Solutions for subsidence in the California Delta, USA, an extreme example of organic-soil drainage gone awry

Abstract

Abstract. The Sacramento-San Joaquin Delta is at the heart of California's water supply system that provides water for irrigation and human consumption. It is also home to subsiding organic soils, decreasing native aquatic species populations, water quality degradation, vulnerable levees (levees are equivalent to dikes) and decreasing agricultural viability. There has been substantial progress in the interdisciplinary understanding and quantification of the nature and effects of subsidence and its mitigation. Because of the need for a drained rootzone, farming of crops such as vegetables, trees, vines, corn and alfalfa, results in an ongoing unsustainable cycle of continuing peat oxidation and deepening of drainage ditches to compensate for elevation loss. Despite substantial evidence for the increasing risks to the State's economy and water supply, the unsustainability of the status quo, and evidence for the benefits of alternatives, there has been limited progress in converting to land uses that can reduce, stop and reverse subsidence. Our overall approach has been to measure land-surface elevation changes; understand, quantify and model subsidence and greenhouse gas emissions from drained organic soil, and evaluate alternate land uses. Subsidence rates vary from less than 0.5 to over 2 cm yr−1, depending primarily on depth to groundwater and soil organic matter content. The primary cause of subsidence is the oxidation of organic matter, which has resulted in elevations of −3 to −9 m on about 100 000 ha. Using the results from micrometeorological measurements and modelling, we estimate that organic-matter oxidation causes an annual emission of over 2×106 t of CO2-equivalent which represents about 21 % of the State's plant-based agricultural emissions. Rewetting of the peat soils is emerging as a viable solution. Rice and wetlands stop and (in the case of wetlands) reverse the effects of subsidence and result in a net greenhouse-gas emission reduction benefit. Wetlands accrete about 3 cm of soil per year, can break the unsustainable subsidence/drainage cycle, reverse the trajectory of increasing hydraulic pressures on levees, reduce the probability of levee failure and seepage onto islands (islands are equivalent to polders), and may provide material for biofuels and animal feed. The recent implementation of a methodology for quantification of the GHG benefit is facilitating land use conversion and participation in the carbon market.

Carbon Sequestration and Subsidence Reversal in the Sacramento-San Joaquin Delta and Suisun Bay: Management Opportunities for Climate Mitigation and Adaptation

“…Key indicators of unsustainable conditions include native species decline (e.g., Moyle et al 2016), decreased arability (Deverel et al 2015), increased pressure on levees and water supply vulnerability (Deverel et al 2016), and large GHG emissions (Hemes et al 2019). For example, using the SUBCALC model described in (Miller et al 2011;Deverel et al 2020). The long-term projections of these restoration trajectories and geomorphic model outcomes are unknown (Mount and Twiss 2005) with several important constraints on rates of subsidence reversal and marketable C sequestration (Bates and Lund 2013).…”

“…Little progress has been made in reducing, stopping, or reversing subsidence of organic soil in the Delta (Madani and Lund 2012). This is the case despite substantial evidence for increasing risks to the state's economy and water supply, the unsustainability of current elevations under the status quo, and evidence for the benefits of alternative land uses (Loucks 2019;Deverel et al 2020). Economic incentives are necessary for land-owners and agricultural producers to convert to more sustainable land uses (Ingebritsen et al 2000) of flooding could quantify the potential of this common intervention.…”

“…With the high permeability of the dunes, restoring the water table for these lands could help protect soil carbon stocks in neighboring parcels. Thus, they could be considered a "no regrets solution" for farmers receiving no other obvious benefit to active management (Deverel et al 2020).…”

“…While the Delta was once a hydrologically connected and diverse landscape (Whipple et al 2012), the 2,000 km 2 landscape has shifted to two dominant land-cover types: agricultural land (~1,750 km 2 ) and open water (~250 km 2 ). Very few parcels of agricultural land are poised for tidal reconnectivity because of historical elevation loss and operational constraints (Deverel et al 2020). Thus, reconnectivity is largely an intervention for less-altered landscapes, such as seasonal wetlands and floodplains, notably those in the west (Suisun) and north Delta zones.…”

“…However, drainage of the peatlands, which began in the mid-19 th century, resulted in the loss of approximately half the Delta's soil carbon stock (Drexler et al 2019). This degradative process is on-going (Deverel et al 2016) and has resulted in subsidence of Delta islands to elevations as low as 9 m below sea level (Deverel et al 2020).…”