I promised an assessment of the Hetch Hetchy reservoir inflows, outflows, and storage, and I intend to deliver something. I looked into the reservoir, from the data available from USGS to the Restore Hetch Hetchy website to the San Francisco Public Utility Commission’s official position on Proposition F to the California Department of Water Resources (DWR) 2006 study on restoration of the Hetch Hetchy valley. There are many things out there with opinions, but I intend to highlight the facts.
- Storage, inflows, and outflows. The Hetch Hetchy reservoir has stored an average of 282,100 acre-feet over the past 5 years, with an extreme maximum storage of 363,300 acre-feet and a minimum storage of 154,200 acre-feet. I can see a couple of jumps in the plots of storage vs. time that suggest the dam is occasionally operated to send water to other locations, so a true picture of the storage in the system would need to include all 8 reservoirs in the system and river flow at multiple locations. Honestly I don’t have time to get to this. But I can say that in the past 5 years, the SF utility system has used at least 363,000 – 154,200 = 209,100 acre-feet stored in the Hetch Hetchy reservoir. The other 7 dams owned by the San Francisco Public Utility Commission (SFPUC) have a combined storage of 537,600 acre-feet [Urban Water Management Plan, 2010]. So just to store the water that was stored in Hetch Hetchy and used in the past 5 years, you would need to use approximately 40% of the remaining reservoir space in the entire 7-dam network, and it’s not like those reservoirs are normally sitting empty.
- Water storage needs. The Restore Hetch Hetchy plan calls for water intake via pipes without the dam. The storage can be maintained by the rest of the reservoirs on the system. “Hydrologic analysis shows that it will be possible to fully meet system demands in 4 out of 5 years. In the driest years, 20% of system demands will need to be met from additional water storage or supply resources.” This is a direct quote from the Restore Hetch Hetchy website. Let’s examine this. One in every five years, this new Hetch-Hetchy-free system would experience a water shortage. Have you ever heard of a utility plan to have water shortages? We have utilities to provide reliability, and even then 20-year, 50-year, and 100-year events can overwhelm the infrastructure we have. A 20-year event is a disaster, rather than an inconvenience, when your planning basis is a 5-year event. Civil engineers like to design for 20-year events, at a bare minimum, with floods like the 1993 Mississippi River flood or the 2005 Hurricane Katrina disaster bringing calls for 500-year and 1000-year designs for water management (in those cases, we’re talking levees rather than dams, but droughts can be equally devastating).
- New supplies. The second half of the above quote addresses where additional water supplies will come from in the dry years: “additional water storage or supply resources.” Let me refresh your memory, San Francisco — California’s water supply is already overallocated. I just drove down I-5 this weekend and saw farmers’ billboards complaining about “Congress-created dust bowl” with cuts in water supply listed. Those weren’t Congress-created cuts, those were drought-created cuts combined with a federal judge upholding the primacy of the Endangered Species Act, specifically related to fish in the Bay-Delta, over California’s water allocations. There isn’t enough water for everything we already want to do. Please, Restore Hetch Hetchy, tell us where the water will come from. Water storage measures and water recycling facilities take time, money, and planning, and can’t happen overnight once the dam removal is underway.
- Climate change. Finally, I will just note that climate change is projected to decrease snowpack (nature’s water storage) in the Sierras and increase the frequency and duration of “extreme” events, both rainfall and drought. To remove excess storage from the water supply system in light of this very real challenge to California’s water supplies is very short-sighted.
That’s enough for now about water flows. Next time, I’ll tell you what the DWR’s own report recommended for the Hetch Hetchy restoration question. Hint: it’s not what’s in Proposition F.