Brady A. Flinchum's research while affiliated with Clemson University and other places

This study explores the impact of deep ( >5 m) critical zone (CZ) architecture on vegetation distribution in a semi-arid snow-dominated climate. Utilizing seismic refraction surveys, we identified a significant correlation between saprolite thickness and LiDAR-derived canopy heights (R²=0.47). We argue that CZ structure, specifically shallow fractured bedrock under valley bottoms, redirects groundwater to locations where trees are established—suggesting they are located in specific 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 and resilience.

This study explores the impact of deep ( >5 m) critical zone (CZ) architecture on vegetation distribution in a semi-arid snow-dominated climate. Utilizing seismic refraction surveys, we identified a significant correlation between saprolite thickness and LiDAR-derived canopy heights (R²=0.47). We argue that CZ structure, specifically shallow fractured bedrock under valley bottoms, redirects groundwater to locations where trees are established—suggesting they are located in specific 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 and resilience.

For decades, seismic imaging methods have been used to study the critical zone, Earth's thin, life‐supporting skin. The vast majority of critical zone seismic studies use traveltime tomography, which poorly resolves heterogeneity at many scales relevant to near‐surface processes, therefore limiting progress in critical zone science. Full‐waveform tomography can overcome this limitation by leveraging more seismic data and enhancing the resolution of geophysical imaging. In this study, we apply 2D full‐waveform tomography to match the phases of observed seismograms and elucidate previously undetected heterogeneity in the critical zone at a well‐studied catchment in the Laramie Range, Wyoming. In contrast to traveltime tomograms from the same data set, our results show variations in depth to bedrock ranging from 5 to 60 m over lateral scales of just tens of meters and image steep low‐velocity anomalies suggesting hydrologic pathways into the deep critical zone. Our results also show that areas with thick fractured bedrock layers correspond to zones of slightly lower velocities in the deep bedrock, while zones of high bedrock velocity correspond to sharp vertical transitions from bedrock to saprolite. By corroborating these findings with borehole imagery, we hypothesize that lateral changes in bedrock fracture density majorly impact critical zone architecture. Borehole data also show that our full‐waveform tomography results agree significantly better with velocity logs than previously published traveltime tomography models. Full‐waveform tomography thus appears unprecedentedly capable of imaging the spatially complex porosity structure crucial to critical zone hydrology and processes.

Plain Language Summary When a material is squeezed, the ratio between the change in height and width is described by an elastic parameter called Poisson's ratio. Most earth materials have a positive Poisson ratio, meaning the material will expand when squeezed (e.g., Playdough or wet sand). Materials with a negative Poisson's ratio rarely occurs naturally and will shrink in all directions when squeezed. Cork is a common material with a Poisson's ratio of approximately zero. Cork is ideal for bottling wine because its width does not change when pushing it into the bottle's narrow neck. Here we use surface‐based measurements to quantify Poisson's ratio from P‐wave (Vp) and S‐wave (vs.) velocities in the top 50 m of Earth's surface. Our results show an unexpected result—material in the CZ has a negative Poisson's ratio. We believe this unexpected behavior is caused by the combination of low effective pressures and small and irregular cracks created during rocks' transformation into soil. The cracks have a greater impact on the material's ability to resist compression. At the same time, most of the rock is still coherent and thus only experiences a minimal loss of shear strength.

In this study, we introduce a novel field‐based method to estimate specific yield (S y ) in fractured, low‐porosity granite aquifers using borehole nuclear magnetic resonance (bNMR). This method requires collecting a bNMR survey immediately following a pump test, which dewaters the near‐borehole fractures. The residual water content measured from bNMR is interpreted as “bound” and represents the specific retention (S r ) while the water drained by the pump is the S y . The transverse relaxation cutoff time (T 2C ) is the length of time that partitions the total porosity measured by bNMR into S r and S y . When applying a calibrated T 2C , S y equals the bNMR total porosity minus S r ; thus, a calibrated T 2C is required to determine S y directly from NMR results. Based on laboratory experiments on sandstone cores, the default T 2C is 33 ms; however, its applicability to fractured granite aquifers is uncertain. The optimal T 2C based on our pumping test is 110 ± 25 ms. Applying this calibrated T 2C on a saturated, A‐type granite at our field site, we estimate the S y to be 0.012 ± 0.005 m ³ m ‐3 which is significantly different from the S y (0.021 ± 0.005 m ³ m ‐3 ) estimate using the default T 2C of 33 ms. This S y estimate falls within a range determined using traditional hydraulic testing at the same site. Using the conventional T 2C (33 ms) for fractured granite leads to an inaccurate S y ; therefore, it is essential to calibrate the bNMR T 2C for the local site conditions prior to estimating S y . This article is protected by copyright. All rights reserved.

Integrated modeling of headwater watersheds in mountain environments is often limited by the lack of hydrological characterization and monitoring data. For the No-Name watershed in the Medicine Bow Mountains in Wyoming (USA), this research integrates regional surface and subsurface hydrological and geophysical measurements to create three-dimensional integrated hydrological models with which interactions between surface water, soil water, and groundwater are elucidated. Data used to build and calibrate the integrated model include a digital elevation model (DEM), stream discharge at the outlet of the watershed, soil-moisture data, weather data, and geophysical surveys including seismic refraction, airborne resistivity, and nuclear magnetic resonance (NMR). Based on interpretation of geophysical measurements, subsurface hydrostratigraphy consists of a top unconsolidated layer, a middle layer of fractured granite and metamorphic bedrock, and a lower protolith. Given that both measurements and interpretations have uncertainty, a sensitivity analysis was carried out to evaluate conceptual model uncertainty, which suggests the following: (1) for predicting stream discharge at the No-Name outlet, the most influential parameters are the Manning coefficient, DEM, hydrostratigraphy and hydraulic conductivity, and land cover. Compared to a lower-resolution DEM, a LiDAR DEM can lead to more accurate predictions of the stream discharge and stream elevation profile. (2) For predicting soil moisture, the most influential parameters are hydrostratigraphy and the associated hydraulic conductivities and porosities. (3) Based on a calibration exercise, the likely values for subsurface hydraulic conductivity at No-Name are ~10–5 m/s (the unconsolidated layer), ~10–6 m/s (fractured bedrock), and ~10–6 m/s or lower (protolith).

For decades, seismic imaging methods have been used to study the critical zone, Earth’s thin, life-supporting skin. The vast majority of critical zone seismic studies use traveltime tomography, which poorly resolves heterogeneity at many scales relevant to near-surface processes, therefore, limiting progress in critical zone science. Full-waveform inversion can overcome this limitation by leveraging more of the seismic waveform and enhancing the resolution of geophysical imaging. In this study, we apply full-waveform inversion to elucidate previously undetected heterogeneity in the critical zone at a well-studied catchment in the Laramie Range, Wyoming. In contrast to traveltime tomograms from the same data set, our results show variations in depth to bedrock ranging from 5 to 60 meters over lateral scales of just tens of meters and image steep low-velocity anomalies suggesting hydrologic pathways into the deep critical zone. Our results also show that areas with thick fractured bedrock layers correspond to zones of slightly lower velocities in the deep bedrock, while zones of high bedrock velocity correspond to sharp vertical transitions from bedrock to saprolite. By corroborating these findings with borehole imagery, we hypothesize that lateral changes in bedrock fracture density majorly impact critical zone architecture. Borehole data also show that our full-waveform inversion results agree significantly better with velocity logs than previously published traveltime tomography models. Full-waveform inversion thus appears unprecedently capable of imaging the spatially complex porosity structure crucial to critical zone hydrology and processes.

Physical, chemical, and biological processes create and maintain the critical zone (CZ). In weathered and crystalline rocks, these processes occur over 10–100 s of meters and transform bedrock into soil. The CZ provides pore space and flow paths for groundwater, supplies nutrients for ecosystems, and provides the foundation for life. Vegetation in the aboveground CZ depends on these components and actively mediates Earth system processes like evapotranspiration, nutrient and water cycling, and hill slope erosion. Therefore, the vertical and lateral extent of the CZ can provide insight into the important chemical and physical processes that link life on the surface with geology 10–100 s meters below. In this study, we present 3.9 km of seismic refraction data in a weathered and crystalline granite in the Laramie Range, Wyoming. The refraction data were collected to investigate two ridges with clear contrasts in vegetation and slope. Given the large contrasts in slope, aspect, and vegetation cover, we expected large differences in CZ structure. However, our results suggest no significant differences in large-scale (>10 s of m) CZ structure as a function of slope or aspect. Our data appears to suggest a relationship between LiDAR-derived canopy height and depth to fractured bedrock where the tallest trees are located over regions with the shallowest depth to fractured bedrock. After separating our data by the presence or lack of vegetation, higher P-wave velocities under vegetation is likely a result of higher saturation.

Forests are increasingly threatened by climate-change-fuelled cycles of drought, dieback and wildfires. However, for reasons that remain incompletely understood, some forest stands are more vulnerable than others, leaving a patchwork of varying dieback and wildfire risk after drought. Here, we show that spatial variability in forest drought response can be explained by differences in underlying bedrock. Our analysis links geochemical measurements of bedrock composition, geophysical measurements of subsurface weathering and remotely sensed changes in evapotranspiration during the 2011–2017 drought in California. We find that evapotranspiration plummeted in dense forest stands rooted in weathered, nutrient-rich bedrock. By contrast, relatively unweathered, nutrient-poor bedrock supported thin forest stands that emerged unscathed from the drought. By influencing both subsurface weathering and nutrient supply, bedrock composition regulates the balance of water storage and demand in mountain ecosystems. However, rather than enhancing forest resilience to drought by providing more water-storage capacity, bedrock with more weatherable and nutrient-rich minerals induced greater vulnerability by enabling a boom–bust cycle in which higher ecosystem productivity during wet years drives excess plant water demand during droughts.