GrokSurf's San Diego

Local observations on water, environment, technology, law & politics

San Diego-Sweetwater intertie could boost local reservoir storage by 100,000 acre-feet of water

Posted by George J Janczyn on April 18, 2011

Since the end of California’s severe 1987-92 drought it has taken a 1993 Army Corps of Engineers reconnaissance study, U.S. Senate and House legislative support from Rep. Duncan Hunter (Sr.), Rep. Susan Davis, and Sen. Dianne Feinstein, a California Proposition 50 grant via the Integrated Regional Water Management Plan, and negotiations between Sweetwater Authority, the City of San Diego, and the U.S. Bureau of Reclamation — plus another major drought — just to reach this stage.

This stage is a proposal for a pre-feasibility study of a four-reservoir intertie between the City of San Diego and Sweetwater Authority systems.

The proposal will be presented today (Apr 18) at the Independent Rates Oversight Committee (IROC) [agenda] and Wednesday (April 20) at the Natural Resources and Culture Committee [agenda] where support will be sought in bringing it to the San Diego City Council. If full Council approval is obtained an agreement will then be executed by the City of San Diego, Sweetwater Authority, and the Bureau of Reclamation.

As described in a San Diego Integrated Regional Water Management document:

“Connecting the San Vicente, El Capitan, Loveland, and Murray Reservoirs would create an enhanced and integrated reservoir system to more efficiently use the reservoirs and increase accessibility to approximately 100,000 acre-feet of surface storage without creating new reservoirs or new storage capacity. The environmental effects of the future conveyance system would be minimal because each reservoir has been in place since the 1940s or earlier, and reservoir footprints would not increase. The Sweetwater Authority is serving as the lead agency for the regional intertie project.”

[Update: at the IROC presentation it was announced that the lead agency has been changed from Sweetwater to San Diego]

Since that report was written, of course, one reservoir footprint is increasing due to the San Vicente Dam Raise Project currently underway that will give it a more significant role within the potential intertie system.

As things are now, overall reservoir capacity is underutilized. To give one example, San Diego’s El Capitan reservoir (110,120 AF) is rarely full because its small imported water supply connection has very limited ability to supplement local runoff. An intertie connection would facilitate more optimal reservoir levels.

[I’ve since learned that another reason El Capitan water levels often appear to be low is that its catchment area produces the greatest amount of runoff of all the reservoirs in the San Diego area, so the city intentionally keeps water levels low there in order to capture the maximum amount of water during wet years and not lose any to spillway overflow].

Further south, Sweetwater’s Loveland Reservoir (25,225 AF) relies solely on watershed runoff which is usually not enough to keep it filled. An intertie would allow adding more water during drier times. On the rare occasions that very wet seasons such as we recently experienced result in spillway overflow, excess runoff at Loveland could be diverted to the larger El Capitan or San Vicente reservoirs instead of being lost.

A bay in the San Vicente Reservoir. Water cascades down the fill chute from the San Diego Aqueduct tunnel portal. Water level has been drawn down while the dam raise project is underway.

San Diego’s much smaller Murray reservoir (4684 AF) was included because of its Alvarado Water Treatment Plant supplying a large part of the city.

The shared connections would permit more water overall to be stored in local reservoirs. That translates into increased reliability in the event of a cutoff of imported supplies due to disaster or other problem. The 1993 Army Corps of Engineers reconnaissance study of San Diego’s water supply (not available online*) also estimates that 23% of the total benefits from an intertie system would be from increased flood control.

Because an intertie system would be an immensely expensive and complicated undertaking, the pre-feasibility study would carry out “an appraisal investigation to determine the prudence of a full-scale feasibility study for the proposed intertie system” according to the Army Corps study.

(A subsequent Army Corps Executive Summary of Lessons Learned from the California Drought (1987-92) says one confirmed lesson is that “local and regional interconnections between water supply systems are effective and flexible options against severe water shortages.”)

If the pre-feasibility study recommends it, then a full feasibility study could be performed as a phase 2 project.

Funding for the pre-feasibility study will be shared between the U.S. Bureau of Reclamation ($344,332), state grant funding from Proposition 50 ($112,832), the City of San Diego ($171,500), and the Sweetwater Authority ($60,000) for a total of $688,664.


* Thanks to Marguerite S. Strand, Assistant General Manager of Sweetwater Authority for allowing me to examine the Army Corps reconnaissance study and answering my questions.


One Response to “San Diego-Sweetwater intertie could boost local reservoir storage by 100,000 acre-feet of water”

  1. K. Svet said

    Great article and background info. This was much more informative than the presentation given at the NR&C Committee. Will be interested in the cost per AF as a new source of water, and seeing how that would compare to ocean desalination and other potential local sources.

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