Issues

A new land surface scheme, SABAE-HW, an extension of the Canadian LSS (CLASS v2.6), is presented. The model is user-friendly and allows specified soil column depth and layers. Diverse simulations under warm and freezing weather conditions reveal the worth of SABAE-HW predictions that are contrasted to results from other models including SHAW.

Acquiring a high level of confidence in the hydraulic properties at field sites is a challenge. We present an integrated approach, using both “hard” and “soft” data sets of field and laboratory scales in conjunction with pedotransfer functions, interpolation algorithms, and numerical modeling to characterize vadose zone hydraulic properties.

We present results from a field evaluation of horizontal transport of miscible solutes in the capillary fringe and shallow groundwater. Bromide tracer applied above the capillary fringe entered and moved horizontally in the capillary fringe until bromide was partially moved into the shallow groundwater by rain-induced fluctuation of the water table.

Field-scale infiltration results conducted at a highly heterogeneous Au heap leach mine revealed only limited preferential flow when the applied fluxes were far below the field saturated hydraulic conductivity. These results confirm the high Au recovery rates commonly seen in these systems.

Macropores have the potential to be hydraulically connected to subsurface drains, thereby impacting contaminant transport. This research modeled laboratory experiments of surface-connected and buried macropores located in the vicinity of a drain using a three-dimensional flow code capable of simulating both soil matrix and macropore flow domains.

Soil matric suction in deep vadose zones is controlled by the net infiltration rate. Measurements using borehole installations are difficult. A new method for measuring soil matric suction within boreholes is presented that requires no electronics and no calibration. Careful consideration of the soil materials and response times is required.

We demonstrate that during unsaturated flow in sediments, colloids can be mobilized by attachment to a moving liquid-gas interface. As hydraulic boundary conditions imposed by sampling devices or column outflow systems will affect liquid-gas interface configurations, colloid mobilization is affected by the type of boundary condition as well.

In situ air permeability measurements were modeled with a three-dimensional finite element flow model. Results showed that anisotropy produces an error factor of 2 for air permeameters with high diameter/height ratios. In situ and ex situ air permeability can be used to infer anisotropy, especially for large diameter/height ratios and small horizontal permeability.

Quantitative relationships were established between the velocities of convection cells, the mass-transfer rate due to convection, and the Rayleigh and Sherwood numbers for a large aperture fracture. Convective flow and the consequently high replacement rate of fracture air could impact evaporation and gas exchange rates with the atmosphere.

Numerical experiments and theoretical interpretation conducted in this study revealed why the extensively used mass conservative approach for discretizing the storage term of the Richards equation, based on a mixed form of the equation, is superior to the traditional chain rule expansion approach and another mass conservative approach using a chord-slope approximation.

A modified thermodynamically based model (TBM) was developed to predict fluid–fluid interfacial areas in porous media for arbitrary drainage-imbibition sequences. The TBM explicitly distinguishes between interfacial areas associated with continuous (free) and isolated (entrapped) nonwetting fluids. It can be easily incorporated in numerical codes.

Wetting liquids and electrical double layers contribute to the bulk electrical conductivity of expandable soils in a rather unusual way. Leached cations in soils offset cationic contributions of a wetting liquid to surface conductivity. Changing sample volume with a wetting liquid shows a pseudobehavior of the electrical formation factor.

Mobile–immobile (MIM) transport parameters estimated from tracer experiments in two sands depended strongly on water content or pore velocity. Dispersivities and mass transfer coefficients in transient flow in the uniform sand were higher than for steady flow, indicating that MIM behavior depends on flow regime and porous medium characteristics.

A field calibration of the neutron moisture meter (NMM) and three capacitance (EM) sensors in a Panoche clay loam soil in the San Joaquin Valley, California, showed that, in contrast to the NMM, EM sensor calibrations changed greatly with depth, often requiring separate calibrations for every 10- or 20-cm depth range, and were relatively inaccurate.

We present a benchmark solution for one-dimensional porous media with heterogeneity involving two soils separated by a sharp interface. The proposed solution allows the inclusion of advection terms in a limited way. A computationally efficient algorithm and several computational results are presented.

The topics addressed in this special section on plant roots and root function include: imaging techniques for observing roots and uptake processes; methods for measuring water potential in the root zone; modeling of uptake mechanisms; species- and biome-level investigations of root development and functioning; and effects of gravity on root growth.

The application of a new MRI method for the determination of water content changes in soils is described. By coregistration of the 3D root architecture in a soil chamber planted with a 4-wk-old Ricinus communis carmencita, we showed that the largest water content changes took place in the regions with highest root densities.

The mechanisms of root water and nutrient uptake in soils have not been studied experimentally in detail because of the difficulty of measuring quantities at the microscale. We used magnetic resonance imaging and microtensiometry to image a seedling's root system and to quantify the dynamics of water migration toward roots and especially at the root–soil interface.

Root hairs develop along the surface of roots to enhance the uptake of soil moisture and nutrients; however, the exact nature of this structural development is not fully understood. A model and magnetic resonance imaging technology demonstrate that root surface area is effectively increased by root hair presence and consequently enhance root water uptake.

Water flow in soil and plant roots is imaged by neutron radiography, a non-invasive method sensitive to the detection of water. For the first time, we image complete root systems and their development jointly with water distribution dynamics during and after irrigation. Our systems consisted of either a lupin or a maize plant in a fine sand medium.

Direct soil water potential measurements by polymer tensiometers help in defining levels of local water stress in root water uptake experiments, especially in dry soil where moisture content measurements become less informative. Observed temporal matric potential patterns showed changing root water uptake behavior under water-stressed conditions.

New technologies are providing crucial information to understand belowground processes. We integrated continuous measurements of fine roots and rhizomorphs with automated measurements of soil respiration. Large changes in root and rhizomorph length occurred in less than 4 days and may contribute to daily variation in soil respiration.

A physically based model is presented that includes a water uptake partitioning mechanism based on root density and matric flux potential, suppressing the need of several empirical parameters. Simulation results show good agreement between model predictions and long-term experimental measurements of soil water content.

The impact of the parameterization of a three-dimensional root-soil water flow model (R-SWMS) was investigated. Radial root conductivity and soil hydraulic functions controlled the distribution of the soil-root water fluxes. It was shown that different water content spatial distributions may result in similar one-dimensional sink term profiles.

In this study a microscopic model was developed that links a three-dimensional soil macroscopic flow model with the water flow in the root system, considering local hydraulic effects. Results show that local effects play a large role, especially when the radial root conductivity regulating root water uptake is larger than the soil hydraulic conductivity.

A transient variably saturated flow model with root water uptake is developed and coupled to a previously developed reactive transport model to evaluate observed upward migration of Pu in 11-yr-old field lysimeters. This analysis strongly suggests that upward migration is the result of Pu uptake into the transpiration stream.

Impacts of chaparral vegetation on root zone hydrology for a 50-year old experiment are described. Water content, matric potential, chloride mass balance, and stable isotope results suggest only minor species-level differences. However, some differences are statistically significant, indicating that vegetation type is affecting hydrologic behavior.

A soil water infiltration and extraction model was designed to test the hypothesis that ecosystem-level root profiles tend to be as shallow as possible and as deep as needed to fulfill evapotranspirational demands. The model was tested succesfully against root data from eight biomes, showing that it allows prediction of global root distributions.

Astronauts need to grow food in space. Growth is directly related to stomatal opening, but stomatal resistance has not been measured under microgravity. I grew wheat in horizontal columns with soil to simulate gravity-free conditions. The roots did not grow into the soil; stomatal resistance was high. Aeroponics may be needed for growth in space.