High Accuracy Estimation and Validation of InSAR-derived Surface Displacements at Temperate Raised Peatlands

Abstract

Peatland surface motion derived from satellite-based Interferometry of Synthetic Aperture Radar (InSAR) is potentially a proxy for groundwater level variations and greenhouse gas emissions from peat soils. Ground validation of these motions at equivalent temporal resolution and accuracy has proven problematic because of limitations of traditional surveying methods. Since 2019, peat surface motion has been measured in-situ using novel camera-based instrumentation at two temperate raised bogs, Cors Fochno and Cors Caron, Wales, United Kingdom. The cameras provide continuous measurements at sub-millimetre precision and sub-daily temporal resolution. From these and Sentinel-1 acquisitions spanning mid-2015 to early-2023, we demonstrate that accurate InSAR-derived peat surface motion can be derived using a combination of interferometric networks comprising long and short temporal baselines. The InSAR time series data closely match the in-situ data at both bogs, in particular the annual peat surface oscillations. Pearson’s values for the correlation of in-situ and InSAR displacements are 0.8-0.9, and 76 % of differences are < ±5 mm (93 % < ±10 mm). Larger differences mainly occur during drought periods. Multi-annual displacement velocities based on InSAR indicate long-term subsidence of the Cors Caron surface (max. -7 mm·yr-1) while that of Cors Fochno exhibits subsidence at the centre and uplift at the margins (-9 mm·yr-1 to +5 mm·yr-1). These long-term peat surface subsidence rates correlate well with peat dome elevation and peat thickness. In addition, the annual oscillations in surface motion are synchronous with or lag slightly behind groundwater level changes. A coarse ratio of 1:20 to 1:10 is observed between annual changes in groundwater levels and peat surface displacements. Satellite-based InSAR can thus enable accurate monitoring of hydrologically driven surface motions of temperate raised peatlands.