Assessing the Potential of Sentinel-1 SAR Backscatter intensity for Monitoring Groundwater Levels in Temperate Raised Bogs

Monitoring of peatland groundwater levels is crucial for effective bog rewetting. Traditional in-situ measurement methods are often costly and impractical in countries with expansive peatland areas. Synthetic Aperture Radar (SAR) may be an approach for remotely estimating groundwater levels over large areas due to its sensitivity to soil properties, such as soil moisture.  In this study, we investigated the relationships between terrain-corrected Sentinel-1 C-band SAR backscatter intensity (γ₀) and groundwater level (GWL) at two temperate raised bogs: (1) a near-intact site (Cors Fochno bog, Wales, United Kingdom; 11 dip wells; Area = ~ 6.3 km²); and (2) an industrially-extracted, ‘bare-peat’ site (Castlegar Bog, Co. Galway, Ireland; 34 dip wells; Area = ~ 3.2 km²). Both sites have recently undergone rewetting measures, primarily bunding and drain blockage. For the industrially extracted Castlegar Bog, initial linear regression analysis between γ₀ and GWL yielded average correlation coefficients (r) of 0.33 and 0.47 for VV and VH polarization, respectively. However, average correlation values increased when the dataset was separated into pre- and post-rewetting periods. Values of 0.55 and 0.64 for VV and VH, respectively, were found before restoration, and 0.43 and 0.54 for VV and VH, respectively, were found after restoration. For the near-intact Cors Fochno bog, SAR intensity exhibited very weak correlation with GWL, with average r values of 0.34 and 0.16 for VV and VH polarizations, respectively. Average correlation values changed to 0.41 and 0.14 for VV and VH after accounting for and filtering out rainfall events preceding each acquisition.  Consequently, our results indicate a limited capability of SAR backscatter intensity to serve as a reliable proxy for GWL in near-intact temperate raised peatlands. We hypothesize that the limited correlation is attributable to two main factors. Firstly, GWL in near-intact sites typically remains approximately 10 cm below the surface with minimal fluctuation, thereby maintaining a consistently saturated peat layer and limiting variance in dielectric properties beyond background noise levels. Secondly, vegetation acts as a buffer, temporarily retaining rainfall in above-ground and near-surface layers, which increases local volumetric water content, leading to surface saturation that affects the SAR backscattering mechanism. On the other hand, these findings indicate that SAR-based monitoring of GWL using C-band data is effective in highly degraded or extracted temperate peatlands, where water table fluctuations are pronounced and where vegetation impacts on the SAR signal are reduced due to extensive bare peat exposure.