By John Hart, Estuary News
In the Sacramento-San Joaquin River Delta, an invisible fire is burning. Peat soils, laid down under water when the region was a vast wetland, are oxidizing, vanishing into the atmosphere at the rate of about 1.5 inches a year. 150 years after levee-building created the modern landscape of agricultural islands between hardened tidal channels, many of those islands have lost 20 feet or more of soil; standing in a sunken cornfield, you might tilt your head back to see a ship chug by on an adjoining river. The subsidence continues. On some tracts, there are 30 or more additional feet of organic soil to go.
Some of the consequences are known. Levees defending hollowed-out islands are under pressure and at risk of failure. Valued farmland is likely to flood. And island collapse could wreck the plumbing of California’s largest water systems, which use Delta channels to shunt Sacramento River water south to the San Joaquin Valley and beyond. (These flows will continue for decades regardless of the fate of the latest Delta tunnel plan.)
Lately attention has shifted to another penalty of peat soil loss: its sizable output of greenhouse gases. Like coal, peat is a store of carbon, and its oxidation releases carbon dioxide to the air. The Delta comprises about 1.4% of California farmland, but generates about 15% of the state’s agricultural greenhouse gas emissions. “This tiny little dot in the middle of California is a chimney pumping carbon into the atmosphere,” says Campbell Ingram, Executive Officer of the Delta Conservancy.
The solution to this knot of problems is, in a way, simple. Soils that evaporate when exposed to the air quit doing it when they are rewetted; in fact, they start building up again, turning from carbon producers to carbon absorbers, or sinks. “When you reflood these peatlands you kind of turn a switch,” says biogeochemist Patty Oikawa of CSU East Bay.
Rewetting doesn’t mean opening levees and abandoning islands, a retreat which would have multiple drawbacks. Rather, wetlands can be restored on subsided lands within existing defenses. Since the 1990s, the Department of Water Resources has been testing the potential on plots on Twitchell and Sherman Islands. Some test fields grew bulrushes (tules), others, rice. Spidery, instrument-laden structures called eddy covariance towers actually measured the flux of gases between ground and atmosphere. The results have quieted early skeptics. The “proof of concept” is in hand.
An Empire of Peat
How much peat could be protected or restored? Some planners look forward to a time when the most vulnerable 150,000 acres of the delta become a damp mosaic: tule wetlands in the soggiest and most deeply subsided areas, rice fields on somewhat higher ground. That’s an ambitious goal. The next step is to scale up from small plots to “whole island demonstration projects.” The two that are farthest along are on Staten Island, owned by The Nature Conservancy, and Webb Tract, one of four islands acquired in 2016 by the Metropolitan Water District of Southern California.
On Staten, the Conservancy has already switched 4,000 acres from corn to rice, and is about to begin creating 1,000 acres of marsh at the southern tip. On the Met-owned Webb Tract, the planned proportions are approximately reversed: the Met foresees 3,000 to 3,500 acres of marsh, 1,000 to 1,500 acres of rice. The difference is instructive.
If fighting climate change were the only goal, every available acre would go into marsh, either managed or (on less subsided areas) tidal. Rice paddies do build soil, and fix carbon, while they are flooded, but lose some of the gain in late summer and fall when the fields are dried for harvest and new planting. Also, some of the biomass they grow is exported in the crop. On an annual basis, rice farming on peat halts, but does not reverse, soil loss. Only a maximal wetland option could actually convert the Delta as a whole from a carbon source into a carbon sink.
Why then include rice at all? First, because the continuation of delta agriculture is deeply rooted in custom and indeed in law. Second, and very importantly, because rice is a profitable crop in its own right, making for a much easier transition. The Nature Conservancy favored rice on Staten because it wants to run a “real” farm, one that pays its own costs—and thus demonstrates that climate- and wildlife-friendly agriculture works anywhere. The Met on Webb is mainly concerned with raising the ground as fast as possible.
The switch to rice from other crops is not so easily accomplished. Fields must be meticulously leveled, berms built, plumbing modified. The state’s Delta Rice Conversion Program, channeled through the Delta Conservancy and the Nature Conservancy, pays farmers $400 to $800 per acre to help the change along. 5,000 acres are initially targeted. The hope is that word will spread. “So much of it comes down to farmers talking to other farmers,” says TNC’s Michelle Passero.
If rice is one economic support for the mosaic vision, the rising carbon market is the second. Activities that reduce the emission of greenhouse gases are now recognized to have a cash value, translatable, after an elaborate vetting process, into credits that can be bought and sold. Some of these are traded in government-run “compliance” markets, the cap-and-trade systems; a far larger volume of credits changes hands in decentralized “voluntary” markets supervised by non-government agencies like the American Carbon Registry.
To date, only the voluntary market is available in the Delta. Since 2017, the Registry has had a detailed—200-page— “Protocol for deltaic and coastal wetlands.” At present prices, wetland projects can earn $15-$20 per ton of carbon kept out of the atmosphere. The number of tons saved when an acre is switched to tules depends on peat content and also on how deep a layer has been kept dry and oxygenating due to drainage systems; the tonnage can be as low as ten or as high as thirty. Translated into dollars per acre, credits could be worth as little as $120 per acre or as much as $600; the Delta Conservancy talks in terms of $150-$350. This is already competitive, says the Conservancy’s Ingram, with the yields of such crops as alfalfa or corn.
Till recently, these prospects have been mostly theoretical. Farmers hear numbers but can’t compare notes with experienced neighbors. The “whole island” pilot projects should begin to change that.
A further powerful shove could come from the California Air Resources Board, which has been urged to bring peat credits into the state’s cap-and-trade system. If this came to pass, credits earned by wetland restorers could join the stock sold at auction to agencies or companies that need to compensate for carbon-generating actions of their own. The going price of “regulatory credits” is now $30 per ton, half again the voluntary market level. With mounting climate concern, the price could go much higher.
Rice farming—or rather the conversion to rice from other crops–is also covered in the American Carbon Registry’s 2017 protocol. Yet farmers have been hesitant to enroll, at least at current prices, because a 40-year commitment is required. To make this step less scary, farmers might sign up in groups, allowing a rotation of members while delivering the promised total carbon benefit. And of course any rice farmer could make the further switch to tule marsh, earning a richer credit.
Attention to the Margins
Restoring Delta peat is plainly good for the atmosphere. How good is it for the Delta as a landscape? Can these efforts help the region survive, as we know it, while the climate continues to warm and the oceans to rise?
At the recent Bay-Delta Science Conference, Steven Deverel, a leading advocate for rebuilding peat, underlined the challenge. To raise land even to the current sea level will take decades: up to 50 years on the Delta margins, 50-100 years in the more subsided middle zone, and as much as two centuries in the deeply degraded core.
Is the western Delta then doomed? Possibly not.
For one thing, it turns out that restoring wetlands along the interior bases of levees pays off in the very short term. In normal Delta agriculture, farmers are constantly pumping fields dry, creating voids that draw in water from adjacent rivers and sloughs. This seepage is a major cause of levee failure. With a wetland band along the levee base, groundwater rises and “hydrostatic pressure” is reduced—and not just a little. “The improvement is exponential,” said Campbell Ingram. This at least buys some time for soil to accrete (raising the water table further).
Deverel also told the conference of a possible accelerator: growing tules in floating masses. According to historical accounts, the original Delta had mobile tule islands, sometimes seen with large animals on them. On Twitchell and on Bouldin Islands, experimenters grew clumps robust enough to support a human being. More importantly, they found that these living rafts added biomass about twice as fast as ordinary tule stands, thickening by two to three inches per year.
The historic Delta, Ingram observes, was itself the product of sea level rise. The post-glacial intrusion of tidewater deep into the Central Valley made a constellation of lands and waters perhaps unique on the planet. If the current, human-generated rise is on the moderate side of the plausible range—and if we act in the Delta very quickly and decisively indeed—a well-loved California landscape may yet have a fighting chance.
This story was produced by Estuary News Group with a grant from the Delta Stewardship Council.
