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DIVISION S-1-SOIL PHYSICS

Soil water content determination using a network analyzer and coaxial probe

^a USDA-ARS Southwest Watershed Research Center, P.O. Box 213, Tombstone, AZ 85638 USA
^b Dep. of Soil Sci., Univ. of Wisconsin-Madison, 1525 Observatory Dr., Madison, WI 53706-1299 USA

gordon{at}tucson.ars.ag.gov

A small-volume dielectric constant and soil water content sensor would be desirable in many laboratory experiments. Phase shift of the reflection coefficient in soil and various solutions was measured with a coaxial probe (CP) and a network analyzer operating at a frequency of 795 MHz. The CP had a measurement depth <1 cm. Five soils, varying widely in texture, bulk density, and organic matter content, were tested and comparison was made with other dielectric methods. Synthesized time domain reflectometry (TDR), resonant waveguide, CP, and conventional TDR measurements were in agreement for the sand. A simple mixing model for known dielectrics accurately predicted measurements of the apparent dielectric (K_a) for mixed solutions. A linear function , fit for the water content of all soil data, had a single measurement root mean square error . The uncertainty improved when individual linear soil calibrations (singe measurement to 0.99) were used and further improved when repeated measurements were averaged . The CP method for measuring K_a is fast, simple, linear, easily repeated, and reasonably accurate, indicating that this instrumentation is useful for studying dielectric behavior of soil and various solutions and for rapid determination of soil water content in a small sample.

Abbreviations: CP, coaxial probe • FDR, frequency domain reflectometry • K_a, apparent dielectric constant • K_m, dielectric constants of mixtures • MPS, mean phase shift • MPSA, mean phase shift in air • n, refractive index • NA network analyzer • TDR, time domain reflectometry • t, Kelvin temperature • _v, volumetric water content

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