Issues

After a brief overview of preferential flow phenomena and relevant modeling concepts, the manuscript introduces 18 papers dealing with improved process descriptions, measuring techniques, and modeling approaches of preferential flow at various scales. The papers indicate a trend towards the use of noninvasive methods and improved modeling formulations.

Hydraulic and mass exchange term parameters for the HYDRUS 2D/3D dual-permeability model were estimated using data from micromorphological images and dye tracer experiments, and numerical inversion of transient infiltration data measured with tension disk infiltrometer and Guelph permeameter methods in the surface horizon of Haplic Luvisol.

A benchmark study was conducted to assess the predictive capacity of two dual-permeability models for structured soil under different land use and land cover. The comparison of simulations with experimental data from dye tracer experiments (dye pattern analysis and TDR measurements) revealed ambiguous results concerning the predictive capacity.

Funneled water flow at the base of trees by the mechanism of stemflow may lead to enhanced leaching of toxic substances to groundwater or surface water. The effect of this surface condition was simulated using HYDRUS-1D and HYDRUS-2D by imposing spatially distributed water flux and aluminum flux (stemflow and throughfall) at the soil surface.

The oxygen-18 isotope, monitored in precipitation, subsurface hillslope discharge, and soil water, was used as a natural tracer to study the role of preferential flow in the formation of shallow subsurface runoff. The dual-continuum model of soil water flow was successfully applied to explain the observed variations of oxygen-18 in the hillslope discharge.

The first-order transfer term, controlling the soil water exchange between the preferential flow domain and the soil matrix, is parameterized with the emphasis on interfacial resistance. This parameterization is used as a framework for the evaluation of the performance of two basic approaches to numerical coupling of the governing flow equations.

Saturation overshoot, in which the water profile is inverted during an infiltration, is (i) observed for many media and boundary conditions, (ii) implicated as the cause of gravity-driven fingering, and (iii) forbidden by the Richards equation. This study investigated how measurements of overshoot are a superb testing ground for proposed extensions of the Richards equation.

Neutron radiography produced two-dimensional time series of water contents during finger flow in a sand box. The series were analyzed with a water content waves formulation (i.e., Stokes flow). Results showed that momentum dissipation in the capillary fringe was three times higher than during finger flow.

A dynamic simulator of the immiscible displacement of two fluids in permeability networks is developed to investigate the sensitivity of the transient flow pattern and upscaled relative permeability curves of heterogeneous soil columns to the permeability distribution and flow parameters such as the capillary number and viscosity ratio.

A new approach to preferential flow emphasizes rapid response to hydraulic inputs at the soil surface – an effect that can be conceptualized as laminar flow in free-surface films along the walls of pores. A mathematical formulation based on these concepts is useful to represent traditionally difficult cases of unsaturated-zone flow.

Soil particles of colloidal size can facilitate the transport of adsorbed contaminants through the vadose zone. This study proposes a new model of particle mobilization in macroporous soils that emphasizes the role of capillary stresses occurring in the macropore walls between two rainfall events.

The two-dimensional spatial distribution of soil organic matter composition at intact surfaces of preferential flow paths was analyzed with Fourier transformed infrared spectroscopy in diffuse reflectance mode. Resulting millimeter-scale maps of the relation between CH and CO functional groups allowed distinguishing earthworm burrows, root channels, and aggregate coatings.

Pore-size data derived from mercury intrusion and nitrogen adsorption exhibited multifractal behavior for two soils. Entropy dimension was much lower for nitrogen adsorption than for mercury intrusion. Other multifractal parameters were also significantly different. Results reflect clustering of the pore system at the primary soil aggregate scale.

Measurements with Electrical Resistivity Tomography (ERT), TDR and effluent concentration were conducted during a step tracer experiment for two undisturbed soil columns. At the column scale the transport was essentially non-preferential in behavior, while at the scale of voxels (ERT measurements) the transport was revealed to be preferential.

A tracer experiment was performed at a forest site having a structured soil. To derive solute transport properties of the soil from breakthrough curves, TDR as well as ERT measurements were conducted. Preferential transport could be detected by both, but ERT turned out to be an appropriate means to image the spatial variability of preferential flow.

Water flow in unsaturated mine soils with embedded lignitic and clayey fragments was studied using neutron radiography and tomography. Flow paths could be visualized in two and three dimensions. Under near-saturated conditions, water moves preferably around fragments within a more continuous pore network that exists locally in the matrix.

Effects of residual air redistribution on changing values of steady state infiltration flow rate were studied. Magnetic resonance imaging (MRI) was used to locate air trapping in heterogeneous soil during monitored recurrent ponding infiltration experiments on an undisturbed sample of coarse sandy loam soil.

Spatial persistence of TDR-measured local soil water flux above and below an A/B horizon interface under varying water application rates was assessed on a pedon-scale transect. The persistence of, and correlation between, the A- and B-horizon soil water flux during infiltration was flux- and scale-dependent.

This study attempted to determine the sources of stream runoff in a watershed. Field measurements suggested that hillside saturated areas connected to the stream during rain events via lateral preferential flowpaths. The lateral flow paths reduced the influence of surface topography and channel topology on sources of stream runoff.

Measurements of water content within engineered surface barriers can provide an indirect measure of impending drainage. We used ground-penetrating radar, time-domain reflectometry, and a neutron-scattering probe to measure seasonal water content at a vegetated capillary barrier on the Hanford Site.

Dynamic effects in capillary pressure were experimentally evaluated via repeated drainage–wetting experiments using a single soil. The obtained dynamic coefficient varied with saturation and was largely controlled by the slope of the capillary pressure–saturation relationship. In addition, the obtained dynamic coefficient was compared with redistribution time.

Two-dimensional soil water distributions during infiltration and redistribution from a dripper source were tracked with continuous images from a color scanner. Moments of the spatial distribution of the digitized soil water contents were found to be a simple and efficient method for describing subsurface wetting patterns.

Water repellency in soil may lead to an erroneous determination of hydraulic soil properties. Thus, the intrinsic hydraulic soil properties, i.e., the hydraulic soil properties that are independent of the soil's wettability are introduced. Additionally, the applicability of ethanol as a testing liquid to measure the intrinsic hydraulic properties is tested.

Oxygen availability is an important control on acid production in commercial-scale sulfur blocks and acidic drainage (pH 0.4–1.0) from these blocks. Advective and diffusive oxygen transport through an extensively fractured block were quantified. Conclusions have global implications for gas transport in sulfur blocks and other types of fractured media.

On stony soils in central-northern Chile, infiltration measurements with single-ring and double-ring infiltrometers resulted in comparable results among measurement locations, but clear under- or overestimations of K_fs were observed when evaluating different calculation methods, thereby identifying unbiased, optimal methods for both infiltrometers.

We characterized cation exchange on vadose zone sediment from the Idaho National Laboratory. Experimental data on exchange of Sr, Mg, Na, Li and NH4 onto Ca-saturated sediment were used to calculate selectivity coefficients. The results are useful for modeling radionuclide transport in the INL vadose zone.

Complete surface ponding of tile-drained fields is an inefficient method of leaching salts because of large differences in infiltration that exist across the field. Simulations with HYDRUS-2D/3D were used to investigate an alternative, water-conserving, partial ponding, leaching strategy for various soil textures and profiles.

Because soil water diffusivity increases rapidly with water content, it makes Richards' equation difficult to solve numerically. There are very few exact analytical solutions with realistic diffusivities available for validation of numerical schemes. We present a new, exact, realistic, easy-to-use analytical solution for this purpose.