Three-dimensional temperature variations measured with fibre-optic sensors and boreholes at a boreal permafrost site in Interior Alaska

Saito, Kazuyuki*; Iwahana, Go*; Ikawa, Hiroki*; Nagano, Hirohiko   ; Busey, R. C.*

A DTS (distributed temperature sensing) system using fibre-optic cable as a sensor, based on the Raman-scattering optical time domain reflectometry, was deployed to monitor a research site (Poker Flat Research Range) in the boreal forest of interior Alaska. Surface temperatures range between -40 C in winter and 30 C in summer at this site. A fibre-optic cable sensor system monitored ground surface temperatures across the landscape at high resolution. The total cable ran 2.7 km with about 2.0 km monitoring a horizontal surface path on a flat area (loop1), and another 5.5 km that covers four 30-m quadrats and hill slope (loop2). Sections of the cable sensor were deployed in vertical coil configurations (1.2m high) to measure temperature profiles from the ground up at 5mm intervals. Measurements were made continuously over two separated periods; a 2-year period from October 2012 to October 2014 (loop1), and a 32-month period from July 2016 and February 2019 (loop1 and loop2). Vegetation at the site consists primarily of black spruce underlain by permafrost, but shows large heterogeneity. We classified land cover types within the study area into six descriptive categories. The measured horizontal temperature data exhibited spatial and temporal changes within the diurnal and seasonal variations, and showed different characteristic behaviours for specific land cover type. Differences in snow pack evolution and insulation effects also co-varied with the land cover types. The hill-slope measurements illustrated the development of the atmospheric boundary layer. The apparatus used to monitor vertical temperature profiles generated high-resolution data for air column, snow cover, and ground surface, to deduce the distribution and seasonal changes in thermal diffusivity of the snow pack and the upper soil layer. This research also identified several technical challenges in deploying and maintaining a DTS system under subarctic environments.