The influence of drainage canals on subsidence in Southeast Asian peatlands

Abstract:

The islands of Borneo and Sumatra in Southeast Asia contain 25 million ha of peatlands which store vast swaths of carbon in soil. Since the 1980’s, most of these peatlands have been deforested and used for plantations of oil palm or acacia, as well as smallholder farming. This change in land use has coincided with a dramatic increase in subsidence averaging about 2 cm/yr. This subsidence threatens low-lying coastal regions and is associated with the release of 150 Mt C/yr. The subsidence is caused not only by the land use change itself, but by the associated building of drainage canals, used for transportation and to promote agricultural growth. The associated hydrologic changes, including lowering the surrounding groundwater tables and drying out of the overlying peat, leads to the subsidence. In this study, we investigate the influence of drainage canals on subsidence rates. To do so, we use two novel remote sensing datasets of the region: a region-wide map of canal occurrence derived from high-resolution optical imagery from Planet cubesats, and maps of subsidence based on interferometric synthetic aperture radar (InSAR) measurements from the Advanced Land Observing System (ALOS) Phased Array type L-band Synthetic Aperture Radar (PALSAR). The spatial patterns of subsidence are clearly influenced by the nearby canals, despite an ~8 year difference in the collection times of these sensors,. The most intensely drained regions are associated with average subsidence rates of 1.9 cm/yr larger than regions with little to no drainage. Models of subsidence rates improve by about 40% when the distance to the nearest canal and local canal length is accounted for. This work motivates additional research in the relationship between drainage canals and subsidence, suggesting policies and practices regarding canal properties may be useful targets for reducing continued subsidence and mitigating carbon emissions.