Time Domain Reflectometry Measurements of Water Content and Electrical Conductivity of Layered Soil Columns

Abstract

The major argument for measuring the soil salt concentration and water content at the same position is their high correlation and spatial variability. Time domain reflectometry (TDR) was used for simultaneous measurement of soil water content, θ (derived from the soil dielectric constant ε), and bulk soil electrical conductivity, σa (from the attenuation of a transmitted pulse), for uniform and layered soil profiles in the laboratory. The purpose of the study was to test the different concepts of travel time and attenuation by using independent estimates of θ and σa. Both parallel (two‐rod) and simulated coaxial (three‐rod) probes resulted in essentially the same ε. The three‐rod probe measurements were easier to interpret and did not necessitate an impedance‐matching transformer. The TDR measurements of θ for layered profiles were not always accurate, especially when wet soil was overlying dry soil, due to erroneous interpretation of the TDR trace. In this case, the pulse travel time should be measured at the inflection point, which is sometimes difficult to identify, and not at the minimum of the TDR trace. This study reports a new method of calculating σa, which is based on a direct measurement of the transmission‐line load by TDR. This new method, which is simpler than the previous ones and independent of multiple reflections, was found to correlate better than the previously published methods with another, independent method for σa measurement (four‐electrode technique).

Using neural networks for calibration of time-domain reflectometry measurements

“…where Z L is the steady-state impedance (in Q,), fy is a temperature correction coefficient, and K p is the cell constant of the TDR probe (Giese & Tiemann, 1975;Topp et al, 1988;Nadler et al, 1991). The term K p is a calibration constant that can be determined by immersing the probe in solutions with known conductivity.…”

Desempenho do tensiômetro, TDR e sonda de nêutrons na determinação da umidade e condutividade hidráulica do solo

“…Dentre as vantagens da técnica, destacam-se: característica não-destrutiva, rapidez na obtenção dos resultados, segurança do operador, além de permitir a multiplexação (Baker & Allmaras, 1990;Herkelrath et al, 1991) e, dessa forma, o monitoramento contínuo da água no solo, tanto no espaço quanto no tempo (Topp et al, 1980). Além disso, permite medidas simultâneas de conteúdo de água e condutividade elétrica no campo (Dasberg & Dalton, 1985;Dalton & van Genuchten, 1986), permite realizar leituras próximo à superfície e, sobretudo, no laboratório (Nadler et al, 1991) Por esse motivo, a criação de novos métodos e equipamentos que possibilitem a análise das propriedades físicas do solo, com maior precisão, rapidez e confiabilidade, é sempre um desafio da pesquisa científica. Neste aspecto, técnicas que envolvam a utilização da instrumentação agronômica com vistas em determinar o conteúdo de água no solo são de grande importância para a pesquisa científica.…”

Using rainfall simulation, TDR, and tracer anions to determine effects of soil properties on nitrate leaching

“…Time domain reflectometry (TDR) is routinely used to measure soil water content and electrical conductivity both in the laboratory and the field (Dalton et al, 1984;Nadler et al, 1991;Noborio et al, 1994;Heimovaara et al, 1995). Extended use of TDR has been studied by some researchers.…”

“…It was mentioned that when the cable resistance, Zcabk = 0.2 O, was accounted for (using a 50 Q coaxial cable approximately 2 m long), a perfect linear relationship was found between solution EC and TDK-measured EC, with the slope, equal to K< , 2.937 m" 1 . The cell constant Kc was obtained by measuring Z when immersing the TDR probe in five different salt solutions of known conductivity ranging from 0 to 1.2 S m" 1 (Nadler et al, 1991;Heimovaara et al, 1995). It is not necessary to take the temperature factor (equation 2-7) into account to calculate the cell constant K c when the reference measurement with the laboratory conductivity meter is done on the same sample at the same temperature.…”