The effects of soil bulk density, clay content and temperature on soil water content measurement using time‐domain reflectometry

Abstract

Time-domain reflectometry (TDR) is increasingly used for field soil water estimation because the measurement is nondestructive and less affected by soil texture, bulk density and temperature. However, with the increase in instrument resolution, the influences of soil bulk density and temperature on TDR soil moisture measurements have been reported. The influence is primarily caused by changes in soil and water dielectric permittivity when soil compaction and temperature varies. The objective of this study is to quantify the influence of soil bulk density and temperature, and to provide the corresponding correction methods. Data collected from sand, sandy loam, loam and clay loam show a linear relationship between the square root of dielectric constant of dry soil and bulk density, and a bulk density correction formula has been developed. The dielectric permittivity of soil solids estimated using this formula is close to that of oxides of aluminium, silicon, magnesium and calcium. Data collection from sandy loam show a noticeable decrease in measured soil moisture with increase in temperature when the volumetric soil water content is above 0Ð30 m 3 m 3 . A temperature-correction equation has been developed, which could provide the corrected soil moisture based on soil temperature and TDR-measured moisture. The effect of clay content has been detected, but it is not statistically significant. High clay contents cause the underestimation of soil water content in the low moisture range and overestimation of soil water content in the high moisture range.

A data-driven method to remove temperature effects in TDR-measured soil water content at a Mongolian site

“…Many researchers (e.g., Gong et al, 2003;Malicki et al, 1996;De Loor, 1968;Karkkainen et al, 2000;Or and Wraith, 1999) have been continuing to model the soil as a mixture of solid particles, water and air to take into account their contribution to e. In some studies, the water is divided into free water and bound water which is near to the surface of solid particles to consider the difference between their dielectrical behaviors (e.g., Or and Wraith, 1999). Gong et al (2003) reported that, as soil texture becomes finer (surface area per unit mass becomes larger) the effect of bound water becomes significant and therefore Topp's calibration equation (Topp et al, 1980) needs to be adjusted. Accordingly the temperature effect on all four phases (solid, gas, free water, bound water) of the soil column needs to be assessed for the total evaluation of the temperature effect.…”

“…Further they have proposed a method to correct e in order to compute the temperature-corrected soil moisture. Gong et al (2003) used direct TDR-measured soil water content to add the temperature correction. Skierucha (2009) also suggested two temperature corrections depending on the value of equilibrium water content and TDR-measured soil moisture.…”

Accuracy of Two Electromagnetic Soil Water Content Sensors in Saline Soils

“…Jacobsen and Schjønning (1993) demonstrated that at higher bulk densities dielectric constant values increased when compared with lower bulk densities. Similarly, Gong et al (2003) and Zheng et al (2011) reported an increase of the VWC with increasing soil bulk density.…”

“…They concluded that soils high in clay content exhibited greater data variation than sandy soils. Topp et al (2000), Gong et al (2003), and Bittelli et al (2008) also indicated an overestimation of the TDR VWC measurements in saline clayey soils.…”

Experiences from in-situ monitoring of pavement under weather conditions change

“…( [7]) All sensors were checked and calibrated in laboratory before installation work. Calibration of TDR probes included the effects of temperature, soil type and soil unit mass upon the measured values ( [16,18,19,20,21,22,23]). The influence of temperature upon the measurement with strain gauges was also checked in laboratory and included into calibration.…”