SAR and InSAR data linked to soil moisture changes on a temperate raised peatland subjected to a wildfire

Abstract

Interferometry of Synthetic Aperture Radar (InSAR) can potentially contribute to the cost-effective regional or global monitoring of the degradation and restoration of peatlands. However, there are uncertainties about the links between InSAR results and peatland ecohydrological parameters, especially soil moisture. Here, we analyse the relationships between the temporal evolutions of InSAR coherence, ground displacements, and in-situ soil moisture measurements for a temperate raised bog at Ballynafagh, Co. Kildare, Ireland, in the period 2017-mid-2021. We also investigate the effects of a wildfire in June–July 2019 on those relationships. InSAR-derived ground displacements from Sentinel-1 C-band radar data indicate long-term subsidence of the intact and Active Raised Bog. Superimposed on the long-term displacement trends are annual oscillations that are linked to variations in rainfall and temperature and that are in phase with changes in soil moisture. We show that InSAR coherence is directly related to the change in soil moisture, with large changes causing coherence decrease or loss. The wildfire removed a 10–20 cm thick mossy vegetation layer across 60–70% of the intact bog area. The SAR backscatter intensity in VV polarisation increased after the wildfire, but the InSAR coherence, the InSAR-derived surface displacements and the soil moisture were not noticeably affected. We therefore infer that C-band radar waves attenuate in the active vegetation layer, but penetrate through it into the upper few cm of the underlying peat. The SAR backscatter occurs primarily at this level in the peat, where its coherence is controlled by the soil moisture. These findings underpin application and interpretation of radar for monitoring of peatlands, even if affected by wildfires, which are forecast to increase in both frequency and intensity due to global warming.