skip to main content


Title: Hydrogeophysical comparison of hillslope critical zone architecture for different geologic substrates
The belowground architecture of the critical zone (CZ) 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 hydrologic fluxes. Hydrogeophysical imaging provides unique insights into the geometries and properties of earth materials that are present in the CZ and beyond the reach of direct observation beside sparse wellbores. An improved understanding of CZ architecture can be achieved by leveraging the geophysical measurements of the subsurface. Creating categorical models of the CZ is valuable for driving hydrologic models and comparing belowground architectures between different sites to interpret weathering processes. The CZ 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 hydrofacies classes to reveal the relative wetness and weathering states. The hydrofacies classifications are robust to the choice of initial mean values used in the classification and noncontemporaneous timing of geophysical data acquisition. These results will lead to improved interdisciplinary models of CZ processes at various scales and to an increased ability to predict the hydrologic timing and partitioning. Beyond the hillslope scale, this enhanced capability to compare CZ 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.  more » « less
Award ID(s):
1818550
NSF-PAR ID:
10331259
Author(s) / Creator(s):
; ; ; ; ; ; ; ; ; ;
Date Published:
Journal Name:
GEOPHYSICS
Volume:
86
Issue:
5
ISSN:
0016-8033
Page Range / eLocation ID:
WB29 to WB49
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    Weathering and erosion processes are crucial to Critical Zone (CZ) evolution, landscape formation and availability of natural resources. Although many of these processes take place in the deep CZ (∼10–100 m), direct information about its architecture remain scarce. Near‐surface geophysics offers a cost‐effective and minimally intrusive alternative to drilling that provides information about the physical properties of the CZ. We propose a novel workflow combining seismic measurements, petrophysical modelling and geostatistical analysis to characterize the architecture of the deep CZ at the catchment scale, on the volcanic tropical island of Basse‐Terre (Guadeloupe, France). With this original workflow, we are for the first time able to jointly produce maps of the water table and the weathering front across an entire catchment, by means of a single geophysical method. This integrated view of the CZ highlights complex weathering patterns that call for going beyond “simple” hillslope CZ models.

     
    more » « less
  2. Abstract

    The structure of the critical zone (CZ) is a product of feedbacks among hydrologic, climatic, biotic, and chemical processes. Past research within snow‐dominated systems has shown that aspect‐dependent solar radiation inputs can produce striking differences in vegetation composition, topography, and soil depth between opposing hillslopes. However, far fewer studies have evaluated the role of microclimates on CZ development within rain‐dominated systems, especially below the soil and into weathered bedrock. To address this need, we characterized the CZ of a north‐facing and south‐facing slope within a first‐order headwater catchment located in central coast California. We combined terrain analysis of vegetation distribution and topography with soil pit characterization, geophysical surveys and hydrologic measurements between slope‐aspects. We documented denser vegetation and higher shallow soil moisture on north facing slopes, which matched previously documented observations in snow‐dominated sites. However, average topographic gradients were 24° and saprolite thickness was approximately 6 m across both hillslopes, which did not match common observations from the literature that showed widespread asymmetry in snow‐dominated systems. These results suggest that dominant processes for CZ evolution are not necessarily transferable across regions. Thus, there is a continued need to expand CZ research, especially in rain‐dominated and water‐limited systems. Here, we present two non‐exclusive mechanistic hypotheses that may explain these unexpected similarities in slope and saprolite thickness between hillslopes with opposing aspects.

     
    more » « less
  3. Climate warming in alpine regions is changing patterns of water storage, a primary control on alpine plant ecology, biogeochemistry, and water supplies to lower elevations. There is an outstanding need to determine how the interacting drivers of precipitation and the critical zone (CZ) dictate the spatial pattern and time evolution of soil water storage. In this study, we developed an analytical framework that combines intensive hydrologic measurements and extensive remotely-sensed observations with statistical modeling to identify areas with similar temporal trends in soil water storage within, and predict their relationships across, a 0.26 km 2 alpine catchment in the Colorado Rocky Mountains, U.S.A. Repeat measurements of soil moisture were used to drive an unsupervised clustering algorithm, which identified six unique groups of locations ranging from predominantly dry to persistently very wet within the catchment. We then explored relationships between these hydrologic groups and multiple CZ-related indices, including snow depth, plant productivity, macro- (10 2 ->10 3 m) and microtopography (<10 0 -10 2 m), and hydrological flow paths. Finally, we used a supervised machine learning random forest algorithm to map each of the six hydrologic groups across the catchment based on distributed CZ properties and evaluated their aggregate relationships at the catchment scale. Our analysis indicated that ~40–50% of the catchment is hydrologically connected to the stream channel, lending insight into the portions of the catchment that likely dominate stream water and solute fluxes. This research expands our understanding of patch-to-catchment-scale physical controls on hydrologic and biogeochemical processes, as well as their relationships across space and time, which will inform predictive models aimed at determining future changes to alpine ecosystems. 
    more » « less
  4. Wymore, A.S. ; Yang, W.H. ; Silver, W.L. ; McDowell, W.H. ; Chorover, J. (Ed.)
    The study of Critical Zone (CZ) biogeochemistry in Intensively Managed Landscapes is a study of transitions. Large-scale anthropogenic inputs in the form of agricultural practices have induced significant shifts in the transport and transformation of water, carbon, and nutrients across the landscape. Disentangling the present-day complexity of physical, biological, and hydrologic CZ processes in intensively managed landscapes requires us to first understand the interplay between underlying natural processes which have occurred over geologic time scales from the overpowering, comparatively abrupt onset of intensive agricultural practices that have dominated the landscape in recent centuries. Modeling provides a unique advantage to extricate such complex processes. Advancements in recent years have improved our ability to elucidate (1) the coevolution of soil organic carbon storage, movement, and decomposition under climate and land cover changes, (2) the impacts of intensive agricultural management practices on age-nutrient dynamics and their consequential modification of rates and landscape fluxes, and (3) the integral regulatory role of vegetation and root exudation on CZ biogeochemical processes. In this chapter, we will review several recent models developed in the Intensively Managed Landscape Critical Zone Observatory in Illinois, USA that advance our understanding of critical transitions in biogeochemical dynamics due to intensive management and discuss future challenges. 
    more » « less
  5. Abstract. Many scientists have begun to refer to the earth surface environment from the upper canopy to the depths of bedrock as the critical zone (CZ). Identification of the CZ as an integral object worthy of study implicitly posits that the study of the whole earth surface will provide benefits that do not arise when studying the individual parts. To study the CZ, however, requires prioritizing among the measurements that can be made – and we do not generally agree on the priorities. Currently, the Susquehanna Shale Hills Critical Zone Observatory (SSHCZO) is expanding from a small original focus area (0.08km2, Shale Hills catchment), to a larger watershed (164km2, Shavers Creek watershed) and is grappling with the prioritization. This effort is an expansion from a monolithologic first-order forested catchment to a watershed that encompasses several lithologies (shale, sandstone, limestone) and land use types (forest, agriculture). The goal of the project remains the same: to understand water, energy, gas, solute, and sediment (WEGSS) fluxes that are occurring today in the context of the record of those fluxes over geologic time as recorded in soil profiles, the sedimentary record, and landscape morphology.

    Given the small size of the Shale Hills catchment, the original design incorporated measurement of as many parameters as possible at high temporal and spatial density. In the larger Shavers Creek watershed, however, we must focus the measurements. We describe a strategy of data collection and modeling based on a geomorphological and land use framework that builds on the hillslope as the basic unit. Interpolation and extrapolation beyond specific sites relies on geophysical surveying, remote sensing, geomorphic analysis, the study of natural integrators such as streams, groundwaters or air, and application of a suite of CZ models. We hypothesize that measurements of a few important variables at strategic locations within a geomorphological framework will allow development of predictive models of CZ behavior. In turn, the measurements and models will reveal how the larger watershed will respond to perturbations both now and into the future.

     
    more » « less