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Development of a Resonant Length Technique for Soil Water Content Measurement

School of Natural Resources, The Ohio State University, 2021 Coffey Rd., Columbus, OH 43210-1085

B. Lowery, E. T. Cooley and G. L. Hart

Dep. of Soil Science, Univ. of Wisconsin, Madison, 1525 Observatory Dr., Madison, WI 53706-1299

*Corresponding author (starr.69{at}osu.edu).

ABSTRACT

Oven drying a soil sample of known volume is one of the few inexpensive methods for measuring volumetric water content (_v). Repetitive application of this technique to field soil sampling is a laborious, time-consuming, and destructive process. Faster and less destructive low-cost methods are needed in all branches of soil science. A technique has been developed that quickly determines _v by monitoring the reflected power amplitude with a parallel-rod waveguide probe inserted into soil. Reflected power reaches a distinct minimum when a resonant condition is reached; at this point, the depth of insertion or resonant length (L_res) is measured and calibrated against _v. An inverse relationship is derived showing that L_res increases with decreasing water content. This measurement does not require a microprocessor, as do most time domain reflectometry (TDR) systems. Second-order polynomials adequately describe the calibration curves between L_res and _v in three soils with coefficients of determination >0.99. Zone of influence varies with _v, but the range of L_res and _v may be tailored to experimental needs by varying the operational frequency or probe length. For a 30-cm probe and 141-MHz operating frequency, L_res varies from 1 to 6 cm with a _v range from 0.20 to 0.0 m³ m^-3. The need for calibration, water content range, and variable zone of influence could be disadvantages; thus, this technique is not a replacement for more complex and expensive methods, such as TDR.

Research supported by USDA-CSREES-NRI and Univ. of Wisconsin, Madison, College of Agricultural and Life Sciences.

Received for publication July 3, 1997.