About

Ciaran Harman is an associate professor of landscape hydrology and the Russell Croft Faculty Scholar in the Department of Environmental Health and Engineering at Johns Hopkins University. His research focuses on how the structure of landscapes controls the movement of water from rainfall to streams and how that structure evolves over time. Harman’s work addresses fundamental gaps that inhibit reliable predictions of streamflow quantity and quality in headwater catchments. He combines theory, experiments, modeling, fieldwork, and data analysis to understand flow and transport across hydrologic scales and their links to the co-evolution of landscape structure. He currently directs the Landscape Hydrology Lab, which aims to address fundamental challenges in hydrologic science and the emerging interdisciplinary field of critical zone science. His lab investigates how water, solutes, and sediments move through landscapes and translates that understanding into mechanistic insights and predictive models. Harman’s group also studies how the hydrology of specific landscapes might have co-evolved with other ecological, geomorphic, and geochemical processes operating concurrently over their histories. His contributions have helped establish the theory of storage selection functions for lumped transport modeling. Harman is a member of several professional organizations, including the European Geosciences Union, American Geophysical Union, Geologic Society of America, and International Association of Hydrologic Sciences. His awards include an NSF Career Award, an Early Career Award from the American Geophysical Union, and an Editors Citation for Excellence in Refereeing for Geophysical Research Letters. He holds undergraduate degrees in arts and engineering from the University of Western Australia and advanced degrees in geography and civil engineering from the University of Illinois at Urbana-Champaign. After postdoctoral research at the University of Arizona, he joined Johns Hopkins University in 2012, where he holds joint appointments in the departments of Environmental Health and Engineering and Earth and Planetary Sciences.

Research topics

• Computer Science
• Mathematics
• Geology
• Environmental science
• Geography
• Meteorology
• Engineering
• Cartography
• Ecology
• Artificial Intelligence
• Machine Learning
• Geotechnical engineering
• Operations research
• Transport engineering
• Data science
• Climatology
• Environmental resource management

Selected publications

• Creating a Critical Zone: Feedbacks Between Bedrock Geology, Hydrology, and Vegetation on an Exposed Bedrock Surface, Panola Mountain, Georgia, USA

  2026-02-26

  articleOpen access

  This preprint is the original submitted manuscript. The final published version is available open-access in the Journal of Geophysical Research: Earth Surface at: doi.org/10.1029/2025JF008424 Most of Earth's present-day terrestrial surface is covered by regolith --- the layers of soil, saprolite, and weathered bedrock that together comprise the critical zone. Recent research has focused on understanding fluxes of minerals, water, and energy through the critical zone under steady state assumptions. However, in eroding landscapes, regolith and soil are produced from the bedrock as it is exhumed. Therefore, at some point in time, every location on the Earth's surface currently mantled by regolith experienced an onset of weathering processes. This initial creation of a critical zone from rock is poorly understood. Here we study initial critical zone formation from exposed bedrock by combining surface and subsurface geophysical observations at a site where regolith appears to be forming from bedrock on a granodiorite outcrop in Panola Mountain State Park, Georgia, USA. Vegetation gains an initial foothold on the outcrop by colonizing microtopographic depressions created by differential weathering of contrasting bedrock compositions. We observe a range of colonization stages, from moss to grasses to small bushes and eventually to large trees. Subsurface signatures of the vegetation include enhanced radar reflectance and reduced seismic velocities, with larger vegetation associated with stronger subsurface signals. Using a space-for-time substitution approach, we propose an evolutionary sequence for critical zone development. While disentangling the chicken-and-egg questions that pervade this topic remains challenging, our results suggest that geological heterogeneity can provide the initial catalyst for colonization, but ultimately vegetation itself plays a strong role in producing subsurface structures we associate with the critical zone.

  PublisherDOI

• Creating a Critical Zone: Feedbacks Between Bedrock Geology, Water Retention, and Vegetation on an Exposed Bedrock Surface, Panola Mountain, Georgia, USA

  Journal of Geophysical Research Earth Surface · 2026-01-01

  articleOpen access

  Abstract Most of Earth's present‐day terrestrial surface is covered by regolith—the layers of soil, saprolite, and weathered bedrock that together comprise the critical zone. Recent research has focused on understanding fluxes of minerals, water, and energy through the critical zone under steady state assumptions. However, in eroding landscapes, regolith and soil are produced from the bedrock as it is exhumed. Therefore, at some point in time, every location on the Earth's surface currently mantled by regolith experienced an onset of weathering processes. This initial creation of a critical zone from rock is poorly understood. Here we study initial critical zone formation from exposed bedrock by combining surface and subsurface geophysical observations at a site where regolith appears to be forming from bedrock on a granodiorite outcrop in Panola Mountain State Park, Georgia, USA. Vegetation gains an initial foothold on the outcrop by colonizing microtopographic depressions created by differential weathering of contrasting bedrock compositions. We observe a range of colonization stages, from moss to grasses to small bushes and eventually to large trees. Subsurface signatures of the vegetation include enhanced radar reflectance and reduced seismic velocities, with larger vegetation associated with stronger subsurface signals. Using a space‐for‐time substitution approach, we propose an evolutionary sequence for critical zone development. While disentangling the chicken‐and‐egg questions that pervade this topic remains challenging, our results suggest that geological heterogeneity can provide the initial catalyst for colonization, but ultimately vegetation itself plays a strong role in producing subsurface structures associated with the critical zone.

  PublisherDOI

• Recent Advances in Tracer‐Aided Mixing Modeling of Water in the Critical Zone

  Reviews of Geophysics · 2025-09-01 · 4 citations

  articleOpen access

  Abstract Safeguarding water resources for society and ecosystems requires a comprehensive understanding of hydrological fluxes within the Critical Zone, Earth's living skin where the atmosphere, hydrosphere, biosphere, and lithosphere meet. For decades, tracer‐aided mixing models have been used to track water flow paths through the Critical Zone, mapping the journey of water particles from atmospheric moisture to groundwater. Recent advances in novel tracer measurements and modeling methodologies offer new insights into hydrological partitioning within the Critical Zone, enabling improved quantification of water fluxes across scales ranging from microscopic to macroscopic. Advanced tracer‐aided modeling approaches enable more rigorous testing of assumptions and improved quantification of uncertainties. In this review, we (a) summarize state‐of‐the‐art tracer and modeling techniques, with an emphasis on stable water isotope tracers, (b) synthesize insights emerging from new approaches, and (c) highlight opportunities to apply these methods in interdisciplinary Critical Zone research.

  PublisherDOI

• Creating a Critical Zone: Feedbacks Between Bedrock Geology, Hydrology, and Vegetation on an Exposed Bedrock Surface, Panola Mountain, Georgia, USA

  2025-04-07

  preprintOpen access

  Most of Earth's present-day terrestrial surface is covered by regolith --- the layers of soil, saprolite, and weathered bedrock that together comprise the critical zone. Recent research has focused on understanding fluxes of minerals, water, and energy through the critical zone under steady state assumptions. However, in eroding landscapes, regolith and soil are produced from the bedrock as it is exhumed. Therefore, at some point in time, every location on the Earth's surface currently mantled by regolith experienced an onset of weathering processes. This initial creation of a critical zone from rock is poorly understood. Here we study initial critical zone formation from exposed bedrock by combining surface and subsurface geophysical observations at a site where regolith appears to be forming from bedrock on a granodiorite outcrop in Panola Mountain State Park, Georgia, USA. Vegetation gains an initial foothold on the outcrop by colonizing microtopographic depressions created by differential weathering of contrasting bedrock compositions. We observe a range of colonization stages, from moss to grasses to small bushes and eventually to large trees. Subsurface signatures of the vegetation include enhanced radar reflectance and reduced seismic velocities, with larger vegetation associated with stronger subsurface signals. Using a space-for-time substitution approach, we propose an evolutionary sequence for critical zone development. While disentangling the chicken-and-egg questions that pervade this topic remains challenging, our results suggest that geological heterogeneity can provide the initial catalyst for colonization, but ultimately vegetation itself plays a strong role in producing subsurface structures we associate with the critical zone.

  PublisherOA PDFDOI

• A Null Model for Global Root Depth Distributions: Analytical Solution and Comparison to Data

  Ecohydrology · 2025-04-01 · 1 citations

  articleOpen access1st authorCorresponding

  ABSTRACT To accurately predict earth system response to global change, we must be able to predict the responses of important properties of that system, such as the depths over which plant roots are distributed. In 2008, H. J. Schenk proposed a model for the depth distribution of plant roots based on a simple hydrological scheme and the assumptions that plants will take up the shallowest water available first and will distribute their roots in proportion to long‐term mean uptake at each depth. Here, we derive an analytical solution to the Schenk model under an idealised climate (in which infiltration events are treated as a marked Poisson process), explore properties of the result and compare with data. The solution suggests that in very humid and arid climates, the soil wetting and drying cycles induced by root water uptake are generally confined to a characteristic depth below the surface. This depth depends on the typical magnitude of rainfall events (most strongly so in arid climates), the typical total transpiration demand between rainfall events (most strongly in humid climates) and the plant‐available water holding capacity of the soil. Root water uptake (and thus predicted root density) in very humid and arid landscapes decreases exponentially with depth at a rate determined by this characteristic depth. However, in a mesic climate, soils may be wet or dry to greater depths below the near‐surface, and the duration spent in each state increases with depth. Consequently, root water uptake and root density in mesic climates more closely resemble a power law distribution. When the aridity index is exactly 1, the characteristic depth diverges and the mean rooting depth approaches infinity. This suggests that the most skewed root depth distributions might occur in mesic environments. We compared this model to another analytical solution and a compiled database of root distributions (159 combined locations). For a larger comparison dataset, we also compared 99th percentile rooting depth to rooting depths modeled by two other frameworks and a database of observed rooting depths (1271 combined locations). Results demonstrate that the analytical formulation of the Schenk model performs well as a shallow bound on rooting depths and captures something of the nonexponential form of root distributions, and its error is similar to or less than that of other modeling frameworks. Errors may be partly explained by the deviation of real climate from the idealisations used to obtain an analytical solution (exponentially distributed infiltration events and no seasonality).

  PublisherOA PDFDOI

• On the role of inherited rock fabric in critical zone porosity development: Insights from seismic anisotropy measurements using surface waves

  Earth Surface Processes and Landforms · 2025-07-01 · 3 citations

  articleOpen access

  Abstract Within Earth's critical zone, weathering processes influence landscape evolution and hillslope hydrology by creating porosity in bedrock, transforming it into saprolite and eventually soil. In situ weathering processes drive much of this transformation while preserving the rock fabric of the parent material. Inherited rock fabric in regolith makes the critical zone anisotropic, affecting its mechanical and hydrological properties. Therefore, quantifying and studying anisotropy is an important part of characterising the critical zone, yet doing so remains challenging. Seismic methods can be used to detect rock fabric and infer mechanical and hydrologic conductivity anisotropy across landscapes. We present a novel way of measuring seismic anisotropy in the critical zone using Rayleigh and Love surface waves. This method leverages multi‐component surface seismic data to create a high‐resolution model of seismic anisotropy, which we compare with a nuclear magnetic resonance log measured in a nearby borehole. The two geophysical data sets show that seismic anisotropy and porosity develop at similar depths in weathered bedrock and both reach their maximum values in saprolite, implying that in situ weathering enhances anisotropy while concurrently generating porosity in the critical zone. We bolster our findings with in situ measurements of seismic and hydrologic conductivity anisotropy made in a 3 m deep soil excavation. Our study offers a fresh perspective on the importance of rock fabric in the development and function of the critical zone and sheds new insights into how weathering processes operate.

  PublisherOA PDFDOI

• Using seismic refraction data to estimate a relationship between landscape curvature and deep critical zone structure in the South Carolina Piedmont, USA

  Repository for Publications and Research Data (ETH Zurich) · 2025-11-01

  otherOpen access

  P-wave velocity profiles from seismic refraction reveal deep critical zone (CZ) architecture along profiles hundreds of meters long. However, extrapolating local velocity measurements to infer CZ architecture at regional scales (1–20 km2) remains challenging. Here, we present a strategy that transforms seismic observations from individual profiles into maps of CZ architecture spanning tens of square kilometers. Data from 15 seismic refraction profiles (approximately 6.6 km total length) collected in weathered crystalline rocks of the South Carolina Piedmont, USA, revealed approximately 400,000 m2 of deep CZ architecture. Using casing depths from four boreholes, we show that the boundary dividing saprolite and fractured rock corresponds to a velocity of 1,870 m/s. Using velocity measurements from an outcrop within the survey area, we identify the bedrock velocity as 4,550 m/s. These velocities define a three-layer CZ structure comprising soil and saprolite, fractured bedrock, and unweathered bedrock. We developed an empirical relationship between CZ structure and minimum and maximum principal curvatures, enabling prediction of CZ architecture over approximately 17 km2. The correlation between seismically inferred CZ structure and principal curvatures at our study site suggests that curvature metrics can be used to predict CZ structure at larger scales in crystalline terrains under subtropical climates. However, the empirical relationship struggled to predict CZ structure where landscape curvatures were near zero, suggesting that other variables likely contribute to local heterogeneity. Given that curvature is an important variable for erosion and groundwater flow, our results suggest it could be a promising metric for predicting CZ structure.

  PublisherDOI

• Exploring the connection between critical zone structure and tree distribution in a semiarid eroding landscape with shallow seismic refraction

  Vadose Zone Journal · 2025-04-28 · 3 citations

  articleOpen accessSenior author

  Abstract This study explores the impact of deep (5–40 m) critical zone (CZ) structure on vegetation distribution in a semiarid snow‐dominated climate. Utilizing seismic refraction surveys, we identified a significant negative correlation between seismically derived saprolite thickness and light detecting and ranging‐derived vegetation heights ( R = −0.66). We argue that CZ structure, specifically shallow fractured bedrock under valley bottoms, provides moisture near the surface where trees are established—suggesting the trees are situated in locations with access to nutrients and water. This work provides a unique spatially exhaustive perspective and adds to growing evidence that in addition to other factors such as slope, aspect, and climate, deep CZ structure plays a vital role in ecosystem development.

  PublisherOA PDFDOI

• Using Seismic Refraction Data to Estimate a Relationship Between Landscape Curvature and Deep Critical Zone Structure in the South Carolina Piedmont, USA

  Journal of Geophysical Research Earth Surface · 2025-11-01 · 1 citations

  articleOpen access

  Abstract P‐wave velocity profiles from seismic refraction reveal deep critical zone (CZ) architecture along profiles hundreds of meters long. However, extrapolating local velocity measurements to infer CZ architecture at regional scales (1–20 km 2 ) remains challenging. Here, we present a strategy that transforms seismic observations from individual profiles into maps of CZ architecture spanning tens of square kilometers. Data from 15 seismic refraction profiles (approximately 6.6 km total length) collected in weathered crystalline rocks of the South Carolina Piedmont, USA, revealed approximately 400,000 m 2 of deep CZ architecture. Using casing depths from four boreholes, we show that the boundary dividing saprolite and fractured rock corresponds to a velocity of 1,870 m/s. Using velocity measurements from an outcrop within the survey area, we identify the bedrock velocity as 4,550 m/s. These velocities define a three‐layer CZ structure comprising soil and saprolite, fractured bedrock, and unweathered bedrock. We developed an empirical relationship between CZ structure and minimum and maximum principal curvatures, enabling prediction of CZ architecture over approximately 17 km 2 . The correlation between seismically inferred CZ structure and principal curvatures at our study site suggests that curvature metrics can be used to predict CZ structure at larger scales in crystalline terrains under subtropical climates. However, the empirical relationship struggled to predict CZ structure where landscape curvatures were near zero, suggesting that other variables likely contribute to local heterogeneity. Given that curvature is an important variable for erosion and groundwater flow, our results suggest it could be a promising metric for predicting CZ structure.

  PublisherOA PDFDOI

• Recent Advances in Tracer‐Aided Mixing Modeling of Water in the Critical Zone

  Universität Zürich, ZORA · 2025-09-01

  articleOpen access

  Safeguarding water resources for society and ecosystems requires a comprehensive understanding of hydrological fluxes within the Critical Zone, Earth's living skin where the atmosphere, hydrosphere, biosphere, and lithosphere meet. For decades, tracer‐aided mixing models have been used to track water flow paths through the Critical Zone, mapping the journey of water particles from atmospheric moisture to groundwater. Recent advances in novel tracer measurements and modeling methodologies offer new insights into hydrological partitioning within the Critical Zone, enabling improved quantification of water fluxes across scales ranging from microscopic to macroscopic. Advanced tracer‐aided modeling approaches enable more rigorous testing of assumptions and improved quantification of uncertainties. In this review, we (a) summarize state‐of‐the‐art tracer and modeling techniques, with an emphasis on stable water isotope tracers, (b) synthesize insights emerging from new approaches, and (c) highlight opportunities to apply these methods in interdisciplinary Critical Zone research.

  PublisherDOI

Recent grants

• Collaborative Research: Hillslope hydrology under glass: Controlled experimental testing of hillslope-scale hydrologic transport theories at Biosphere2

  NSF · $230k · 2014–2018

• Collaborative Research: Network Cluster: Bedrock controls on the deep critical zone, landscapes, and ecosystems

  NSF · $531k · 2020–2026

• CAREER: proposal to link catchment hydrologic transport to the evolved architecture of the critical zone

  NSF · $633k · 2017–2022

• COLLABORATIVE RESEARCH: COUPLED HYDROLOGICAL AND GEOCHEMICAL PROCESS EVOLUTION AT THE LANDSCAPE EVOLUTION OBSERVATORY

  NSF · $189k · 2014–2019

Frequent coauthors

• Murugesu Sivapalan

  67 shared

• P. A. Troch

  University of Arizona

  48 shared

• Katherine R. Barnhart

  United States Geological Survey

  25 shared

• Gregory E. Tucker

  23 shared

• David G. Litwin

  Helmholtz Centre Potsdam - GFZ German Research Centre for Geosciences

  21 shared

• Minseok Kim

  20 shared

• Sally Thompson

  17 shared

• Phillip Blaen

  University of Birmingham

  17 shared

Education

• PhD, Civil and Environmental Engineering

  University of Illinois at Urbana Champaign

  2011

Awards & honors

• NSF Career Award (2017)
• Early Career Award from the American Geophysical Union (2016…
• Editors Citation for Excellence in Refereeing for Geophysica…
• 2021 recipient of AGU James B. Macelwane Medal