TWITCHELL ISLAND, California – Bryan Brock stared out at a rice field on Twitchell Island, nestled between the meandering river paths of the Sacramento-San Joaquin Delta. Brock, a senior engineer with California Department of Water Resources’ West Delta Program, rubbed his goatee and pointed at foot-tall emerald stalks. The plots were drenched in about 4in of water.
Medium-grain rice was planted here in 2009 as a research project to see if rice could help the Delta survive the impacts of subsidence. The results have yielded both good and bad news.
The Central Valley is no stranger to subsidence. But unlike other areas that are sinking due to overpumping of groundwater, the Delta’s subsidence is a force of nature. Since most of the soil is peat, which contains decomposing vegetation, microbes eat away at the carbon in the dead plants and emit carbon dioxide, shrinking the soil level by about 1in per year, Brock said.
The Delta’s agriculture economy has made the situation worse. Planting crops in the Delta seemed like an obvious decision for newcomers to the region following the Gold Rush. Pumping out the estuary and throwing up levees alongside nearby rivers left plots with rich soil and bountiful harvests.
But generations later, those same plots that were started on unsteady peat soil are sinking further below the waterline, putting the long-term fate of farming in the Delta at risk. Currently there are more than 100 crops grown in the region on 500,000 acres of farmland. But the threat of massive flooding looms over an area where corn fields now sit 25ft below the water level of the San Joaquin River, held back by dirt-and-rock levees.
“In the deep areas where it’s really subsided, it’s getting really challenging to farm,” said Bruce Linquist, lead researcher for the rice pilot project and an agronomy researcher at the University of California, Davis. “There’s a lot of holes out there where you have water constantly coming up.”
The Twitchell Island rice pilot program began as an endeavor to find a solution that would help reverse subsidence while offering farmers a moneymaking solution. The results were mixed.
Rice fields can capture a significant amount of carbon dioxide emitted by the microbes in peat soil, acting as a carbon sequestration sink, Brock said. One of the rice fields on Twitchell Island used to grow corn, and during that time the underground microbes gave off 8 tons of carbon dioxide equivalent per year for each acre. “By flooding it and growing rice, I think we only have an emission of 1 ton per acre per year,” he said. That’s a climate win.
Researchers also found that rice fields slow down – and sometimes completely stop – the microbial feeding that causes subsidence because of the wet environment, Brock said. While this halts subsidence, it can’t reverse it, so rice fields are still seen as a subpar option to wetland restoration, which is usually preferred by environmental groups. By planting cattails and other wetland plants, the plants stop microbial subsidence and can reverse subsidence when the plants die and decompose, creating new soil.
While the pilot project showed some promising benefits, rice was not the panacea researchers had hoped.
Despite the ecological benefits, the economics of planting rice in the Delta simply don’t pencil out for many farmers in the Delta, Brock said. The pilot project’s rice crop yields paled in comparison to rice grown near Sacramento, and sometimes costs exceeded revenues.
Linquist said rice could have its place in the Delta one day, even though the trial crops didn’t do particularly well. The Sacramento Valley is one of the nation’s largest producers of rice, according to the United States Department of Agriculture – but usually those fields are in clay soil, Brock said, not the Delta’s unpredictably sinking soil.
“Really, the yield potential in the Delta is not bad,” Linquist said. “If you can deal with the elements, you can get a good yield of rice. One of the big issues affecting that is cold temperatures at night.”
Another issue is weeds, and blackbirds that arrive in “the millions” and knock off rice grains when they land on the stalks, Brock said.
But one of the biggest problems is properly managing the water levels. Rice likes to be alternated between wet and dry periods. Because the Delta is sinking, and because it was naturally a flooded estuary, water has to be constantly pumped off fields. That environment keeps the rice wet year round, limiting growth potential.
Rice would be a nice option for ecological reasons, but it’s tough to convince a farmer to replace corn fields that earn significantly more money than rice. Plus, specialized equipment to harvest and maintain rice is often prohibitively expensive.
Researchers, ecological restoration advocates and state officials are also pursuing other restoration efforts in the Delta. Wetlands restoration (and the added paycheck that comes with carbon sequestration farming) and duck habitat creation for hunting are among the ecological methods that also make more economic sense than rice at the moment, Brock said.
And then there’s the simple fact that farmers will prefer to plant corn or use their fields for more profitable cattle grazing until the unstable, wet ground forces their hand.
“The long-term picture is to stack as many potential moneymaking opportunities as you can,” Brock said of the vision for the Delta’s future. “And to at least stop subsidence.”