Hydrogeophysical comparison of hillslope critical zone architecture for different geologic substrates

Abstract

The belowground architecture of the Critical Zone consists of soil and rock in various stages of weathering and wetness that acts as a medium for biological growth, mediates chemical reactions, and controls partitioning of hydrological fluxes. Hydrogeophysical imaging provides unique insights into geometries and properties of the earth materials that are present in the Critical Zone and beyond the reach of direct observation besides sparse wellbores. Improved understanding of Critical Zone architecture can be achieved by leveraging geophysical measurements of the subsurface. Creating categorical models of the Critical Zone is valuable for driving hydrological models and comparing belowground architectures between different sites to interpret weathering processes. The Critical Zone architecture is revealed through a novel comparison of hillslopes by applying facies classification in the elastic-electric domain driven by surface-based hydrogeophysical measurements. Three pairs of hillslopes grouped according to common geologic substrates – granite, volcanic extrusive, and glacially altered, are classified by five different hydro-facies classes to reveal relative wetness and weathering states. The hydro-facies classifications are robust to the choice of initial mean values used in the classification and non-contemporaneous timing of geophysical data acquisition. These results will lead to improved interdisciplinary models of Critical Zone processes at various scales, and to an increased ability to predict hydrologic timing and partitioning. Beyond the hillslope scale, this enhanced capability to compare Critical Zone architecture can also be exploited at the catchment scale with implications for improved understanding of the link between rock weathering, hydrochemical fluxes, and landscape morphology.

Hydrogeophysical Inversion of Time‐Lapse ERT Data to Determine Hillslope Subsurface Hydraulic Properties

“…The subsurface zoning method of Parsekian et al. (2021) may provide an avenue to test more complex domains as their method can resolve up to 4 unique subsurface regions whose spatial distribution are determined based on ERT and seismic tomograms.…”

“…(2018) and Parsekian et al. (2021). The ERT array was 82.5 m long with 56 electrodes spaced 1.5 m apart and was installed at a roughly 100 m long south‐facing hillslope in the eastern portion of the watershed with an average slope angle of 21.8°.…”

“…The No Name hillslope was defined as a two‐layer system based on the two‐layer classification of the hillslope presented in Parsekian et al. (2021). The surface of the model domain was defined using elevations obtained from 0.5 m resolution LiDAR data collected for No Name in 2014.…”

“…More complex subsurface domains may give more information regarding whether the benefits of coupled hydrogeophysical inversion can be realized to a greater extent in this field system. The subsurface zoning method of Parsekian et al (2021) may provide an avenue to test more complex domains as their method can resolve up to 4 unique subsurface regions whose spatial distribution are determined based on ERT and seismic tomograms.…”

“…No Name lies at elevations of 2888-3074 m above sea level in the Snowy Range approximately 50 km west of Laramie, Wyoming, USA. Electrical resistivity data were collected from a time-lapse ERT system (DAS-1 resistivity meter, Multi-Phase Technologies, Reno, NV) as described inThayer et al (2018) andParsekian et al (2021). The ERT array was 82.5 m long with 56 electrodes spaced 1.5 m apart and was installed at a roughly 100 m long south-facing hillslope in the eastern portion of the watershed with an average slope angle of 21.8°.…”