Best Practice for Measuring Permafrost Temperature in Boreholes Based on the Experience in the Swiss Alps

Abstract

Temperature measurements in boreholes are the most common method allowing the quantitative and direct observation of permafrost evolution in the context of climate change. Existing boreholes and monitoring networks often emerged in a scientific context targeting different objectives and with different setups. A standardized, well-planned and robust instrumentation of boreholes for long-term operation is crucial to deliver comparable, high-quality data for scientific analyses and assessments. However, only a limited number of guidelines are available, particularly for mountain regions. In this paper, we discuss challenges and devise best practice recommendations for permafrost temperature measurements at single sites as well as in a network, based on two decades of experience gained in the framework of the Swiss Permafrost Monitoring Network PERMOS. These recommendations apply to permafrost observations in mountain regions, although many aspects also apply to polar lowlands. The main recommendations are (1) to thoroughly consider criteria for site selection based on the objective of the measurements as well as on preliminary studies and available data, (2) to define the sampling strategy during planification, (3) to engage experienced drilling teams who can cope with inhomogeneous and potentially unstable subsurface material, (4) to select standardized and robust instrumentation with high accuracy temperature sensors and excellent long-term stability when calibrated at 0°C, ideally with double sensors at key depths for validation and substitution of questionable data, (5) to apply standardized maintenance procedures allowing maximum comparability and minimum data processing, (6) to implement regular data control procedures, and (7) to ensure remote data access allowing for rapid trouble shooting and timely reporting. Data gaps can be avoided by timely planning of replacement boreholes. Recommendations for standardized procedures regarding data quality documentation, processing and final publication will follow later.

Changes in Ground Temperature and Dynamics in Mountain Permafrost in the Swiss Alps

“…In the case of the Hoernli hut borehole (Figure 3) increasing positive deviations from 0 • C clearly indicate sensor drift. Duplicate measurements were therefore carried out in the borehole using additional miniature UTL-3 loggers (section "3.1 Ground Temperature Measurements in Boreholes") from 2016 onwards for comparison with the measurements of the installed thermistor string (Figure 3; Noetzli et al, 2021). The UTL-3 loggers measured an active layer thickness of 1.5 to 2.0 m between 2016 and 2019, which is in the range of the active layer thickness observations before 2014 (section "4.2.2.1 Active Layer Thickness").…”

Permafrost Biases Climate Signals in δ18Otree-ring Series from a Sub-Alpine Tree Stand in Val Bever/Switzerland

“…At lower altitudes (~1800 m asl) on the north-facing slope, geophysical soundings and ground surface measurements confirmed the occurrence of thin isolated, sporadic At lower altitudes (~1800 m asl) on the north-facing slope, geophysical soundings and ground surface measurements confirmed the occurrence of thin isolated, sporadic permafrost lenses at shallow depth down to~20 m [59,60,[73][74][75][76][77] (Figure 2B). Compared to permafrost sites in the periglacial zone (e.g., [78][79][80]), the active layer thickness is relatively shallow (1-3 m) and ground temperatures are just below zero degrees (−0.2 • C at 8 m depth). This subalpine permafrost occurrence is approximately 500 m below the expected lower limit of discontinuous permafrost, frequently represented by rock glaciers at 2300 m asl in the Upper Engadin [57,77,81,82].…”

Attributing observed permafrost warming in the northern hemisphere to anthropogenic climate change

“…First, permafrost monitoring is based on in-situ ground temperatures measured in boreholes (Noetzli et al 2021). Their installation and long-term operation at remote sites in cold climates are challenging.…”