We Can Have a Vibrant Economy AND Healthier Ecosystems

The State Water Resources Control Board is in the process of updating the Bay Delta Water Quality Control Plan as required by the Clean Water Act. The State aims to achieve the co-equal goals of ecosystem restoration and ensuring a reliable water supply. Phase 1 of the Plan focuses on the San Joaquin River and its three main tributaries –the Tuolumne, Stanislaus and Merced Rivers –and southern Delta salinity standards.

The Bay Delta Plan calls for requiring a percentage of unimpaired flow –what would occur in the absence of dams and diversions –to flow down the three main tributaries and into the San Joaquin River between February and June. The current recommendation is 40% of unimpaired flow, with flexibility to adjust it between 30% and 50%, depending on whether biological and environmental goals are met.

The San Francisco Public Utilities Commission (SFPUC) manages the Hetch Hetchy Water System, which provides water to 2.7 million people in San Francisco, San Mateo, Santa Clara and Alameda Counties. The SFPUC has opposed the revised Bay Delta Plan, claiming it would severely impact the Bay Area economy. This is not the case.

Could the SFPUC run out of water?

The Tuolumne River Trust (TRT) has modeled what would happen if the six-year drought-of-record (1987-92) were to reoccur and the revised Bay Delta Plan were in effect. Even assuming water demand rebounded to pre-drought levels, the SFPUC could manage the drought with water left over in storage. This means that if the last 100 years of precipitation repeated, the SFPUC would not run out of water.

TRT 6-Year Drought Model

(223 mgd baseline, 40% unimpaired flow Feb-June)

TRT 6-Year Drought Model[1]

Note that the model assumes there would be no rationing in years 1 and 2 of the drought (since we wouldn’t know we were in a drought yet), rationing would be 10% in years 3 and 4, and 20% in years 5 and 6, for an average of only 10% rationing.

Why is the SFPUC concerned?

The SFPUC has not challenged TRT’s drought model, and in fact, its own modeling has accepted the outcome. However, the SFPUC is planning for what it calls a “Design Drought” –an 8.5-year drought that arbitrarily combines the drought-of-record (1987-92) with the driest two-year period on record (1976-77).

Regardless of how much water the SFPUC has in storage, it considers every year to be the beginning, or middle, of its Design Drought. The SFPUC states:

Our Level of Service objective for water supply is to survive the drought planning scenario (1987-92 followed by 1976-77) with no more than 20% rationing from a total system demand of 265 MGD [million gallons per day]…We need to plan for each year as if it is the beginning of our drought planning scenario.[2]

In anticipation of the Design Drought, the SFPUC claims that under the Bay Delta Plan’s 40% unimpaired flow, with demand at 223 MGD, rationing would have to begin at 39% for the first three years, and then increase to 49% for the next three years. This would leave 576,000 acre-feet in storage at the end of the six-year drought –enough to last more than two years. If history repeated, all of its reservoirs would refill after the drought. The excessive rationing would have been unnecessary.

When lobbying others, the SFPUC has misled influential people and groups by not explaining its extremely conservative 8.5-year Design Drought, nor that it’s figures are based on 2040 demand projections aimed at accommodating aggressive development as forecast by Plan Bay Area, leaving the impression that the Bay Delta Water Quality Control Plan would have a much more severe impact than under a realistic scenario.

What if we did experience a longer drought?

In a worst-case scenario, the SFPUC could purchase water from an agricultural irrigation district for about the same amount as it charges its customers. According to a recent study by the Sacramento Area Council of Governments:

Additionally, while all agriculture contributes significant value to the regional economy, specialty crops generated around $2,700 per acre annually –more than three times the value per acre than non-specialty crops.[3]

It takes about 3.5 acre-feet of water to produce an acre of crops, so according to the figure above, one acre-foot of water would generate $771 in income. Then there’s the “multiplier effect”–revenue generated through processing and distributing agricultural products.

Specialty Crop Cluster Direct Output Value

Specialty Crop Cluster Direct Output Value[4]

Using the above percentages for the multiplier effect, one acre-foot of water used to grow specialty crops generates $2,659. Lower value crops generate considerably less. The SFPUC currently charges its wholesale customers about $2,200 per acre-foot.

In other words, the SFPUC could purchase water from an agricultural irrigation district at a price that could allow farmers, processors and distributors to make as much money as if crops were being produced, but they wouldn’t have to do any work.

What are the SFPUC’s water rights on the Tuolumne?

The Modesto and Turlock Irrigation Districts, which share the Tuolumne with the SFPUC, have senior water rights on the River dating back to 1887. As the junior water rights holder, the SFPUC is required to let the first 2,400 cubic feet per second (cfs) (imagine 2,400 basketballs full of water) flow past its dams to satisfy the Irrigation Districts’water rights. Between mid-April and mid-June, the Irrigation Districts are entitled to the first 4,000 cfs.

Tuolumne River Water Entitlements

The SFPUC’s water rights are poor in dry years, but exceptional in normal and wet years. According to the Substitute Environmental Document (SED) for Phase 1 of the Bay Delta Water Quality Control Plan:

The 1922-2003 average calculated volume of water potentially available to CCSF [City and County of San Francisco] under the Raker Act was about 750 TAF/y [thousand acre-feet per year].[5]

The SED explains the SFPUC’s water demand as follows:

According to a SFPUC planning document, an average of 244 TAF/yis diverted from the Tuolumne River…based on data from 1989-2005.[6]

In other words, in an average year the SFPUC has the right to capture three times as much water from the Tuolumne as it uses. This allows storage to replenish quickly after a drought. At the height of the recent drought, the SFPUC had enough water in storage to last three years. 2016 was an average water year, and the SFPUC was able to rebuild its storage to 80%. Then came 2017 –the second wettest year on record in the Tuolumne River watershed –and the SFPUC had the right to capture enough water to last 12 years. Obviously, they didn’t have enough storage capacity to capture even a fraction of that water, so most of it had to be “dumped”into the River to prevent future flooding downstream.

SFPUC Tuolumne Storage

SFPUC Tuolumne Storage Graph[7]

You’ll note that the largest block of water in the above graph is labeled “Water Bank.”This is water the SFPUC can “pre-pay”to the Irrigation Districts when its reservoirs are full and it still has the right to capture more water. The SFPUC helped pay for the construction of Don Pedro Reservoir, which is owned and operated by the Irrigation Districts and is downstream of the SFPUC’s reservoirs, in exchange for the ability to “bank”water there. The SFPUC cannot directly access this water, but the set-up allows it to capture water at Hetch Hetchy that it otherwise wouldn’t be entitled to, and subtract an equal amount from its water bank. The water bank got low during the drought, but the SFPUC’s reservoirs remained close to full.

Knowing that its water supply depends on storing water during normal and wet years for future use during a string of dry years, the SFPUC has invested in storage. When full, the SFPUC’s reservoirs and water bank can store enough water to last six years.

SFPUC Storage Capacity

SFPUC Storage Capacity

To increase water supply even further, conservation groups have proposed that the SFPUC partner with the Irrigation Districts to capture excess water in very wet years and use it to recharge groundwater in the San Joaquin Valley for future agricultural use during dry years. Like the Don Pedro water bank, the SFPUC would not have access to this water, but could benefit from a groundwater water bank.

How does the SFPUC’s policy of hoarding water harm the environment?

During the recent drought, the SFPUC released only as much water from its reservoirs into the Tuolumne River as was required by a 1995 flow schedule. This schedule is based on baseflows –a minimum amount of water that must remain in the River, depending on water-year type (how wet) and the time of year. While people were inconvenienced by the drought, fish and wildlife experienced extreme conditions for five years.

In 2017, all of the water we conserved during the drought had to be “dumped,”providing one excessively good year for the environment at the expense of five terrible years. Had the Bay Delta Plan been in effect, the Tuolumne River would have benefitted, and the SFPUC still would have been able to replenish all of its reservoirs and much more.


Prepared by Tuolumne River Trust, June 21, 2018.

[1]Storage measured in thousands of acre-feet (TAF). One acre-foot equals 326,000 gallons.

[2]SFPUC Board meeting, January 10, 2017

[3]“Food System Multipliers for Specialty Foods,”Sacramento Area Council of Governments, July 25, 2016.


[5]750,000 acre-feet equals 670 million gallons per day.

[6]244,000 acre-feet equals 218 million gallons per day.

[7]This graph, courtesy of the SFPUC, does not include Bay Area storage.