastructure. Almost all California rivers have already been dammed, and the remaining potential sites are not desirable when one considers the economic expense relative to the amount of water that could be gained. 

One possibility is underground storage.  Sierra storm water could conceivably be diverted to open fields where it could percolate into groundwater aquifers.  This could have the dual benefits of recharging badly overdrawn groundwater, and restoring some of the wetland habitat that has been lost in the Central Valley.  This method could yield more storage capacity than new surface reservoirs, but water rights issues could be a problem. 

Flooding open Central Valley fields with Sierra storm water could help recharge groundwater and restore lost wetland habitat. Photo Credit: Bob Wick, Bureau of Land Management.

Coastal cities could also increase their resilience to loss of Sierra water by making more resourceful use of local water supplies such as rainfall, rather than allowing much of it to drain into the ocean.  This would entail policies and infrastructure that would enable storm water capture, water recycling, and water conservation. 

Individual actions also add up, and by serving as an example to your family, friends, and neighbors, you can maximize your own contribution.  Consider the following:

  • Save water and energy at home.  You will contribute to your community’s water resilience, lower your carbon footprint and save on utility bills.
  • Check with your local utility about energy efficiency upgrade programs and rebates on efficient appliances.
  • Find out if your home is compatible with a rain-capture or grey water system to provide water for outdoor uses.
  • Consider switching to native, drought-tolerant landscaping.  Native gardens save water, and provide habitat for local wildlife.
  • Utilize transportation alternatives.  The largest share of California’s greenhouse gas emissions comes from our cars.  One can drive less, take public transit, or consider walking or biking for short trips. 
  • Spread the word about climate change impacts by discussing the subject with your family, friends and peers.
  • Participate in California’s political process, by engaging with your local representatives by phone or social media, and remember to vote!