Practical Aspects of Salinity Effect on TDR‐Measured Water Content A Field Study

Abstract

It is possible to use the reflected electromagnetic time‐domain reflectometry (TDR) pulse for measuring water content (θ), for conventional probes, only when the soil electrical conductivity (EC) is low. The exact limiting level, above which the reflection is unnoticeable, depends on soil texture, salinity, θ, and probe geometry. Recently, the validity of θTDR, measured in saline soils, was questioned and overestimation was attributed to it. This study tested such a possible bias by simultaneously measuring the soil‐water content by the TDR and neutron scattering techniques, in two field experiments. The experiments lasted 150 d, and were irrigated with saline waters. In the Zeelim sandy site, peanut (Arachis hypogaea L.) was sprinkler irrigated at a rate of 6500 to 7500 m3 ha−1 with 1.9 dS m−1 irrigation waters. In the Nirim loamy site, cotton (Gossypium hirsutum L.) was drip‐irrigated at a rate of 3010 to 4580 m3 ha−1 with waters of EC ranging 3.6 to 4.0 dS m−1 The objective of this study was to observe if salinity increased θTDR relative to θneutron Measured θneutron values were found to be up to 0.02 or 0.08 m3 m−3 above and below θTDR for the sandy or loamy tested soils, respectively, with no significant effect of the salinity. Similarly, both, under‐ and over estimation were related to the effect of salinity on θTDR in over 20 previously published studies comparing θTDR to θgravimetric under wide ranges of clay, organic matter, and salt contents. The EC effect, expressed as bulk soil EC (σa), may depend also on texture and geometric parameters like particle aspect ratio. At field capacity, for the two soils tested, no detectable effect of salinity was found.