Two- and three-parameter calibrations of time domain reflectometry for soil moisture measurement
1997
Yu, C. | Warrick, A.W. | Conklin, M.H. | Young, M.H. | Zreda, M.
Time domain reflectometry (TDR) is widely used to measure and monitor soil water. The commonly used calibration curve is the third-degree "universal polynomial" of Topp et al. [1980]. The most common refinement is calibration to a specific soil but still using four parameters (coefficients) from fitting a third-degree polynomial. Here we demonstrate that a three-parameter expression, (...) fits as close as or more closely than the four-parameter polynomial (where the three parameters a, b, and alpha are determined by fitting water content to the dielectric coefficient). This form is consistent with the well-known mixing model. For an isotropic soil with homogeneous water distribution this expression is further simplified to two parameters by taking alpha = 0.5. When alpha is 0.5, its calibration is equivalent to the linear calibration between volumetric soil water content and the travel time along the waveguide. In addition, the simple three-parameter expression can be easily inverted without losing accuracy with regard to the original calibration. The TDR calibration expressed in a three-parameter form not only achieves a good fit but also conveys a physical connotation.
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