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Improved Calibration of Time Domain Reflectometry Soil Water Content Measurements

Dep. of Water Resources, Wageningen Agricultural Univ., Wageningen, the Netherlands

S. Dasberg

Institute of Soils and Water, ARO, Volcani Center, Bet Dagan, Israel

*Corresponding author.

ABSTRACT

Time domain reflectometry (TDR) is becoming a widely used method to determine volumetric soil water content, , from measured effective relative dielectric constant (permittivity), , using the empirical () Topp-Davis-Annan calibration equation. This equation is not adequate for all soils. The purpose of this study was to compare the Topp calibration equation with a theoretical (Maxwell-De Loor) and an empiricial (fitting exponent ) mixing model for the four components: solid phase (s), tightly bound water (bw), free water, and air. Water content permittivity were measured, gravimetrically and by TDR, on packed columns of 11 soils ranging from loess to pure bentonite. Measured specific surfaces were S = 25 to 665 m² g^–1 and bulk densities _b = 0.55 to 1.65 g cm^–3. Topp yielded accurate () values only for the four soils with _b > 1.30 g cm^–3, including illite (S = 147 m² g^–1). Maxwell-De Loor gave similar accuracy for seven soils, including attapulgite (S = 270 m² g^–1, _b = 0.55 g cm^–3), assuming a monomolecular tightly bound water layer (thickness = 3 x 10^–10 m; _bw = _bS), _bw = 3.2, and _s = 5.0. The () curve of these soils had the same shape as Topp. Two gibbsite soils with dissimilar curves required _bw = 3.2 and _s = 16 to 18, and two smectite soil materials required _bw = 30 to 50 and _s = 5.0, to obtain good fits. Deviations from Topp appear generally due more to the lower _b and thus higher air volume fraction at the same associated with fine-textured soils than to tightly bound water with low . Both effects, as well as apparent anomalous behavior such as decreasing effective with increasing _s, can be accomodated by the Maxwell-De Loor equation. This makes it a better calibration equation than Topp. The empirical model is sensitive to the unpredictable value of and cannot accomodate anomalous behavior.

This study was carried out at Wageningen Agricultural University.

Received for publication April 28, 1992.

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