About that 40 percent number

The Stanislaus River near Calaveras Big Trees. Record file photo

It seems simple enough: The State Water Resources Control Board, arbiter of equitable water use in California, wants to leave 40 percent of the water in three streams feeding the San Joaquin River south of Stockton.

To the surprise of no one who follows this stuff, it’s more complicated than it sounds.

Let’s ignore, for the moment, that the board has actually proposed a range of river flows from 30 percent to 50 percent, starting at 40 percent. I can only handle so much complexity, so we’ll just examine that 40 percent number.

What, exactly, does it mean? You could look at it two ways: One, that even after the board requires higher flows, that more than half of these rivers — the Stanislaus, the Tuolumne and the Merced — will still be diverted for human use. The science says that if you really want to restore salmon runs, it should be the other way around.

Or, you can look at how little water has traditionally been left in these rivers during the driest of years — as low as 5 percent or 6 percent on the Tuolumne in extreme cases, according to one commentator at today’s hearing. Cast in that light, 40 percent starts to look like a pretty big number.

That’s why we’ve seen a host of extremely critical opinion pieces published by newspapers around these parts, particularly the Modesto Bee, which ran a column earlier this week likening the water board to a drug addict obsessed with getting a fix (water, in this case) at any price.

Forty percent. Interpret this number with caution. Context is important.

For one thing, some folks may be thinking that the water board intends to increase flows by 40 percent. That’s not the case. It’s an increase to 40 percent.

An increase from what? According to the water board,¬†about 21 percent of the water has historically been left in the Tuolumne River. On the Merced, it’s about 26 percent. And the Stanislaus is already close to 40 percent.

But one fish biologist suggests examining these numbers more closely. Jon Rosenfield, with environmental group The Bay Institute, points out that the numbers in the previous paragraph are based on the years 1984-99 only.

If you look at 1995-2014 — which makes sense, since 1995 is the last time that water quality standards were significantly updated — flows have been 28 percent, 30 percent and 43 percent on the Tuolumne, Merced and Stanislaus respectively. according to Rosenfield.

That’s right: Using those other years as a baseline, flows on the Stanislaus have already topped 40 percent. Rosenfield says the water board’s plan could result in less water flowing down the Stan.

Across the three rivers, the biologist told me in an email last month, “The increase in flows is incremental and… entirely inadequate to restore viable salmon populations.”

Could even this “incremental” change cause serious harm to humans? The water board has said that increasing flows on the rivers will cost water users about 288,000 acre-feet of water per year on average. Of course, averages are just that — averages.

In wet years there’ll be plenty of water for people while still meeting the new flow target. No problemo. It’s in drier years when conflicts could emerge. State officials acknowledged at today’s hearing that farmers will likely need to pump more groundwater to compensate for a decrease in surface water, among other consequences. But when it comes to quantifying those impacts the water board and water users are miles apart.

I don’t pretend to have the expertise needed to vet those numbers myself, and I certainly won’t try here. All I know is that nothing is as simple as I’d like it to be. Not even a nice round number like 40 percent.

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