The effect of soil electrical conductivity on moisture determination using time-domain reflectometry in sandy soil

Abstract

The effect of soil electrical conductivity on moisture determination using time-domain reflectometry in sandy soil. Can. J. Soil Sci. 80: [13][14][15][16][17][18][19][20][21][22]. A series of laboratory experiments was conducted, in order to systematically explore the effect of soil electrical conductivity on soil moisture determination using time domain reflectometry (TDR). A Moisture Point MP-917 soil moisture instrument (E.S.I. Environmental Sensors Inc., Victoria, BC, Canada) was used to measure propagation time (time delay) of a step function along a probe imbedded in fine sand with different moisture and salinity. The volumetric soil water content was independently determined using a balance. With the help of the diode-switching technique, MP-917 could detect the reflection from the end of the probe as the electrical conductivity of saturated soil extract (EC e ) increased to 15.29 dS m -1 . However, the relationship between volumetric soil water content and propagation time expressed as T/T air (the ratio of propagation time in soil to that in air over the same distance) deviated from a linear relationship as the conductivity exceeded 3.72 dS m -1 . At the same water content, the time delay in a saline soil was longer than that in a non-saline soil. This leads to an over-estimation of volumetric soil water content when the linear calibration was applied. A logarithmic relationship between volumetric soil water content and T/T air has been developed and this relation includes soil electrical conductivity as a parameter. With this new calibration, it is possible to precisely determine the volumetric water content of highly saline soil using TDR. A l'aide de la technique brevetée MoisturePoint utilisant des rupteurs à diodes, MP-917 pouvait détecter la réflexion à la fin de la sonde, jusqu'à un niveau de conductivité électrique de 15.29 dS m -1 . Cependant, la relation entre humidité volumétrique et temps de propagation (exprimé sous la forme de T/T air : le rapport du temps de propagation dans le sol sur le temps de propagation dans l'air, pour la même distance et le même signal) s'écartait d'une relation linéaire pour les valeurs de conductivité supérieures à 3.72 dS m -1 . A teneur en humidité égale, le temps de propagation dans une eau saline était plus long que dans une eau non saline. En conséquence, l'humidité volumétrique était sur-estimée quand l'équation linéaire habituelle était appliquée. Une relation logarithmique entre teneur en eau et T/Tair a été établie, et cette relation varie avec la conductivité électrique. Avec cette nouvelle calibration, il est possible de déterminer avec précision l'humidité volumétrique de sols relativement salins par la méthode de TDR. Mots clés:Refléctométrie en Domaine Temporel, temps de propagation, conductivité electrique apparente (σ), humidité volumétrique (θ), permittivité relative ou constante, diélectrique (ε r ), vitesse de propagation V p Time-domain reflectometry has been used to measure volumetric water content of soil and other porous materials since 19...

Precision and Accuracy of Time Domain Reflectometry and Capacitive Probes to Determine Soil Electrical Conductivity in Cranberry Production – Technical Note

“…Moreover, to achieve such results, TDR data needed to be filtered, omitting values of EC a that corresponded with unrealistic values of θ w (e.g., θ w = 0.80 cm 3 cm −3 and higher for sand; data not shown). This phenomenon was probably due to frequent irrigation events pushing down the concentrated solute front, thereby causing higher values of θ w and EC p (Dalton 1992;Sun et al 2000). Finally, a linear regression slope (value of 5.00 and 17.22 for the two salinity ranges) that significantly differed from the manufacturer recommended value of 6.4 indicated that a specific calibration would be needed to improve accuracy, as raised by previous authors in sandy soils (Ward et al 1994;Caron et al 1999).…”

“…A high soil electrical conductivity is also known to affect the accuracy and resolution of TDR water content measurements (Hook et al 2004). In a sandy matrix, TDR-based water content measurements were overestimated when EC p > 8 dS m −1 (Dalton 1992;Sun et al 2000). Moreover, depending on the geometry of the probes, differences in the measurement volume can also result in inconsistent results due to variable salt distribution over space (Robinson et al 2003).…”

Using neural networks for calibration of time-domain reflectometry measurements

“…Jacobsen & Schj0nning, 1993;Hook & Livingston, 1996), electrical conductivity (e.g. Dalton, 1992;Sun et al, 2000;Persson et al, 2000), and temperature (e.g. Pepin et al, 1995;Persson & Berndtsson, 1998).…”

Laboratory and field evaluation of five soil water sensors

“…This is within the range of PPs operating temperature (0-70°C). The electrical conductivity of the soil was less than 0.35 mS cm -1 , and was not expected to have an effect on the PP readings (Sun et al 2000).…”