San Francisco gets most (85%) of its water from the Tuolumne River. Its water rights on the Tuolumne, however, are "junior" to those of the Turlock and Modesto Irrigation Districts.
The peculiar nature of the allocation of these water rights leads to a "feast or famine" situation for San Francisco - one that has driven its considerable investments in surface storage.
Most of the time, San Francisco derives far more water under its rights than it can use. Its supply in dry years, however, is well below what it needs to adequately supply the City and its customers in parts of San Mateo, Santa Clara and Alameda Counties.
To determine the allocation of Tuolumne River water rights under California law, hydrologists calculate what the daily average flow would be at La Grange if there were no dams upstream.
How much of the Tuolumne's water is allocated to San Francisco depends on the river's average daily flow. For most of the year, the Districts receive any and all flow up 2416 cubic feet per second, as measured at La Grange, just below Don Pedro Reservoir.
After a wet start to winter in 2016, the weather turned dry and warm. In February, the Tuolumne's flow level was driven by a small amount of rain and some melting snow. Hydrologists calculated the Tuolumne River's natural flow at La Grange for the month to be 144,143 acre-feet. (An acre-foot is the amount of water needed to cover an acre one foot deep, and is the standard unit of measure for water consumption on a large scale).
For February, 90% of this flow was allocated to the Turlock and Modesto Irrigation Districts. Only 10% was allocated to San Francisco as the junior water rights holder. At this rate, San Francisco's would need to use reserves to meet customer needs in in 2016.
In early March, however, the weather turned in San Francisco's favor. Over the first 10 days of the month, the Tuolumne's flow was 124,979 acre-feet, with San Francisco receiving 62% of the total. On March 6 alone, San Francisco received about 34,990 acre-feet or about 15% of its average annual use of Tuolumne River water - not bad for one day. See Figure 1.
For most of the year, the Districts receive the first 2416 cubic feet per second (or 4792 acre-feet per day) of the Tuolumne River's flow - as measured on a daily basis. San Francisco receives flows in excess of that amount.
Presently, 2016 does not appear it will be as wet as predicted by our "Godzilla" El Nino, but it is already much better than 2014 and 2015.
Figure 2, below, shows how the Tuolumne's flow has been allocated between San Francisco and the Districts since 1971, with the current drought shown at the far right. Whenever San Francisco's share of water rights falls below its average use of Tuolumne supplies, it needs to draw on reserves. When San Francisco's share of water rights exceeds its use, it can replenish its reserves if necessary.
Tuolumne River flows, like those of all rivers in California, are highly variable. Overall, San Francisco uses a small fraction of the river.
With more wet weather due this month and significant snowpack in the Sierra, San Francisco's allocation of Tuolumne River supply in 2016 is highly likely to meet system needs and, to at least some extent, replenish reserves.
Next week I will provide an estimate how full we can expect San Francisco's surface reservoirs to be by the end of the spring snowmelt, and whether it will be appropriate to say that San Francisco's system has recovered from the last 4 years of drought.
It is essential for us to understand San Francisco's system as it exists today, so that, with system improvements, we can be certain that not one drop of supply is lost when Hetch Hetchy Valley is restored.
P.S. Tickets are still available for our Annual Dinner. Please join us if you can. See below:
Tickets Now on Sale for the Restore Hetch Hetchy Annual Dinner
Berkeley City Club - April 9, 2016
For more information, call 510-893-3400.
Meet our special guests and count on a truly special evening.