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Development of a Resonant Length Technique for Soil Water Content Measurement
Author(s) -
Starr G. C.,
Lowery B.,
Cooley E. T.,
Hart G. L.
Publication year - 1999
Publication title -
soil science society of america journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.836
H-Index - 168
eISSN - 1435-0661
pISSN - 0361-5995
DOI - 10.2136/sssaj1999.03615995006300020004x
Subject(s) - reflectometry , water content , calibration , soil water , sampling (signal processing) , content (measure theory) , range (aeronautics) , power (physics) , amplitude , soil science , inverse , time domain , environmental science , materials science , volume (thermodynamics) , analytical chemistry (journal) , optics , mathematics , geology , physics , chemistry , geotechnical engineering , geometry , composite material , computer science , mathematical analysis , detector , chromatography , quantum mechanics , computer vision , statistics
Oven drying a soil sample of known volume is one of the few inexpensive methods for measuring volumetric water content (θ 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 θ 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 θ 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 θ v in three soils with coefficients of determination >0.99. Zone of influence varies with θ v , but the range of L res and θ 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 θ v range from 0.20 to 0.0 m 3 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.