California’s historic drought curtailments: “very deep” or no change on Tuolumne?

Early season storms provided substantial water to San Francisco under their junior water rights on the Tuolumne River. By the second week of June, however, the snowpack has largely melted and none of the river’s flow will likely be available to the City anyway – so the State Board’s curtailment order is likely to have no effect on its supply.  Overall, San Francisco is doing pretty well water supply-wise this year, far better than most of California.

Due to our ongoing serious drought, on June 7 the State Water Board expanded its “Delta Watershed Curtailment Status List” to include San Francisco’s water rights on the Tuolumne River as well as some rights of the Turlock and Modesto Irrigation Districts. Erik Ekdahl, deputy director of the agency’s Division of Water Rights, described the restrictions — known as curtailment orders — as “significant” and “very deep.”

The story was covered with provocative headlines by both the Sacramento Bee (California tells San Francisco, Valley farmers to halt water diversions as drought worsens) and the San Francisco Chronicle (California orders thousands of farms and cities, including San Francisco, to stop pumping water during drought).

It does not appear, however, that the curtailment order will have any effect on San Francisco, and it may not even have any effect on the Turlock and Modesto Irrigation Districts (together, Turlock and Modesto use about 4 times as much Tuolumne River water as San Francisco and its Bay Area customers).

First, note that the order applies only to the limited snowmelt that still remains, not to water already stored in reservoirs or underground. Second, it’s highly unlikely that any river flows for the remainder of the water year would be available to San Francisco under their rights – the natural flow of the river would need to be over 2416 cubic feet per second (it’s possible but highly unlikely to be at all significant, see chart above). And thanks to the large storms earlier in the year, San Francisco is doing pretty well water supply-wise, far better than most of California.

It’s also not clear that the curtailment order will have any effect on the Turlock and Modesto Irrigation Districts. While some of the Districts’ water rights are curtailed, their longstanding “riparian” rights are not. See table below – extracted from the State Board’s massive spreadsheet, which  identifies curtailment orders for 4571 out of the 16472 listed water rights in the Bay-Delta watershed.

No doubt many water rights in the Central Valley will indeed be affected, so the little flow available will go to those with the most senior water rights, and perhaps some will be left over to provide a sliver of relief for the Bay-Delta.

In many years, California’s cattywampus water rights system does not work very well for San Francisco. This year, with a significant ongoing drought, the City and its customers in other Bay Area cities are doing reasonable well.