Estimating active layer thickness and volumetric water content from ground penetrating radar measurements in Barrow, Alaska

Using Ground Penetrating Radar for Permafrost Monitoring from 2015–2017 at CALM Sites in the Pechora River Delta

This paper describes the results of ground penetrating radar (GPR) research combined with geocryological data collected from the Circumpolar Active Layer Monitoring (CALM) testing sites in Kashin and Kumzha in August 2015, 2016, and 2017. The study area was located on the Pechora River delta. Both sites were composed of sandy ground and the permafrost depth at the different sites ranged from 20 cm to 8–9 m. The combination of optimum offset and multifold GPR methods showed promising results in these investigations of sandy permafrost geological profiles. According to direct and indirect observations after the abnormally warm conditions in 2016, the thickness and water content of the active layer in 2017 almost returned to the values in 2015 in the Kashin area. In contrast, the lowering of the permafrost table continued at Kumzha, and lenses of thin frozen rocks that were observed in the thawed layer in August of 2015 and 2017 were absent in 2016. According to recent geocryological and geophysical observations, increasing permafrost degradation might be occurring in the Pechora River delta due to the instability of the thermal state of the permafrost.

Validation of Permafrost Active Layer Estimates from Airborne SAR Observations

In permafrost regions, active layer thickness (ALT) observations measure the effects of climate change and predict hydrologic and elemental cycling. Often, ALT is measured through direct ground-based measurements. Recently, synthetic aperture radar (SAR) measurements from airborne platforms have emerged as a method for observing seasonal thaw subsidence, soil moisture, and ALT in permafrost regions. This study validates airborne SAR-derived ALT estimates in three regions of Alaska, USA using calibrated ground penetrating radar (GPR) geophysical data. The remotely sensed ALT estimates matched the field observations within uncertainty for 79% of locations. The average uncertainty for the GPR-derived ALT validation dataset was 0.14 m while the average uncertainty for the SAR-derived ALT in pixels coincident with GPR data was 0.19 m. In the region near Utqiaġvik, the remotely sensed ALT appeared slightly larger than field observations while in the Yukon-Kuskokwim Delta region, the remotely sensed ALT appeared slightly smaller than field observations. In the northern foothills of the Brooks Range, near Toolik Lake, there was minimal bias between the field data and remotely sensed estimates. These findings suggest that airborne SAR-derived ALT estimates compare well with in situ probing and GPR, making SAR an effective tool to monitor permafrost measurements.

Sensitivity of soil freeze/thaw dynamics to environmental conditions at different spatial scales in the central Tibetan Plateau

An enhanced understanding of environmental controls on soil freeze/thaw (F/T) dynamics at different spatial scales is critical for projecting permafrost responses to future climate conditions. In this study, a 1-D soil thermal model and multi-scale observation networks were used to investigate the sensitivity of soil F/T dynamics in the central Tibetan Plateau (TP) to environmental conditions at local (~10 km)-, medium- (~30 km), and large (~100 km)- scales. Model simulated soil temperature profile generally agrees well with the observations, with root-mean-square errors (RMSE) lower than 1.3 °C and 2.0 °C for two in-situ networks, respectively. Model simulated maximum frozen depths (MFD) closely related to elevation (R² = 0.23, p < 0.01), soil moisture content (R² = 0.25, p < 0.01), and soil organic carbon (SOC) content (R² = 0.18, p < 0.01); however, the impact of SOC on MFD may be due to the close correlation between SOC and soil moisture. The main factors affecting MFD vary with scale. Among the environmental factors examined, topography (especially elevation) is the first-order factor controlling the MFD at the large-scale, indicating the dominance of thermal control. Aspect shows sizeable impacts at the medium-scale, while soil moisture plays an important role at the local-scale. Soil thaw onset shows a close correlation with the examined environmental factors including soil moisture, while freeze onset seems to be influenced more by other factors. Besides the well-known thermal effect, our study highlights the importance of soil moisture in affecting soil F/T dynamics at different scales in the central TP region, and reliable soil moisture products are critical to better project the response of the TP frozen ground to future warming at finer scale.