Time domain reflectometry: a seminal technique for measuring mass and energy in soil
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
References (36)
- et al.
Practical considerations for using a TDR cable tester
Soil Technology
(1994) - et al.
The response of sap flow in apple roots to localised irrigation
Agricultural Water Management
(1997) - et al.
A method for measuring soil moisture by time-domain reflectometry
J. Hydrology
(1986) - et al.
System for automating and multiplexing soil moisture measurement by time domain reflectometry
Soil Sci. Soc. Amer. J.
(1990) - et al.
Comments on “Time domain reflectometry measurements of water content and electrical conductivity of layered soil column”
Soil Sci. Soc. Amer. J.
(1993) - et al.
Measuring water exchange between soil and atmosphere with TDR-Microlysimetry
Soil Sci.
(1994) - et al.
Roots: The big movers of water and chemical in soil
Soil Sci.
(1997) - et al.
Time domain reflectometry: simultaneous measurements of soil water content and electrical conductivity with a single probe
Science
(1984) - Dalton, F.N., 1992. Development of time-domain reflectometry for measuring soil water content and bulk soil electrical...
- et al.
Determination of the complex permittivity from thin-sample time domain reflectometry: Improved analysis of the step response wave form
Adv. Mol. Relaxation Processes
(1975)
Comments on “Time domain reflectometry measurements of water content and electrical conductivity of layered soil columns”
Soil Sci. Soc. Amer. J.
A computer-controlled 36-channel time domain reflectometry system for monitoring soil water contents
Water Resour. Res.
Automatic, real-time monitoring of soil moisture in a remote field area with time domain reflectometry
Water Resour. Res.
Errors in converting time domain reflectometry measurements of propagation velocity to estimates of soil water content
Soil Soc. Amer. J.
Field measurement of solute travel times using time domain reflectometry
Soil Sci. Sco. Amer. J.
Improving the calibration of dielectric TDR soil moisture determination taking into account the solid soil
Eur. J. Soil Sci.
Effects of liquid-phase electrical conductivity, water content, and surface conductivity on bulk soil electrical conductivity
Soil Sci. Soc. Amer. J.
Solute transport under transient flow conditions estimated using time domain reflectometry
Soil Sci. Soc. Amer. J.
Cited by (141)
Soil moisture at 30 m from multiple satellite datasets fused by random forest
2023, Journal of HydrologyEstimating soil water and salt contents from field measurements with time domain reflectometry using machine learning algorithms
2023, Agricultural Water ManagementMeasurement of water content at bare soil surface with infrared thermal imaging technology
2022, Journal of HydrologyAccuracy calibration and evaluation of capacitance-based soil moisture sensors for a variety of soil properties
2022, Agricultural Water ManagementAdvances in fiber optic sensors for soil moisture monitoring: A review
2022, Results in OpticsTime and frequency domain reflectometry for the measurement of tree stem water content: A review, evaluation, and future perspectives
2021, Agricultural and Forest Meteorology