Executive Summary:
Hydrology and Critical Zone

The critical zone (CZ) extends from Earth’s deepest groundwater to its tallest tree and thus is host to the complex biological, geochemical, and hydrological processes that drive terrestrial biogeochemical cycles, break bedrock down into life-sustaining soil, and move water across and through landscapes. This outermost skin of Earth supports terrestrial ecosystems and supplies essential food, fiber, and water to human populations and economies, which have in turn dramatically altered the form and function of the CZ in ways that remain poorly understood. While humans interact with the CZ mostly at the surface, much of it is buried at depth and remains difficult to access and study without drilling and imaging. Here we outline major research questions that can be answered through drilling, sampling, and imaging of the soil, weathered rock, and aquifers that comprise Earth’s CZ.

Scientific Theme 1: Controls on subsurface CZ structure. The depth, porosity, permeability, and strength of subsurface CZ materials mediate the flow of water, nutrients, and energy through the CZ. Therefore, quantifying the processes that drive subsurface CZ structure is fundamental to understanding Earth system processes. However, there is still not a unifying theory that can explain spatial variations in weathering across landscapes. Below are the major research questions related to characterizing CZ structure that can be advanced through subsurface drilling and imaging:

·  How do bedrock properties (e.g., composition, foliation, fracture density) dictate chemical, physical, and biological weathering processes?

·  How do preexisting bedrock fractures influence the depth of subsurface weathering by guiding reactive fluids?

·  What is the role of biologic processes (e.g. microbes, mycorrhizae, roots) in mediating CZ development, geochemical weathering, biogeochemical cycling, and hydrologic functions?

Scientific Theme 2: Controls of CZ structure on hydrologic, ecological, and geomorphic functions. The three-dimensional structure of the subsurface CZ and its composition, material properties, and pressure distributions control how water moves in, out, and through it. These fluxes have significant impacts on water resources, ecosystems, and geomorphic hazards. The shallow subsurface is the focus of the vast majority of studies, leaving a major gap in understanding the role of the deep CZ in subsurface and surface processes. Key questions that can be answered using drilling include:

·  How does the capacity to store water (porosity, fracture aperture and density) and transmit water (permeability, anisotropy) evolve during weathering? How does this affect transit times from rainfall to streams, and the persistence of streamflow during droughts?

·  How does solute generation in the deep CZ affect surface water chemistry?

·  How does vegetation access and use stored groundwater and nutrients in the deep subsurface? How deep do roots and associated fungal hyphae penetrate, and what is their role in redistributing water, nutrients, and carbon within the CZ? How does the deep CZ structure determine an ecosystem’s resilience to drought?

Scientific Theme 3: Interactions between humans and the deep CZ. The critical zone supplies life-sustaining resources vital to human society, including replenishing soil and freshwater, while also generating landslides, floods, sinkholes, subsidence, and other damaging land surface hazards. Human activities have transformed Earth’s surface and deep CZ in multiple ways, affecting resource sustainability and potentially magnifying hazards. Major related problems that require drilling-based investigation include:

·  Characterizing the depth and extent over which human activity has altered the CZ and the timescale of response and recovery.

·  Understanding how the quantity and quality of water resources will respond to climate change and increased human use. Studies should be focused across a wide range of environments including mountain headwater systems, coastal and alluvial aquifers, and urban and agricultural settings.

·  Informing models of CZ structure and hydrologic function that enable better forecasting of land surface hazards (landslides, debris flows, flash floods, sinkholes) under a range of conditions, including following wildfires and groundwater extractions.

·  Reconstructing past changes in the CZ from paleo-records, from the time of earliest human origins to soil erosion over recent decades, to place anthropogenic changes in the context of “baselines” which are otherwise difficult to define.

Scientific Theme 4: Synthesis of CZ processes over spatial and temporal scales. There is a need to expand CZ studies to understudied but ecologically and biogeochemically important regions such as the tropics and tectonically active mountain ranges. Synthesis across temporal scales is critical, especially during this current climate crisis. Drilling can help investigate these CZ processes by:

·  Incorporating insights from past and future drilling with preexisting geospatial datasets to model continental scale questions surrounding CZ processes (e.g. spatial heterogeneity of unweathered bedrock, relationship between CZ structure and land surface hazards).

·  Combining shallower scale CZ work with deeper zones; for example, research on fracturing, CZ structure and meteoric water circulation could inform deeper hydrology that addresses problems related to aquifer resources both on and off-shore. 

·  Incorporating past and modern climate records to understand how climate change controls CZ development, ecologic and hydrologic processes, hydrologic resource availability, and geohazards.

Resources and tools needed to address these questions

To tackle the important questions outlined above funding and resources for additional boreholes placed in strategic locations are needed. In addition, we have outlined three additional community needs that will improve and standardize drilling technologies, promote collaborations to optimize the amount of data collected on existing drilling projects, and synthesize new and existing drilling data.

Need 1: Development and standardization of drilling, logging, and sampling techniques: The development of common drilling and sampling techniques in the CZ is critical to a holistic understanding of CZ processes. The development and standardization of drilling and sampling can be achieved through:

·  Resources and funding to facilitate drilling in difficult to access areas such as steep landscapes, in valley bottoms, and away from well-established roads.

·  Development of new drilling techniques to maximize sample recovery in weathered rock and material from fracture zones within unweathered bedrock.

·  Development of standard protocols for drilling techniques, sampling methods, and geophysical logging that make drilling projects accessible to new researchers and allow for comparison of measurements between sites.

·  Resources, funding, and expertise to facilitate logging and instrumentation of existing boreholes.

Need 2: Interdisciplinary collaboration around drilling projects: CZ science is a collaborative endeavor requiring multidisciplinary observations. Drilling projects have the opportunity to bring together researchers from multiple disciplines to collect complementary data and answer difficult research questions. However, such collaboration requires concerted effort throughout the research process. Interdisciplinary collaborations can be achieved through:

·  Resources documenting the benefits and limitations of drilling and borehole completion techniques for complementary research questions.

·  Strategies for making the most of existing drilling projects including strategies for sampling diverse materials (e.g., rocks, roots, water, microbes) throughout the drilling process and conducting geophysical logging and instrumenting boreholes following drilling.

·  Funding opportunities for additional analyses and instrumentation on existing drilling projects including projects outside of the CZ community.

Need 3: Synthesis of new and existing measurements: There is an increasingly pressing need to synthesize knowledge gained from investigations at individual sites into a more general framework of understanding about the deep CZ. Towards this end the community would benefit from:

·  More drilling campaigns aimed at testing emerging CZ hypotheses at additional sites.

·  Compiling existing borehole, core, and geophysical data into a publicly-available database following best practices (e.g. FAIR).

·  Improve inter-comparability between sites through community efforts to obtain consistent datasets by conducting standardized analyses on archived cores and new and existing boreholes.

·  Interdisciplinary workshops for achieving better inter-comparability between sites and synthesizing data across sites using conceptual frameworks, computational models, and data-analytics.

·  Short courses and summer schools to build community and knowledge amongst the next generation of CZ scientists.

Suggested Citation

Callahan, R., Flinchum, B., Aronson, E., Atwood, A., Dere, A., Donaldson, A., Dugan, B., Harman, C., Hayes, J., Hodges, C., Ma, L., Sutfin, N., Michael, H., Person, M., Riebe, C., West, J., Zimmer, M., 2024. Hydrology and Critical Zone Science Planning for Continental Drilling and Coring 2024. https://sites.google.com/umn.edu/csdscienceplanning/home/hydrology-and-critical-zone-executive-summary

Hydrology and Critical Zone Working Group

Emma Aronson, University of California Riverside

Abra Atwood, Woodwell Climate Research Center

Russell Callahan, University of Connecticut

Ashlee Dere, University of Nebraska Omaha

Amanda Donaldson, University of Texas Austin

Brandon Dugan, Colorado School of Mines

Brady Flinchum, Clemson University

Ciaran Harman, Johns Hopkins

Jorden Hayes, Dickinson College

Caitlin Hodges, Oklahoma University

Lin Ma, University of Texas El Paso

Nicholas Sutfin, United States Geological Survey

Holly Michael, University of Delaware

Mark Person, New Mexico Tech

Cliff Riebe, University of Wyoming

Josh West, University of Southern California Dornsife

Margaret Zimmer, University of Wisconsin Madison

Hydrology and Critical Zone Community Editors

Holly Barnard, University of Colorado Boulder

Susan Brantley, Penn State University

Melisa Diaz, University of Colorado Boulder

David Dralle, United States Forest Service Research and Development

Mark Engle, University of Texas El Paso

Ken Ferrier, University of Wisconsin Madison

Daniel Gavin, University of Oregon

Steven Holbrook, Virginia Tech

Mong-Han Huang, University of Maryland

Rachel Lauer, University of Calgary

Jennifer McIntosh, University of Arizona

Seulgi Moon, University of California Los Angeles

Joel Moore, Towson University

Daniella Rempe, University of Texas Austin

Kamini Singha, Colorado School of Mines

Claire Welty, University of Maryland Baltimore County

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