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Infiltration from constant-head well permeameters and ring or disk infiltrometers takes place under conditions of three-dimensional transient and steady state flow. Under positive head conditions (i.e., ponded infiltration), both the saturated and unsaturated components of hydraulic conductivity (i.e., field-saturated hydraulic conductivity and matric flux potential, respectively) determine the early-time transient and steady-state flow rates. Under negative head conditions (i.e., tension infiltration), the early-time transient and steady-state flow rates are determined only by the appropriate range of the unsaturated hydraulic conductivity (i.e., the tension flux potential). Several approaches are presented and discussed for determining the saturated and unsaturated components of hydraulic conductivity from well permeameter and infiltrometer measurements.
In this paper we discuss several factors that we consider as we work with scientific researchers in bringing ideas for new devices to the market. The two main subjects presented are, first, how we at Soilmoisture Equipment worked with the originators of the Guelph Permeameter to bring the instrument to market, and second, the difficulties faced when introducing a new device that is envisioned to be used to satisfy regulatory requirements.
Determination of saturated hydraulic conductivity (Ksat) of the vadose zone may be required to solve many agricultural, environmental, and engineering problems. Modifications to the constant-head well permeameter technique in the past decade have made it a convenient method for determining Ksat in situ. Recently, a compact permeameter with accessories was introduced to measure Ksat of the vadose zone from the soil surface to depths exceeding 10 m. This permeameter, the Compact Constant Head Permeameter (CCHP; Ksat, Inc., Raleigh, NC), has been modified for commercial production. The CCHP is composed of four constant-head tubes, a 4-L water reservoir, a l-L flow measuring reservoir, a water dissipating unit, and a base with a three-way valve. The four constant-head tubes are used to maintain a constant level of water in an auger hole up to 2 m below the surface. Additional constant-head tubes or a special flow measuring reservoir can be attached to the unit for Ksat measurement below 2-m depths. The CCHP is portable, versatile, and easy to operate. To measure Ksat , a 4- to 10-cm diam. cylindrical hole is bored to the desired depth and a constant head of water greater than five times the radius of the hole is maintained at the bottom of the hole. After determining the steady-state flow rate of water, Ksat is calculated by the Glover solution or another appropriate equation. Various comparisons have shown that the Ksat values calculated by the Glover solution are, for most practical cases, comparable with the values obtained by other available equations and approaches. The Glover solution is simple and requires only one measurement of the steady state rate of water flow for determining Ksat.
Field applications of the borehole permeameter are presented for sand, loam, and clay soils under deep water table conditions. In general, the tests are shown to provide useful results for most practical purposes over the range of hydraulic conductivities from 10−7to 10−2 cm s−1. It is important to consider the effects of capillarity, especially in clay soils. Special problems can occur due to air entrapment, although in some cases pre-treatment by carbon dioxide gas flooding can minimize this effect. Temperature can also influence infiltration data, especially with mariotte siphon-type permeameters in low-permeability soil. The chemical characteristics of the infiltration water are also important. In shallow water table conditions, additional work is required to validate the existing analytical solutions; however, reasonable results were obtained in the field for a loam soil. In our experience a down-hole stock tank valve produced consistently good control on water levels and allowed accurate flow measurements by calibrated reservoirs and flow meters over a rather wide range of field conditions.
In this chapter we describe the development of in situ techniques to measure the hydraulic properties of the soil surface and to assess quantitatively the contributions of preferential flow paths to, and the impact of soil management practices on water entry into field soils. Both confined, one-dimensional techniques (tension infiltrometers, rainfall, sprinkler, and drip infiltrometers) and unconfined, three-dimensional methods (the disk permeameter and multi-sized surface sources) are discussed. The assumptions and theory underpinning their use are presented. Applications of the three-dimensional techniques to measure sorptivity, S0; hydraulic conductivity, K0; and the hydraulic conductivity relation, K(Ψ), are given. Measurements of the soil structural parameters; mean characteristic pore size, λm; and macroporosity area, Am, are described. The relevance of these parameters to water entry, erosion, and plant growth are demonstrated. The limitations of all techniques are also discussed.
We describe here the steps taken to commercialize the disk permeameter. Commercialization took place against a background of rapidly changing organizational culture within a government strategic research organization, CSIRO, and reflected the widespread adoption of the corporate management strategy for science throughout the English-speaking world. These changes transformed a prototype instrument distributed gratis into a commercially manufactured version sold worldwide. The rapid shifts in science management impacted directly on the commercialization procedure adopted. We describe the search for and selection of a commercial partner, the development of prototypes, and the preparation of advertising material and instruction manuals. The technical back-up and support required are discussed and the financial returns are mentioned. We make some judgements on the scientific costs and benefits of the whole process. The dilemmas for the management of commercialization in a primarily publicly funded strategic research organization are discussed. It is hoped that this account offers insights applicable beyond the parochial Australian context of the disk permeameter.
This chapter reviews the design, calibration, and operation of tension and ponded infiltrometers. Analysis of unconfined saturated and unsaturated infiltration data obtained from use of these instruments is also reviewed. Unsaturated hydraulic conductivities are easily determined from these data. Unsaturated hydraulic conductivity measurements can provide a rapid and sensitive indicator of the effects of soil compaction, soil texture, management, and root growth on soil hydraulic properties. The hydraulic conductivity function near saturation is also useful in predicting solute movement.
The measurement of bulk soil dielectric constant and electrical conductivity, using time-domain reflectometry (TDR), is a relatively new method for simultaneous determination of soil water content and salinity. These measurements are made with identical sampling volumes using a single probe. Soil water properties are determined by measuring transit time and dissipation of an electromagnetic pulse launched along a parallel wire probe buried in the soil. The historical development and principles of TDR measurements are discussed in relation to specific applications. Constraints on the separation, diameters, and lengths of TDR probes are given as a function of water content and pore water electrical conductivity. Time-domain reflectometry electrode insertion techniques are shown for field sampling requiring a single measurement as well as for permanent installations requiring continuous monitoring. A cable tester quick disconnect system is also described for laboratory column and field studies.
The intent of this chapter is to provide the reader a better understanding of the paths and processes that led from original academic research in the field of Time Domain Reflectometry (TDR) to a series of applied instruments which use this new knowledge to measure volumetric soil moisture. In particular, this chapter describes the development of a TDR product called TRASE from background awareness, through alternate instrument involvement, to the final manufactured product. The effort spans a number of years and covers early, preexisting products and processes available at the time of our involvement and leads down an indirect route to the final construction of this specialized equipment. As with any product developed in a growing applied field such as TDR analyses of soil moisture, there continues to be ongoing development of new products, accessories, and findings that will further affect new research. The results of this work indicated that, to assure optimum results in meeting small business sponsored development, a crucial link between academic institutions and the manufacturing world would be a better match of objectives and processes. Additionally, any company involved in such a complex endeavor must be certain from the outset that system quality and performance specifications are precisely determined and that time and cost projections be realistically outlined. Lastly, the project must be continually monitored for out-of-bounds conditions to assure a satisfactory result on time.
Time domain reflectometry (TDR) has been used to monitor soil-water content for the past decade. During this time there have been marked advances in both the theoretical understanding and practical methodology and technology of the TDR technique. In this chapter we examine critically some of the key issues. We look at empirical and theoretical relationships between volumetric water content, θ, and apparent dielectric constant, Ka, factors influencing the accuracy and resolution of measurements, the volume of soil sampled by TDR probes and their spatial weighting functions, the effect of probe geometry and orientation, and the impact of soil electrical conductivity on TDR measurements. In addition, we give comparative tests of the use of TDR in the field under simulated rainfall, and under prolonged wetting and drying by evapotranspiration on a daily and hourly basis. Finally we discuss applications to other porous materials. We conclude that TDR is best suited for use in lighter textured soils. Measurements are extremely sensitive to the soil closest to the probe wires, the breadth of soil sampled being proportional to wire diameter. This means that probe insertion must be carried out carefully, and imposes limitations on the minimum diameter of probe wires. Soil electrical conductivity, through saline soil-water or surface conduction in clay soils, attenuates the TDR signal, limiting the technique to low conductivity soils. The use of a “universal” empirical θ(Ka) relation gives water balances in the field to within ± 10% of that found using weighing lysimeters on a daily basis. Finally we conclude that individual calibration curves are required when TDR is used to monitor water content in other porous materials such as coal.
The saturated paste extract method is widely accepted as an accurate indication of the degree of salinization of a soil sample. This method basically measures the electrical conductivity of a carefully prepared sample of the soil and, while accurate, is time consuming to carry out because of the preparation procedure. It is also a “point-sample” method, leaving doubt as to whether the measurement is representative. To overcome these limitations extensive investigation into the use of conventional in situ direct current (DC) resistivity techniques has been carried out to determine conditions under which this technique accurately measures the degree of soil salinization. However, conventional DC resistivity measurements are also relatively slow to carry out. Instruments using inductive electromagnetic techniques are now widely used to map terrain conductivity. These devices, which electromagnetically induce small currents in the ground, measure the magnetic field strength generated by these currents to determine terrain conductivity. They are well suited to assessing soil salinity since they respond to more conductive (and thus more saline) soils and, furthermore, do not require electrical contact with the ground. Exploration depth of these devices is determined by the spacing between the transmitter and receiver coils; commercially available instruments measure from a depth of approximately 1 m to several tens of meters. Their chief advantage is that large areas can be surveyed quickly, and thus in considerable detail. A significant disadvantage is that variation of conductivity with depth is not well resolved. Because of various factors other than soil salinity which can affect the bulk soil conductivity, saturated paste extract samples must still be taken, but now at much reduced intervals and at locations now known to be statistically significant. The theory of operation of electromagnetic ground conductivity meters is briefly described, comparative survey results (with both saturation paste extract and DC resistivity measurements) are shown, and survey results over the depth range referred to above are presented.
This paper describes principles, apparatus, reagents, procedures, calculations, and comments on various instrumental methods for determining soil salinity. These methods include the determination of soil salinity from saturation paste conductivity and from bulk soil electrical conductivity.
This chapter describes the history and use of thermal conductivity sensors to measure matric suction. The technique uses an indirect measurement which is correlated to matric suction. The sensors were initially developed and used primarily with irrigation control systems. More recently, attempts have been made to apply the sensors to geotechnical engineering problems. This has resulted in the development of more refined calibration procedures. The thermal conductivity sensors have now been applied on several engineering projects, but more case histories are still required for their complete evaluation.
Various regions of the world are covered with soils which are often categorized as “problematic” soils from a geotechnical engineering standpoint. Soils in this category are (i) swelling soils, (ii) collapsing soils, and (iii) residual soils. Typically, their mechanical behavior does not adhere to classical soil mechanics behavior.
Common to all these problematic soils is the fact that their pore-water pressures are negative, relative to atmospheric conditions and most often, the soils are unsaturated. The negative pore-water pressures can be referenced to the pore-air pressure, [i.e., (ua − u/w] resulting in the definition of the term matric suction. Matric suction is now recognized as one of the stress state variables controlling the mechanical behavior of unsaturated soils (or soils with negative pore-water pressures). The development of techniques and devices for measuring matric suction is important to the advancement of soil mechanics for unsaturated soils. Of particular need is a device which will measure matric suction in situ.
Direct measurements of negative pore-water pressures are limited to less than one atmosphere below zero gauge pressure because of cavitation of water in the measuring system. An indirect method for measuring matric suction involving thermal conductivity measurements is described in this chapter. This method makes use of the thermal properties of a standard porous medium, which is placed in contact with the soil. The resulting device is called a thermal conductivity sensor to measure matric suction. This chapter considers the potential of these sensors for use in geotechnical engineering.
Soil matric potential, a measure of the holding strength of the soil matrix for water, is a critical variable in water management and agriculture. One of the methods of measurement, the heat dissipation method, was proposed about half a century ago and slowly has been developed into a commercial measurement system capable of providing real time and accurate measurements between −10 and −1400 kPa. When adequate pore size distribution is provided in the ceramic media, measurements as low as −1000 to −1500 kPa are achievable. Research has shown that the accuracy of the measurement is dependent on the calibration of the soil matric potential sensor (SMPS) against known standard pressures and independent of salinity, temperature, and soil texture. The response time of the SMPS to change in pressure plate pressures is less than 24 h between −10 and −100 kPa but increases rapidly to several days for soil matric potential lower than −120 kPa. The latest SMPS model uses a passive sensing element (thermocouple). The SMPS can be monitored automatically and remotely with many data acquisition systems, and can provide real-time information about soil water. The SMP sensors were used successfully to monitor soil matric potential and temperature profiles in field soil in real time and to control irrigation systems automatically by initiating irrigation when a preset soil matric potential threshold was exceeded. The major advantage of real-time automated irrigation scheduling lies in its ability to correct for weather changes instantaneously, thus minimizing potential plant water stress and maximizing yields and water use efficiency.