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DIVISION S-1-SOIL PHYSICS

Practical Aspects of Salinity Effect on TDR-Measured Water Content

A Field Study

^a Dep of Physical Chemistry of the Soil, Soil and Water Inst., ARO, Volcani Center, Bet Dagan, 50-250, Israel
^b Lab. for Pest Management Research, Agricultural Engineering Institute, ARO, Bet Dagan, 50-250, Israel
^c Hevel Maon Enterprises, M. P. Hanegev, Israel

vwnad{at}volcani.agri.gov.il

It is possible to use the reflected electromagnetic time-domain reflectometry (TDR) pulse for measuring water content (), 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, , and probe geometry. Recently, the validity of _TDR, 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 _TDR relative to _neutron. Measured _neutron values were found to be up to 0.02 or 0.08 m³ m^-3 above and below _TDR 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 _TDR in over 20 previously published studies comparing _TDR to _gravimetric under wide ranges of clay, organic matter, and salt contents. The EC effect, expressed as bulk soil EC (_a), 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.

Abbreviations: BD, bulk density • EC, electrical conductivity • EM, electromagnetic • TDR, time domain reflectometry

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