Field‐scale flow and transport studies are frequently conducted to assess and quantify various environmental and agricultural scenarios. The utility of field‐scale flow and transport studies, however, is frequently limited by our inability to characterize the heterogeneous distribution of hydraulic properties at these sites. In this study, we present an integrated approach, using both “hard” and “soft” data sets of field and laboratory scales in conjunction with pedotransfer functions, interpolation algorithm, and numerical modeling to characterize the hydraulic properties of the vadose zone. The approach is demonstrated at two 5‐ by 5‐m field plots selected for research on the transport and fate of nutrients and pathogens. We used hard data to quantify the magnitude of the hydraulic properties at selected locations in these plots and included laboratory and field measurements of the hydraulic properties from undisturbed cores and the instantaneous profile method, respectively. More abundant soft data included inductive electromagnetic readings and approximate particle‐size distribution information. The nearest neighbor interpolation algorithm was used to generate a heterogeneous realization of the saturated hydraulic conductivity on these plots. Numerical modeling of steady‐state water infiltration and redistribution experiments was used to compare laboratory‐ and field‐scale hydraulic properties and to refine our conceptual model of the vertical and lateral flow at this site. Good agreement between simulated and measured water contents and water pressure heads was obtained, indicating that field‐scale hydraulic properties were accurately quantified for these conditions. This article provides a real‐world example of how to combine information and approaches to tackle the difficult challenge of characterizing the hydraulic properties at a field site.
Abstract An improved method has been developed for determining the distribution of bulk soil electrical conductivity, EC a , through the soil from electromagnetic measurements taken at the soil surface with the Geonics Limited EM‐38 device. Induced electromagnetic conductivity readings taken with the EM‐38 device's coil configuration oriented parallel and then perpendicular to the soil surface provided sufficient information, when used with equations derived from geophysical instrumentation data, to produce a soil electrical conductivity‐depth profile. The simplicity of this method further enhances the praiticability of the newly developed electromagnetic technique for field measurements of salinity and for saline seep diagnosis.
Abstract The measurement of soil salinity is a quantification of the total salts present in the liquid portion of the soil. Soil salinity is important in agriculture because salinity reduces crop yields by reducing the osmotic potential, making it more difficult for the plant to extract water, by causing specific‐ion toxicity, by upsetting the nutritional balance of plants, and by affecting the tilth and permeability of a soil. A discussion of the principles, methods, and equipment for measuring soil salinity is presented. The discussion provides a basic knowledge of the background, principles, equipment, and current accepted procedures and methodology for measuring soil salinity in the laboratory using electrical conductivity of aqueous extracts from soil samples and measurement of total dissolved solids in the saturated soil extract. Attention is also given to the use of suction cup extractors, porous matrix or salinity sensors, electrical resistivity, and electromagnetic induction to measure salinity in soil lysimeter columns and small field plots (< 10 by 10 m). Land resource managers, producers, extension specialists, Natural Resource Conservation Service field staff, undergraduate and graduate students, and university, federal, and state researchers are the beneficiaries of the information provided.
This chapter contains sections titled: Introduction Background Information for NPS Pollutants in the Vadose Zone Oint Agu Chapman/SSSA Outreach Conference Overview Putting the Pieces of the NPS-Pollution Assessment Puzzle Together Future Directions
A new inductive electromagnetic device (EM) for measuring soil electrical conductivity (ECa) was tested. Practical and accurate methods are given for measuring ECa by soil depth intervals through soil profiles using a succession of EM readings made at various heights above ground. In contrast with other devices and methods for field salinity appraisal, readings are obtained without any soil-to-instrument contact. The device is well suited for field investigations of soil salinity.
Abstract The information age has ushered in a global awareness of complex environmental problems that do not respect political or physical boundaries: climatic change, ozone layer depletion, deforestation, desertification, and pollution from point and nonpoint sources. Among these global environmental problems, point and nonpoint source pollution represent a perfect example of a complex multidisciplinary problem that exists over multiple scales with tremendous spatial and temporal complexity. A point source of pollution discharges to the environment from an identifiable location, whereas a nonpoint source of pollution enters the environment from a widespread area. The ability to accurately assess present and future point and nonpoint source pollution impacts on ecosystems ranging from local to global scales provides a powerful tool for environmental stewardship and guiding future human activities.
There has been renewed interest in the application of functional models to the transport of nonpoint source pollutants at polypedon (i.e., farm) and watershed scales because of the ease of their coupling to a geographic information system and to the accepted organizational hierarchy of pedogenetic modeling approaches. However, very little work has been done to evaluate the performance of a functional transient-state model for the transport of a reactive solute over an extensive study period. Subsequently, the functional model TETrans (Trace Element Transport) was evaluated for model performance with boron (B) transport data collected from a meso-scale soil lysimeter column over a 1000-day study period (i.e., 40 irrigations). Because the ability to simulate water flow has been evaluated previously for TETrans, the focus of this evaluation centered around the performance of various functional models of B adsorption used as subroutines within the TETrans model, including the (1) Freundlich, (2) kinetic Freundlich, (3) Langmuir, (4) temperature-dependent Langmuir, and (5) pH-dependent Keren adsorption isotherm equations. Model performance was evaluated with statistical functions, specifically the Average Absolute Prediction Error, the Root Mean Square Error, the Reduced Error Estimate and the Coefficient of Residual Mass, and graphic displays of observed and predicted B concentration profiles. Even though no single adsorption isotherm equation, when coupled to TETrans, could be considered poor in its performance, results indicated that the order of model performance was the pH-dependent Keren equation first, followed by the temperature-dependent Langmuir and kinetic Freundlich equations, the Freundlich equation, and, finally, the Langmuir equation. Overall, the TETrans model was able to simulate the transport of B with deviations because no functional adsorption equation incorporated all the influences of pH, ionic strength, temperature, and kinetic effects into a single equation. The inability to correctly predict one of the measured peaks in B concentration near the soil surface suggests that problems with the timing of the sample collection may have occurred for the shallowest sampling depth.
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