Issues

Geological action of moving water is central to hydrogeology, a field that seeks to understand the evolution of the earth's crust. Focusing on physical, chemical, and biological effects of water motion, hydrogeology can help enhance the field of pedology. In the evolving earth sciences, pedology and hydrogeology are components of a larger whole.

The impact of suction cups on the natural flow field and the matric potential was anlayzed for stationary conditions in different soils using numerical simulations. Changes in matric potential defined the suction cup activity domain. Particle tracking was used to delineate the suction cup sampling area and suction cup extraction domain.

Matric pressures were measured in the induction zone during nonponding infiltration with unstable flow. A Green-Ampt model described the pressure distribution prior to finger formation while a lateral flow model applied after fingers reached the underlying capillary fringe. Ponded infiltration analysis is shown to provide poorer predictions.

The spatial variability of the mass of a chloride tracer after 34 yr of transport was used to assess the spatial variability in transport processes at the pedon scale (encompassing many pedons). The spatial variability of the soil layering significantly influenced both the vertical and horizontal transport of the chloride.

The multicomponent reactive transport model CW2D was developed as a module for HYDRUS-2D to model the biochemical transformation and degradation processes in subsurface flow constructed wetlands for wastewater treatment. Model simulations are compared with experimental observations, and suggestions for module improvement are discussed.

First-order analysis was used to investigate the effects of several characteristics of a bimodal, spatially heterogeneous, unsaturated formation, on solute breakthrough curves, under steady-state, gravity-dominated flow conditions. Implications of the results with respect to the problem of groundwater contamination are briefly discussed.

The main finding of colloid trapping at air-water-solid (AWS) contact lines reported by Crist et al. (2004)(2005) is an experimental artifact. Collecting images along surfaces open to the atmosphere causes drying of thin films, formation of AWS contact lines, and the observed advection of colloids to and deposition at AWS contact lines.

Experimental evidence distinctly shows that, contrary to assertions in the comment of Wan and Tokunaga (2005), colloid retention indeed occurs proximal to grain surfaces in unsaturated porous media where air, water films, and water menisci meet without the aid of evaporation.

A prefractal water retention equation was derived by assuming some scale-invariant fraction of the pore space does not drain during monotonic drying. The three parameter model was tested by fitting it to primary drainage curves obtained from pore-scale simulations on random Sierpinski carpets and laboratory measurements on different textured soils.

Results are presented for a model of tritium transport from a low-level radioactive waste facility in which source thermal and gas-advection mechanisms were tested. The temperature dependence of Henry's law equilibrium constant and isotopic fractionation was taken into account in the tritium transport model.

The novel cell uses concentric porous stainless steel walls that provide solution exchange as well as sample water content adjustment for saturated and unsaturated measurements of permittivity and electrical conductivity without the need to repack samples.

The co-editors summarize the special section on Soil Water Content Sensing, which grew out of a 2004 SSSA-CSSS joint meeting session, itself motivated in part by a five-year International Atomic Energy Agency effort to compare modern methods of soil water sensing, results of which are being published separately by the IAEA.

Capacitance probe measurements were made as a function of water content for 12 soils representing textures ranging from sand to clay, and compared with TDR measurements. Apparent dielectric measurements for one of the heavy textured soils were also compared with real permittivity measurements made with a network analyzer.

Capacitance sensors have become a popular tool for measuring soil moisture. This paper reports on an investigation of the performance of a multisensor capacitance system in a shrinking-swelling tropical soil at a range of temperatures.

Water content determination with time domain reflectometry has required site-specific calibration for soils high in smectite clays. In the present study a temperature term added into the calibration for such a soil was found to improve results slightly, but the calibrations were found to be inadequate still for many site-depth combinations.

A TDR calibration was developed for bulk electrical conductivity, effective frequency of TDR pulse, and pulse travel time, all determined from TDR waveforms. The method provided a common calibration for three contrasting soils (17, 30, and 48% smectitic clay contents), with good accuracy and temperature sensitivity.

The performance for profiling soil water content of the TRIME-tube TDR system from Imko was extensively investigated under real field conditions in France and Tunisia. Additionally, its calibration was also examined through laboratory experiments performed in reference media and in a natural soil.

The performance and usefulness of the soil moisture sensors CS616 and Aquaflex for timing irrigation were evaluated in drip and fertigated potatoes using a TDR sensor as a standard. Comparisons with TDR were promising; however, contrary to manufacturer claims recalibration was needed to obtain accurate soil moisture measurements.

Performance differences in the growing number of electromagnetic sensors designed to estimate porous media water content suggest the need for a standardized sensor characterization methodology. The authors outline a method for characterizing and evaluating EM sensors via comparison with network analyzer measurements in reference fluids.

Many systems are available to estimate the volumetric water content of porous media via electromagnetic measurements. The authors compare seven different systems by demonstrating their accuracy in lossless media and evaluating the effects of dielectric relaxation, electrical conductivity, and temperature on the electromagnetic measurement.

Few soil water sensors have received independent testing. We investigated the performance of the Hydra Probe on 20 different soils and in four fluids. The instrument accurately measured dielectric properties of the solutions. We also developed improved calibration equations, including one that incorporated the effects of dielectric loss.

Thermo-TDR probes can be used to measure soil thermal and electrical properties to estimate soil water content. This paper shows how well soil water content can be estimated with thermo-TDR probes.

Four methods for measuring total suction in soils are described and compared by laboratory testing. The study focused on accuracy assessment and investigation of factors influencing the accuracy of each of the techniques. The difficulties and limitations of each technique for measuring total suction are described.

Ground penetrating radar (GPR) is an emerging nonintrusive method of estimating soil moisture content. Numerical modeling is used to estimate the sampling depth of the GPR direct ground wave. Soil layering and moisture content can have significant impacts on soil moisture readings by GPR.

A one-dimensional model of gas tracer transport in variably saturated porous media was used to investigate the effect of nonuniformity of water saturation distribution and other system parameters on bulk water saturation measurement accuracy using the partitioning interwell tracer test. Experiments were performed to verify the modeling results.

A method for determining soil surface moisture from soil color recorded by a digital camera was developed. Five different soil types were tested to determine the influence of different soil textures on model performance. Various future applications of the method are also presented.

An extensive experimental investigation highlights the importance of gravity-driven fingering as a mechanism for preferential flow in fractures. Gravity-driven fingers formed across a wide range of conditions, suggesting that they will be ubiquitous in natural systems and must therefore be explicitly considered in flow and transport models.

A radiative transfer model is applied to estimate soil reflectance of wet soils. The model describes light scattering in complex geometries and thus is optimally suited to account for the distributed water phase in soil samples.

Soil surface fluxes of carbon dioxide and oxygen are calculated from continuous concentration measurements employing low-cost infrared sensors for carbon dioxide and galvanic cells for oxygen, and gaseous diffusion coefficient estimation based on soil water content. Laboratory and greenhouse soil column gradient-based fluxes are compared with surface chamber measurements.

A sand chamber packed with medium sand and containing layers of coarse sand and silt was used to simulate the use of soil vazpor extraction (SVE) to remove carbon tetrachloride (TCE) from a simulated spill of DNAPL. Experiments with SVE were performed with moist air and dry air to determine the effect of air moisture content on TCE removal.

Estimates of saturated flow are often based on measurements made on small volumes of soil, but small volumes give low values compared with larger volumes. This bias appears to be an artifact of the lateral confinement of the sample, and not a matter of insufficient representative elementary volume or greater variability among smaller samples.

Mixed hybrid finite element methods have become popular in recent years for modeling groundwater flow. Improvements in this numerical scheme dealing with a better estimation of the equivalent hydraulic conductivity are depicted in this article especially for the simulation of sharp infiltration fronts in the vadose zone.

We present continuous high-resolution root zone drainage measurements made with three types of water flux meters in a disturbed volcanic soil. The instruments performed well for one growing season, after which only one of the three types continued to function. Measured water fluxes were compared with modeled fluxes based on HYDRUS-1D.

Analytical solutions are formulated to describe water content distributions in unsaturated, rooted soils with variable surface fluxes. The solutions presented here have relatively simple and applicable forms compared with other approaches and provide a useful means of evaluating the accuracy of sophisticated numerical schemes.