A Novel Low-Cost, High-Resolution Camera System for Measuring Peat Subsidence and Water Table Dynamics

Chris D Evans,A Jonay Jovani-Sancho,Martha Ledger,Lip Khoon Kho,Mark E Harrison,Kitso Kusin,Alistair Grinham,Alan D Radbourne,Adi Jaya,Stephanie Evers,Nathan Callaghan,Sofie Sjogersten,Zak Mitchell,Susan E Page,Jennifer Williamson

doi:10.3389/fenvs.2021.630752

Abstract

Peatlands are highly dynamic systems, able to accumulate carbon over millennia under natural conditions, but susceptible to rapid subsidence and carbon loss when drained. Short-term, seasonal and long-term peat surface elevation changes are closely linked to key peatland attributes such as water table depth (WTD) and carbon balance, and may be measured remotely using satellite radar and LiDAR methods. However, field measurements of peat elevation change are spatially and temporally sparse, reliant on low-resolution manual subsidence pole measurements, or expensive sensor systems. Here we describe a novel, simple and low-cost image-based method for measuring peat surface motion and WTD using commercially available time-lapse cameras and image processing methods. Based on almost two years’ deployment of peat cameras across contrasting forested, burned, agricultural and oil palm plantation sites in Central Kalimantan, Indonesia, we show that the method can capture extremely high resolution (sub-mm) and high-frequency (sub-daily) changes in peat surface elevation over extended periods and under challenging environmental conditions. WTD measurements were of similar quality to commercially available pressure transducers. Results reveal dynamic peat elevation response to individual rain events, consistent with variations in WTD. Over the course of the relatively severe 2019 dry season, cameras in deep-drained peatlands recorded maximum peat shrinkage of over 8 cm, followed by partial rebound, leading to net annual subsidence of up to 5 cm. Sites with higher water tables, and where borehole irrigation was used to maintain soil moisture, had lower subsidence, suggesting potential to reduce subsidence through altered land-management. Given the established link between subsidence and CO2 emissions, these results have direct implications for the management of peatlands to reduce high current greenhouse gas (GHG) emissions. Camera-based sensors provide a simple, low-cost alternative to commercial elevation, WTD and GHG flux monitoring systems, suitable for deployment at scale, and in areas where existing approaches are impractical or unaffordable. If ground-based observations of peat motion can be linked to measured GHG fluxes and with satellite-based monitoring tools, this approach offers the potential for a large-scale peatland monitoring tool, suitable for identifying areas of active carbon loss, targeting climate change mitigation interventions, and evaluating intervention outcomes.

Highlights

Peatlands are the most carbon rich terrestrial ecosystems on earth, storing an estimated total of 637 Gt C (Page et al, 2011; Dargie et al, 2017), which is equivalent to three quarters of all the CO2 currently in the atmosphere (860 Gt C; Friedlingstein et al, 2019)
The gradient of the relationship is strongly influenced by five data points from the strongly subsiding Misik Oil Palm site, where we found that water table depth (WTD) at the camera site was on average 25 cm deeper than at the manual pole sites, which suggests that higher subsidence rates at the camera site may be real
In accordance with assessments of CO2 and CH4 fluxes from high-latitude peatlands (Couwenberg et al, 2011; Tiemeyer et al, 2020), as well as analyses of long-term subsidence from tropical peatlands (Hooijer et al, 2012; Evans et al, 2019) our analysis suggests that both subsidence and greenhouse gas (GHG) emissions vary greatly within each land-use category, depending on the water management

Summary

Introduction

Peatlands are the most carbon rich terrestrial ecosystems on earth, storing an estimated total of 637 Gt C (Page et al, 2011; Dargie et al, 2017), which is equivalent to three quarters of all the CO2 currently in the atmosphere (860 Gt C; Friedlingstein et al, 2019). This carbon store is under unprecedented and intensifying pressure from human activities, including land conversion and drainage for agriculture and plantation forestry, grazing, and the use of fire for land clearance or management (Turetsky et al, 2015; Page and Baird, 2016). The tropical peatlands of Southeast Asia have undergone very rapid conversion, with 60% of the original area of peat swamp forest cleared between 1990 and 2010, the majority of which is under drainagebased cultivation (Wijedasa et al, 2018)

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