Issues

This special section focuses on soil processes and their interactions with ecological services. The main objective of this special issue is to improve synergies among soil science, hydrology, and ecology, thereby facilitating and fostering interdisciplinary approaches toward understanding emergent ecosystem behavior.

A new distributed model was developed for calculating vertical and lateral water and heat exchange in complex terrain. Model results for a small catchment near Boise, ID, showed that 11 to 16% of incoming yearly precipitation is transformed into streamflow, compared with measured values that ranged between 14 and 34%.

Savanna ecosystems have long fascinated ecologists due to the codominance of trees and grasses. We use geophysical imaging to explore the relationship between trees and grass in relation to soil properties across the landscape.

Seasonally dependent changes in soil microclimate along a grassland-forest continuum of deciduous mesquite exhibited substantial spatiotemporal variability in response to plant phenology, highlighting how microclimate along the grassland-forest continuum is sensitive to canopy architecture and seasonality.

Temporal dynamics of vadose zone water were monitored in four semi-arid plant communities to assess differences in patterns and quantity of use. The work supports the premise of two vadose zone water pools that have vastly different ecological and hydrological function.

Ecohydrologic aspects of woody plant canopy and intercanopy patches are compared for a semiarid woodland using environmental tracers. Significant patch-scale differences are indicated where evapotranspiration is higher and the evaporation/transpiration ratio much lower in canopy patches than in intercanopy patches.

Researchers in many disciplines are beginning to grasp the importance of visualization and characterization of pore-scale processes, but much work is left to be done. The five papers comprising this special section explore pore-scale research relevant for flow and transport in the vadose zone.

We developed a method using a single epifluorescence microscope to track fluorescent colloids in three dimensions based on the out-of-focus image diameter. We also present results of two mini-studies as examples of application of this method for measuring colloid transport in an idealized porous medium consisting of two contacting glass beads embedded in a chamber.

In vertical multiphase displacement, saturation overshoot is the macroscopic manifestation of a microscopically sharp wetting front. We measured the flux above which saturation overshoot occurs for seven different fluids. We found that this transition from a diffuse to a sharp front scales only with the invading fluid's viscosity.

A dynamic pore network model was utilized for extensive parametric studies of the dependency of gas flow patterns on the buoyancy or capillary forces balance, quantified as the local Bond number. Classifications of flow are provided and implications of several fascinating observations for the performance of in situ air sparging are systematically explored.

We developed a generalized geometric basis for computing pore-filling events during drainage and imbibition by calculating the position of fluid–fluid interfaces on grains of arbitrary wettability. We show that a change in wettability of a few disks alters the pattern of pore-filling events and, in turn, the pressure–saturation curve and topology of the trapped phases.

A model for velocity and solute transport with adsorption at the pore scale is presented. The model allows for multiple flow directions between pore bodies and is used to simulate the transport of an adsorbing constituent through a three-dimensional pore network. The results are used to upscale pore-scale parameters to the core scale.

A steady flow analysis is developed for measuring field-saturated hydraulic conductivity and matric flux potential in the vadose zone using cased or lined boreholes. The analysis applies for single, dual, or multiple ponded head infiltration through a permeable section in the casing, such as a well screen, open base, or pneumatic packer.

round-penetrating radar (GPR) was used for identifying underground limestone features in Yucatan, México. Limestone layers, soil pockets, and caves located up to 4 m depth were clearly identified by using a 200-MHz antenna. The main sources of GPR signal attenuation were the higher clay and water contents of soil pockets and limestone bedrock.

Salt cementation may change the properties of granular media due to drying under the effects of climate conditions. We investigated the small-strain stiffness of salt-cemented soils by measuring elastic waves during salt cementation. The experimental data were examined by comparing them with an analytical solution based on micromechanical theory.

We simulated strontium-90 transport and fate in a variably saturated subsurface using TOUGHREACT under different mechanisms for perched-water formation. Different mechanisms led to differences in the peak and travel time of a mobile fraction of Sr. The effective distribution coefficient and retardation factor for Sr changed substantially with time.

Simulating flux breakthrough curves of three surface-applied tracers to a single pumping well suggested that a seepage face along the soil–well interface in a variably saturated, unconfined aquifer can result in early arrival of the solutes.

We studied the significance of the dynamic effect in capillarity in heterogeneous porous materials using numerical simulation of a laboratory experiment. The accumulation time of air at a material interface was investigated as a function of the ratio between air-entry pressure values of the adjacent sands.

Two dielectric methods (operating at 70 MHz and 1 GHz) were applied to monitor water content profiles in covered and uncovered buckets. Positive correlations were found between temperature and water content in the 5- to 10-cm depth, and these were reproduced by simulating the temperature effect on the soil liquid water content dynamics with HYDRUS-1D

Expression of soil water repellency depends on soil water content; however, only a limited amount of predictive description is available to date. In this study, based on experimental data, a simple two-region model was developed to predict the soil water repellency across the entire range of water content from air dryness to complete wetness.

Two dual permeability models, Mixing Cell and HYDRUS 1D, were used to simulate the long term nonequilibrium bromide leaching to depths of 4 and 7 m at two field sites on the Canterbury Plains, New Zealand. Both models indicated macropore flux was significant in transporting solute through the profile, ranging from 5 to 59% of the total flux.

Stained patterns resulting from dye tracer experiments in two European beech forest soils were used to discriminate surface-controlled flow and matrix-controlled flow of water. A log-linear analysis revealed associations between forest density, soil depth, and flow type, which may be utilized in managing water-related services of forest soils.

Aqueous foam transport in unsaturated sediment was studied in column experiments. Findings on foam injection pressure and distribution, foaming liquid uptake, and the influence of foam quality and sediment permeability on transport are presented and discussed with respect to vadose zone remediation using the remedial amendment foam-delivery approach.

A newly developed, combined penetrometer-moisture probe (CPMP), together with electrical resistivity imaging (ERI) and ground-penetrating radar (GPR), was used to determine soil depth, soil type, and water distribution of a forested slope. The combined use of ERI and the CPMP rapidly and accurately quantified those properties.

Bromide adsorption as a function of solution pH and background electrolyte concentration was determined for a set of reference minerals and soils. Adsorption was observed for pH <7, indicating that bromide would not act as a conservative tracer for such conditions. Therefore caution is advised when using bromide as a tracer under such conditions.

A new immiscible liquid, HFE-7100, is presented for directly extracting moisture from sediments using high-speed centrifugation. Unmodified samples of interstitial water chemistry will offer a comprehensive investigation of aquifer geochemistry and provide a new tool for long-term environmental change monitoring in the unsaturated zone.