Measurement of volumetric water content by TDR in saline soils

Abstract

Time-domain reflectometry (TDR) evaluates the bulk dielectric constant, K , of the soil by measuring the travel time of an electromagnetic pulse through a sensor, and through it estimates the volumetric water content. We show that for saline soils the effects of conductivity and frequency on the travel time cannot be neglected and that, as a result, TDR systematically overestimates the water content in saline soils. Simultaneously the bulk electrical conductivity of soils can be estimated by TDR. The equivalent impedance after multiple reflections is related to the bulk electrical conductivity, o. This relation differs from sensor to sensor and requires calibration for each individual sensor.A method is proposed for correcting the volumetric water content in saline soils. First, the bulk electrical conductivity, o, is estimated from the equivalent impedance at a specific equivalent distance of cable, several times the actual length of the sensor. The zero-salinity dielectric constant, K O , of this soil is obtained by correcting the apparent K as a function of the measured bulk electrical conductivity. The volumetric water content is estimated from KO. The correction of K is a function of the equivalent frequency of the electromagnetic pulse. The imaginary part of the dielectric constant is primarily due to ohmic losses. The model, which calculates the velocity of propagation of the electromagnetic pulse and which takes into consideration the imaginary part, performs reasonably well. An empirical approach based on calibration gave slightly better results.

Water content as a function of apparent dielectric permittivity in a Fibric Limnic Humisol

“…The degree of humification was similar to that of the O co horizon (Table 2), but the organic matter may come from other plant species and have different electrical characteristics. In this horizon, the salinity was too low (Table 2) to influence the Ka value (Wyseure et al 1997) as in the two other horizons. The particulate structure, the type of organic matter and the characteristics of the porous system of this horizon seem to be the principal factors affecting the difference between the O hp horizon and the O f and O co horizons.…”

“…For some unclear reasons, the organic matter content did not seem to affect the variability as much as the organic matter characteristics (particle size and humification level) ( Table 2). The salinity of these two horizons was different (Table 2), but too small to influence Ka values (Wyseure et al 1997). The use of a single relationship for these two horizons overestimated u (Fig.…”

Accuracy of Two Electromagnetic Soil Water Content Sensors in Saline Soils

“…The differences between the observed salinity ranges for which a universal calibration could be applied in this study and that reported by Inoue et al (2008) might be attributed to the differences in soil texture and the instrument used. Wyseure et al (1997) found the TDR measurement errors exceeded acceptable levels in different saline soils when EC of the saturated paste extract exceeded 8 to 10 dS m -1 , while Sun et al (2000) stated a 10 dS m -1 limit in saline fine sands. In a study that examined TDR probes in sands over 0 to 40 dS m -1 pore water EC range, Hook et al (2004) reported that the effect of soil salinity on VWC errors was not apparent until the EC reached 25 dS m -1 .…”

Calibration of a frequency-domain reflectometer for determining soil-water content in a clay loam soil

“…Evett et al (2005) found that TDR measurements may be affected by soil salinity, soil temperature, clay type and clay content. The TDR technique may overestimate soil-water content in saline soils because the apparent dielectric constant also depends on the electrical conductivity of the soil (Wyseure et al, 1997). For example, Wyseure et al (1997) used a dielectric-based technique to estimate the electrical conductivity.…”

“…The TDR technique may overestimate soil-water content in saline soils because the apparent dielectric constant also depends on the electrical conductivity of the soil (Wyseure et al, 1997). For example, Wyseure et al (1997) used a dielectric-based technique to estimate the electrical conductivity. Miyamoto and Maruyama (2004) found that by coating the TDR rods, more accurate measurements in a heavily fertilised paddy field was possible.…”