Towards accurate quantification of ice content in permafrost of the Central Andes – Part 1: Geophysics-based estimates from three different regions

Abstract

Abstract. Increasing water scarcity in the Central Andes due to ongoing climate change recently caused a controversy and debate on the significance of permafrost occurrences for the hydrologic cycle. The lack of comprehensive field measurements and quantitative data on the local variability in internal structure and ground ice content further exacerbates the situation. We present field-based data from six extensive geophysical campaigns undertaken since 2016 in three different high-altitude regions of the Central Andes of Chile and Argentina (28 to 32∘ S). Our data cover various permafrost landforms ranging from ice-poor bedrock to ice-rich rock glaciers and are complemented by ground truthing information from boreholes and numerous test pits near the geophysical profiles. In addition to determining the thickness of the potential ice-rich layers from the individual profiles, we also use a quantitative four-phase model to estimate the volumetric ground ice content in representative zones of the geophysical profiles. Our analysis of 52 geoelectrical and 24 refraction seismic profiles within this study confirmed that ice-rich permafrost is not restricted to rock glaciers but is also observed in non-rock-glacier permafrost slopes in the form of interstitial ice, as well as layers with excess ice, resulting in substantial ice contents. Consequently, non-rock-glacier permafrost landforms, whose role for local hydrology has so far not been considered in remote-sensing-based approaches, may be similarly relevant in terms of ground ice content on a catchment scale and should not be ignored when quantifying the potential hydrological significance of permafrost. We show that field-geophysics-based estimates of ground ice content, while more labour intensive, are considerably more accurate than remote sensing approaches. The geophysical data can then be further used in upscaling studies to the catchment scale in order to reliably estimate the hydrological significance of permafrost within a catchment.

Towards accurate quantification of ice content in permafrost of the Central Andes – Part 2: An upscaling strategy of geophysical measurements to the catchment scale at two study sites

“…The bedrock is in general highly fractured as a result of the numerous fault systems that cross the area (Devine et al, 2019). The results of the geophysical surveys presented in Part 1 (see Hilbich et al, 2022a) point to largely homogeneous subsurface conditions with significant ground ice occurrences (mostly in terms of a thin, icerich layer varying in thicknesses of approximately 2-5 m). Furthermore, ground ice is expected to be present as well in the highly fractured and hydrothermally altered bedrock at greater depths .…”

“…Due to the lack of in situ investigations and subsurface information, commonly a mean volumetric ice content of 40 % to 60 % is assumed for the entire rock glacier area (Brenning, 2005;Rangecroft et al, 2015;Bodin et al, 2010;Azócar and Brenning, 2010;Rangecroft et al, 2015;Jones et al, 2019). This can be problematic as the ground ice content of rock glaciers has been shown to vary considerably from case to case (Arenson and Jakob, 2010;Hauck et al, 2011;Mollaret et al, 2020;Halla et al, 2021;Hilbich et al, 2022a) and also along longitudinal profiles of a single rock glacier (Jones et al, 2019;Halla et al, 2021). Furthermore, while rock glaciers are the focus of several studies assessing the hydrological importance of permafrost in semiarid regions, knowledge about the permafrost distribution outside of rock glaciers is still extremely limited (Arenson and Jakob, 2010;Duguay et al, 2015) even though ice-rich permafrost has been shown to be present in areas devoid of rock glaciers (García et al, 2017;Schaffer et al, 2019).…”

“…In order to remove this bias and estimate the total ground ice volume more accurately, Hilbich et al (2022a) presented an extensive geophysical data set from several permafrost regions in the Central Andes of Chile and Argentina consisting of electrical resistivity tomography (ERT) and refraction seismic tomography (RST) measurements, which can be used for the detection of permafrost and the quantification of the ground ice content. In the absence of boreholes, geophysical investigations are a feasible and cost-effective technique to detect and quantify ground ice occurrences within a variety of landforms and substrates (e.g., Hauck and Kneisel, 2008a;Hilbich et al, 2009Hilbich et al, , 2011Monnier and Kinnard, 2013;Mewes et al, 2017;Pellet et al, 2016;Mollaret et al, 2019;Halla et al, 2021;Mollaret et al, 2020).…”

“…In this study we aim to bridge the scale gap between the individual geophysical profiles and the catchment scale by establishing an upscaling strategy, which is then applied to two study sites in the Central Andes of Chile and Argentina. The geophysical data and analyses are hereby based on the companion paper (Hilbich et al, 2022a). We present first estimates of total ground ice contents (and water equivalents) of (a) a rockglacier-dominated site (Site A) and (b) a rock-glacier-free site (Site D).…”

“…In addition, ground ice contents in permafrost areas outside of rock glaciers are usually not considered, resulting in a significant uncertainty regarding the volume of ground ice in the Andes and its hydrological role. In Part 1 of this contribution, Hilbich et al (2022a) present an extensive geophysical data set based on electrical resistivity tomography and refraction seismic tomography surveys to detect and quantify ground ice of different landforms and surface types in several study regions in the semi-arid Andes of Chile and Argentina with the aim to contribute to the reduction of this data scarcity.…”