A synthesis dataset of near-surface permafrost conditions for Alaska, 1997-2016

Synthesis We compile a first integrated near-subsurface permafrost dataset for Alaska from the three most reliable sources monitoring permafrost conditions over several decades: the Geophysical Institute Permafrost Laboratory at the University of Alaska Fairbanks (GI-UAF), National Park Services in Alaska (NPSA), and the USGS. The developed permafrost dataset for Alaska (PF-AK, version 0.1) includes measured air and ground temperatures, snow depth and soil volumetric water content for 72 permafrost monitoring stations in Alaska. The resulting dataset consists of monthly air and ground temperatures, volumetric water content, and snow depth during 1997 - 2016. The ground-measurement depths vary station by station within the GI-UAF and NPSA networks, typically ranging from the ground surface (i.e., 0 m) to 1 m below the ground surface.

Air temperature These networks utilize radiation-shielded thermistors (Campbell Scientific CSI 107 temperature probes) to monitor air temperature. In the GI-UAF and NPSA network, the air temperature sensors were installed at 1.5 or 2 m above the ground surface, whereas the USGS network monitors air temperature at 3.0 m above the ground surface in order to minimize damage by wildlife. Despite the installation of air temperature sensors at different heights, the measurements of air temperature are considered comparable on a monthly scale assuming efficient mixing of the near-surface atmosphere.

Ground temperature To monitor near-surface ground temperature, the networks use a probe with several thermistors embedded into a single rod, typically 1 to 1.5 m long. The MRC probe is designed to record temperatures ranging from -30 to 75 °C. An ice-bath calibration is a required procedure before installation of these probes. The ice-bath calibration includes placing the sensors into an insulated container filled with a mixture of ice shavings and distilled water, measuring the temperature, and recording the offset from 0°C. This measured offset is then used to correct the temperature measurements. The average accuracy for the MRC probe is ± 0.01 °C. For the USGS network, the MRC sensors are installed inside a tight-fitting fluid-filled 125-cm-long plastic tube to measure ground temperatures at 5, 10, 15, 20, 25, 30, 45, 70, 95, and 120-cm depth. The ground-measurement depths vary station by station within the GI-UAF and NPSA networks, typically ranging from the ground surface (i.e., 0 m) to 1 m below the ground surface. It is important to note that most of the installed probes heave with time, and heaving depths are adjusted accordingly by subtracting the heaving values yearly. The released data account for the heave and have corrected ground temperatures.

Soil water content Both the USGS and the GI-UAF networks measure liquid soil moisture using a Hydra Probe sensor developed by Stevens Water Monitoring Systems Inc. The Stevens Hydra Probe has a reported accuracy of ± 0.03 m³/m³. Each Volumetric Water Content (VWC) sensor was calibrated in accordance with the soil texture in a laboratory while uncertainties associated with the sensor’s sensitivity were still existing under specific conditions, e.g. for peat.

Snow Depth Snow depth is measured once per hour with an SR50 ultrasonic distance sensor (Campbell Sci. Inc.) at all of the stations. This downward-looking sensor is mounted on a cross-arm 2.5 m above the ground surface for the USGS and NPSA networks, and 1.5 m above the ground surface for the GI-UAF network respectively. The factory evaluated accuracy is ± 0.01 m or 0.4% of the distance to the ground surface. However, vegetation at the ground surface causes some uncertainty in shallow SR50 snow-depth measurements.