Relationship between ground levelling measurements and radar satellite interferometric estimates of bog breathing in ombrotrophic northern bogs

Understanding the seasonal oscillation of peatland surface height initiated by changes in water table level (known as 'bog breathing') is key to improving spatial models of the water balance of bogs and their greenhouse gas exchanges with the atmosphere. Bog breathing has been studied locally via point-based measurements by telmatologists as well as over wider areas by the remote sensing community, in the latter case often without or with limited ground-truth validation. We aim to bring the two disciplines together by assessing the feasibility of validating Synthetic Aperture Radar (SAR) data from the Sentinel-1 satellite with in situ ground levelling data from nanotopes with different drainage status in two hemiboreal raised bogs. We demonstrate the continuous measurement of bog breathing using automatic ultrasonic levelling devices which shows that, during one growing season, bog breathing amounted to 11.6-14.7 cm in hollows, 6.9-7.5 cm in hummocks and 9.5-11.6 cm in haplotelmic nanotopes. Accounting for such relatively large vertical surface deformations remotely using the SAR Differential Interferometry (DInSAR) technique is prone to estimation errors owing to the so-called estimation ambiguity that occurs when deformation exceeds half the wavelength of the radar signal (2.77 cm for Sentinel-1). We approach the ambiguity problem by estimating deformation between consecutive SAR acquisitions (time separation 6 or 12 days) only. Remote and in situ measurements of bog breathing correlate moderately to very strongly (rs = 0.82-0.93 in hummocks) even though, from time to time, all nanotopes except hummocks show surface deformations in just a single day that exceed the ambiguity threshold. This indicates that DInSAR surface deformation estimates contain useful information despite under-estimating larger changes, and DInSAR has high potential for the assessment of bog breathing. Our findings imply that DInSAR estimates in peatlands without ground validation should be interpreted with caution. To take full advantage of the plentiful data from Sentinel-1, the introduction of contextual information (e.g. temperature, precipitation and/or evapotranspiration data) could guide ambiguity resolution, as we demonstrate up to moderate correlation (rs = 0.59 in a hollow) between precipitation and bog surface height.