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Abstract. Land surface models (LSMs) are critical components of Earth system models (ESMs), enabling the simulation of energy and water fluxes that are essential for understanding climate systems. Soil hydraulic parameters, derived using pedotransfer functions (PTFs), are crucial for modeling soil–plant–water interactions; they introduce uncertainties in soil moisture simulations. However, a key knowledge gap exists in understanding how specific soil hydraulic properties contribute to these uncertainties and in identifying the regions most affected by them. This study conducts an intra-model sensitivity analysis within the Community Land Model version 5 (CLM5), examining how alternative soil parameter settings influence soil moisture variability across the contiguous United States (CONUS) using empirical orthogonal function (EOF) analysis. The EOF analysis revealed dominant spatial and temporal patterns of soil moisture across multiple experimental configurations, highlighting the impact of soil parameter variability on hydrological processes. The results showed significant discrepancies in soil moisture simulations, particularly in the central Great Plains, which may be attributed to the combination of arid climatic conditions and limitations in modeling saturated hydraulic conductivity and soil water retention curves. Seasonal soil moisture dynamics showed broad similarity to ERA5-Land patterns, with differences in magnitude and phase, indicating the importance of refined parameterization, particularly in the representation of infiltration and drainage processes. Comparisons with ERA5-Land, used here solely as a model-based reference for pattern consistency, revealed stronger similarity in regions with consistent climatic gradients but persistent differences in hydrologically complex areas, particularly in areas with arid climates, such as the Great Plains, where hydrological processes remain difficult to represent. Because CLM5 is forced by GSWP3, whereas ERA5-Land is an offline HTESSEL replay forced by ERA5, differences reflect both forcing and structural contrasts in addition to parameter effects. This research demonstrates the necessity to refine soil parameter representations, utilize high-resolution datasets, and consider climatic variability to inform the model development of LSMs. Importantly, these findings also pave the way for future efforts that incorporate dynamic soil properties into LSMs. This work illustrates the influence of soil properties on simulated variability. While the analysis documents their importance, a future direction will be to develop approaches that allow these properties to vary dynamically within LSMs. This study contributes to ongoing efforts toward more integrated modeling frameworks that capture soil–hydrology–climate interactions. 

Abstract Headwater catchments play a vital role in regional water supply and ecohydrology, and a quantitative understanding of the hydrological partitioning in these catchments is critically needed, particularly under a changing climate. Recent studies have highlighted the importance of subsurface critical zone (CZ) structure in modulating the partitioning of precipitation in mountainous catchments; however, few existing studies have explicitly taken into account the 3D subsurface CZ structure. In this study, we designed realistic synthetic catchment models based on seismic velocity‐estimated 3D subsurface CZ structures. Integrated hydrologic modeling is then used to study the effects of the shape of the weathered bedrock and the associated storage capacity on various hydrologic fluxes and storages in mountainous headwater catchments. Numerical results show that the weathered bedrock affects not only the magnitude but also the peak time of both streamflow and subsurface dynamic storage. 

This dataset contains the codes and data used in the manuscript “Influence of Subsurface Critical Zone Structure on Hydrological Partitioning in Mountainous Headwater Catchments” submitted to Geophysical Research Letters. The software requirement are summarized in requirement.txt; hydrologic modeling input data are in the folder TLnewtest2sfb2; the observation data used in the simulation are indicated as comments in the python scripts. Note that the hydrologic modeling was run in HPC (Linux system) with parallel computing. Below are the abstract of the manuscript: “Headwater catchments play a vital role in regional water supply and ecohydrology, and a quantitative understanding of the hydrological partitioning in these catchments is critically needed, particularly under a changing climate. Recent studies have highlighted the importance of subsurface critical zone (CZ) structure in modulating the partitioning of precipitation in mountainous catchments; however, few existing studies have explicitly taken into account the 3D subsurface CZ structure. In this study, we designed realistic synthetic catchment models based on seismic velocity-estimated 3D subsurface CZ structures. Integrated hydrologic modeling is then used to study the effect of the shape of the weathered bedrock bottom on various hydrologic fluxes and storages in mountainous headwater catchments. Numerical results show that the shape of the weathered bedrock bottom not only affects the magnitude but also the peak time of both streamflow and subsurface dynamic storage.” 

Abstract. Soil organic carbon (SOC) is often retained more effectively in aspen-dominated forests compared to coniferous forests in North America, yet the reasons why are unclear. A potential driver could be differences in SOC protection mechanisms. Over decades to centuries, chemical (e.g., mineral association) and physical (e.g., aggregation) processes can work to preserve SOC stocks, which can vary across cover types. To investigate this hypothesis, we evaluate controls on SOC concentrations in the Coal Creek watershed (CO, USA), a montane ecosystem dominated by quaking aspen and Engelmann spruce and underlain by granite and sandstone. We examined a combination of biological, chemical, physical, and environmental conditions to evaluate potential abiotic and biotic mechanisms of SOC preservation at multiple depths. As expected, we observed greater SOC concentrations under aspen compared to spruce. Growing-season soil moisture, temperature, and CO2 and O2 varied with slope position and aspect, and thus forest cover type. Dissolved organic carbon (DOC) was lower under aspen compared to spruce. Exo-enzyme data indicate that aspen soil microbes likely access more organically bound resources; consistent with this, soil organic N exhibited higher δ15N values, hinting at a greater degree of organic matter processing. Finally, aspen soils exhibited greater root abundance, and aspen mineral soils revealed smaller mean aggregate diameters compared to conifer sites. Our data suggest enhanced biotic activities in aspen-dominated forest soils that promote both chemical and physical protection of SOC in aspen- relative to spruce-dominated forests, which may have implications for DOC export. 

ABSTRACT Woody encroachment—the expansion of woody shrubs into grasslands—is a widely documented phenomenon with global significance for the water cycle. However, its effects on watershed hydrology, including streamflow and groundwater recharge, remain poorly understood. A key challenge is the limited understanding of how changes to root abundance, size and distribution across soil depths influence infiltration and preferential flow. We hypothesised that woody shrubs would increase and deepen coarse‐root abundance and effective soil porosity, thus promoting deeper soil water infiltration and increasing soil water flow velocities. To test this hypothesis, we conducted a study at the Konza Prairie Biological Station in Kansas, where roughleaf dogwood (Cornus drummondii) is the predominant woody shrub encroaching into native tallgrass prairie. We quantified the distribution of coarse and fine roots and leveraged soil moisture time series and electrical resistivity imaging to analyse soil water flow beneath shrubs and grasses. We observed a greater fraction of coarse roots beneath shrubs compared to grasses, which was concurrent with greater saturated hydraulic conductivity and effective porosity. Half‐hourly rainfall and soil moisture data show that the average soil water flow through macropores was 135% greater beneath shrubs than grasses at the deepest B horizon, consistent with greater saturated hydraulic conductivity. Soil‐moisture time series and electrical resistivity imaging also indicated that large rainfall events and greater antecedent wetness promoted more flow in the deeper layers beneath shrubs than beneath grasses. These findings suggest that woody encroachment alters soil hydrologic processes with cascading consequences for ecohydrological processes, including increased vertical connectivity and potential groundwater recharge. 

Abstract Forest plantations are widespread globally. Young forest plantations (hereafter young forests) differ from natural old‐growth forests (hereafter old forests) in above‐ and below‐ground structures, shaping water and carbon cycling processes. While above‐ground differences are well studied, below‐ground hydrology and biogeochemical processes remain poorly understood. Here we asked:How do hydrological flow paths and dissolved carbon processes belowground differ between young and old forests?Using a process‐based hydro‐biogeochemical model (BioRT‐HBV) constrained by streamflow and dissolved organic and inorganic carbon (DOC and DIC) data, we analyzed three pairs of young‐old forests at the H.J. Andrews Experimental Forest, Oregon, USA. Detailed simulations for a 57‐year‐old plantation (WS01) and a naturally regenerated ∼500‐year‐old forest (WS02) showed that the young forest had lower streamflow and smaller deep groundwater contributions (20%) than the old forest (30%). DOC was mainly produced in shallow soil but diverged with depth: transformed into DIC in the young forest and further produced in the old forest, yielding contrasting export patterns of flushing (DOC increases with discharge) and dilution (DOC decreases with discharge). These differences likely stem from variations in subsurface structures, supported by deeper, denser roots in old forest. Extending the analysis to two additional pairs showed (a) higher DOC and DIC concentrations in all old forests; (b) consistent DIC dilution patterns but variable DOC patterns. Numerical experiments indicate that these diverse DOC behaviors result from interactions among forest age, geology, and hydrological connectivity, and other factors, highlighting the overlooked role of forest development in subsurface carbon cycling. 

Abstract Plants allocate significant amounts of carbon belowground to support water and nutrient acquisition, and a large proportion of that carbon is allocated to the production of root exudates. One utility of exudates is to aid in phosphorus acquisition, but the amount of phosphorus that can be liberated by different exudates is largely unknown. We used three common exudates—oxalic acid, malic acid, and acid phosphatase (APase)—to extract phosphate from 25 soil profiles across the United States. These extracts represent the “maximum efficiency” of specific exudates at liberating phosphorus from a wide range of soil types, providing an upper boundary to constrain biogeochemical and land‐surface models aimed at representing exudate‐related nutrient fluxes. Our findings reveal significant variability in the efficiency of exudates to liberate phosphate across locations and soil depths. Oxalic acid exhibited the highest potential extraction efficiency overall, and its efficiency often increased in deeper horizons—likely driven by higher proportions of mineral‐bound phosphorus in deeper soil layers. APase had the lowest overall potential extraction efficiency, and its efficiency typically declined with depth and had stronger associations with soil organic carbon. Malic acid potential extraction efficiency was intermediate, and depth trends were often more similar to APase than oxalic acid. Malic acid and APase showed increasing extraction efficiency at sites with higher mean annual temperature and precipitation. This study highlights the varying efficiencies of exudate compounds in liberating soil phosphorus across ecosystems and provides a framework for understanding nutrient dynamics under changing climate or land‐use conditions. 

ABSTRACT Understanding soil organic carbon (SOC) response to global change has been hindered by an inability to map SOC at horizon scales relevant to coupled hydrologic and biogeochemical processes. Standard SOC measurements rely on homogenized samples taken from distinct depth intervals. Such sampling prevents an examination of fine‐scale SOC distribution within a soil horizon. Visible near‐infrared hyperspectral imaging (HSI) has been applied to intact monoliths and split cores surfaces to overcome this limitation. However, the roughness of these surfaces can influence HSI spectra by scattering reflected light in different directions posing challenges to fine‐scale SOC mapping. Here we examine the influence of prescribed surface orientation on reflected spectra, develop a method for correcting topographic effects, and calibrate a partial least squares regression (PLSR) model for SOC prediction. Two empirical models that account for surface slope, aspect, and wavelength and two theoretical models that account for the geometry of the spectrometer were compared using 681 homogenized soil samples from across the US that were packed into sample wells and presented to the spectrometer at 91 orientations. The empirical approach outperformed the more complex geometric models in correcting spectra taken at non‐flat configurations. Topographically‐corrected spectra reduced bias and error in SOC predicted by PLSR, particularly at slope angles greater than 30°. Our approach clears the way for investigating the spatial distributions of multiple soil properties on rough intact soil samples.