Time Domain Reflectometry Calibration for Uniformly and Nonuniformly Wetted Sandy and Clayey Loam Soils
1992
Dasberg, S. | Hopmans, J. W.
Although application of time domain reflectometry (TDR) has been successful in many reported studies, questions still arise with regard to the versatility of the method when used among different-textured soils and in heterogeneous soil profiles. This study was initiated to obtain calibration curves for soil water content determinations by TDR for two soil types in uniform and layered profiles. Measurements were taken in the laboratory for a Hanford sandy loam (coarse-loamy, mixed, nonacid thermic Typic Xerorthent) and a Yolo clay loam (finesilty mixed, nonacid thermic Typic Xerorthent) soil, using a two-rod parallel with pulse transformer and a three-rod coaxial probe. Different configurations of 5-cm-thick wet and dry soil layers were tested to study the influence of variations in soil water content on the signal. The calibration data of the coarse-textured Hanford soil matched the established calibration curve applied by TDR users. The dielectric constants of the fine-textured Yolo soil were significantly lower than those of the Hanford at equal water-content values. The TDR data of both soils were successfully fitted to a four-component mixing model that includes bound and free water, the solid and gas phases. Results indicated that bound water has a dielectric constant close to that of ice. The presence of a wet layer above dry soil may causes difficulties in signal interpretation.
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