[ghshephard]

Most of San Francisco's water comes from Hetch Hetchy, which is fed by the Tuolumne River.

Wikipedia tells me this about Tuolumne: "The river's water has been a source of contention for many years. About 15% of the total flow is diverted to San Francisco from Hetch Hetchy Reservoir, which floods an eponymous valley in the Sierra Nevada once compared to Yosemite Valley for its natural beauty. Further downstream, about one-half of the river flow is diverted at La Grange Dam to irrigate farmland in the Central Valley."

It looks to me like there is agriculture competition for San Francisco's water.

[timr]

There's agriculture competition for the river, not the reservoir. The city of SF owns the reservoir.

But it's not very informative to say that there's competition for the river, because there's obviously competition for water somewhere, since it all comes from the same places. My point is that the water rights are secured for most of the major cities in California, and we're not in the position that if we cut back our domestic usage, the farmers will use it all up. We're conserving so that we don't drain our reservoirs.

[ghshephard]

"There's agriculture competition for the river, not the reservoir. "

Given that in this case, the "River" feeds into the "Reservoir" - the two water bodies can be viewed as the same entity for purposes of water sourcing. Given that only 15% of the Tuolumne is needed for the Reservoir, and 50%+ is required for Agriculture, the major drain on the Tuolumne is not San Francisco residents, but Agriculture. San Francisco may have rights to the Tuolumne, but I don't know whether they are Sr. to the Central Valleys, or, what happens if San Francisco has to turn off the water downstream in order to keep Hetch Hetchy full.

But - I do see your point, it's not the case that all water comes from the same place - there's lots of reservoirs that can't be used for Agriculture in the Central valley. For example, Crystal Springs, a 70k cubic meter reservoir in San Mateo, is entirely for urban use. There is no agriculture competition for it. Every gallon we conserve there, is another gallon conserved for city use.

But - it's important to realize, Cities and Agriculture are in competition for the Sierra Nevada water. And every gallon that goes to the farms, is one less gallon that goes to the residents. As the cities conserver more, and more, reducing showers to 5 minutes, and switching to Ultra-Low-Flush .5 gallon flow Toilets, that water is going to become more valuable to city residents, than it will be to agriculture.

I'm not suggesting we don't cut back residential usage - I'm a huge fan of conservation (and an even bigger fan of efficiency) - but, I am saying that we could go to emergency rationing of 50 gallons/day (down from 329 gallons currently), and agriculture could still use up all the Sierra Nevada water. The inverse isn't true - if Agricultural use somehow cut its water use by the same percentage, it would be difficult for residential use to consume the existing supplies.