Time domain reflectometry: a seminal technique for measuring mass and energy in soil

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Abstract

Soil water exerts a strong influence on the transfer and storage of solutes, heat, air, and even water itself, within the soil profile. Soil water also dominates the mass and energy balance of the soil–atmosphere interface. Over the last decade or so, the development and continuing refinement of the time-domain reflectometry (TDR) technique for in situ, nondestructive measurement of water, ionic solutes and air has revolutionized the study and management of the transfer and storage of mass and energy within the soil profile. TDR-measured water content has been applied successfully to water balance studies ranging from the km scales of small watersheds to the mm scale of the root–soil interface. TDR-measured ionic solute status, which applies to the same sample volume as the water content measurement, has been used successfully on soil column, field plot and whole field scales for in situ determination of solute transport parameters, such as pore water velocity and dispersivity. TDR-measurement of air-filled porosity in space and time has given new insights into the mechanisms controlling aeration and gaseous exchange in the crop root zone. The combined water content – solute mass measurement capability of TDR has made this technique a very powerful tool for characterizing solute leaching characteristics, as well as for evaluating solute transport theories and solute transport models. The portability of TDR instrumentation coupled with the simplicity and flexibility of TDR soil probes has allowed the separation of water and solute content measurement error from soil variability, resulting in the capability for determining the mechanisms behind the spatial and temporal variability in field-based soil water content distributions and solute leaching patterns. The usefulness and power of the TDR technique for characterizing mass and energy in soil is increasing rapidly through continuing improvements in operating range, probe design, multiplexing and automated data collection.

Introduction

Although most biological mass and energy in the crop root zone is ultimately derived from incident solar radiation, it is the amount, distribution, availability and interactions of water, solutes and air in the root zone that exert the major influence over how this mass and energy is stored and transferred. For example, when soil water content is high but not excessive, plants can grow well because transpiration and photosynthesis are not restricted, and greater masses of nutrient are available to plants through dissolution. Excessive water contents, on the other hand, can impede crop growth by restricting the exchange of soil air with the atmosphere and by keeping the soil temperature low. Soil water moves faster under high water contents (higher hydraulic conductivity), which results in more rapid drainage and perhaps more extensive leaching of dissolved crop nutrients. Lack of nutrients, in turn, impedes crop growth through reduced fertility. Low water contents can restrict water and nutrient availability to crops, as well as allow excessive temperatures in the root zone. High or low soil temperatures, as well as low soil air contents, can impede crop growth regardless of the soil water content and nutrient availability. The interactions among soil water, solute and air are clearly very complex, and their impact on the soil-based mass and energy balances associated with crop production is of crucial importance.

Despite the importance of water, ionic solutes and air in the mass and energy balances of the soil profile, it has been only in the last 20 years that rapid, in situ, nondestructive measurement of soil water content, ionic solute concentration and soil air content has become possible in the form of time domain reflectometry (TDR) (Dalton, 1992; Topp et al., 1994; Zegelin et al., 1992). This radar-based technology has revolutionized our ability to characterize the storage and movement of water, solute and air in the soil profile. With TDR it is now practical to monitor simultaneously the soil water, ionic solutes and air (indirectly) in both space and time with high accuracy and relatively low equipment and labour costs. This capability is in turn providing better evaluations of the impacts of agricultural practices on the soil–plant–atmosphere continuum, as well as greater understanding of the mechanisms controlling the mass and energy balances within the soil profile and at the soil–atmosphere interface. This paper gives a brief overview of the operating principles of TDR, as well as a review of how TDR is used to measure water, solute and air in the soil profile. Selected studies will also be reviewed which demonstrate how TDR has improved our understanding of the roles water, solutes and air play in the mass and energy balance of the soil root zone.

Section snippets

Measurements with TDR

The high dielectric constant or relative permittivity of water (about 80) compared to that of the other soil components (1 for air and 2–5 for soil solids) makes determination of the relative permittivity an attractive way to measure water content. The TDR approach, which is a radar technique applied within the soil, is used to determine the soil's bulk relative permittivity. In TDR, a fast rise step voltage pulse is propagated along a transmission line in the soil. The voltage pulse propagates

Mass balance and monitoring water content using TDR

TDR serves effectively for monitoring hydrological water balance, measuring agricultural or forest water use efficiency, and monitoring changes in water content for irrigation scheduling. This type of monitoring requires rapid, reproducible recovery of data from a number of representative locations. These requirements led to the development of automated analyses of the TDR trace for water content (Zegelin et al., 1989; Baker and Allmaras, 1990; Heimovaara and Bouten, 1990; Herkelrath et al.,

Monitoring solute mass transfer with TDR

Since both water content and bulk electrical conductivity (EC) of the soil are determined on the same measured volume, bulk EC is readily converted to pore water EC using the measured water content. If pore water EC is due to a single ionic (electrolytic) solute then pore water EC can be converted to solute concentration.

Using vertical TDR rods, Kachanoski et al. (1992)measured one-dimensional transport of an ionic tracer solute under steady-rate, saturated flow in a laboratory column and in

Measuring oxygen concentration in soil

The transport of oxygen form the atmosphere into the soil is essential to root respiration, microbial activity and all oxidation processes. Given that water and air share the same pore space in the soil, the amount and movement of water exerts a strong influence on the amount and movement of air. Thus the ability to determine accurately the oxygen concentration and fluxes in the root zone depends critically on the ability to measure water content. Using non-destructive and repeatable

Summary and conclusions

The development of TDR has greatly enhanced our ability to measure and monitor the storage and movement of water, solute and air in the crop root zone. Evaluations of TDR for hydrological mass balances have shown TDR comparable to weighing lysimeters, Bowen ratio and rain gauges but with an improved response time and greater operational flexibility. For solute transport the TDR is proving to be very effective and causes less disruption of the flux pattern than solution samplers. The application

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