### Abstract

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, θ(v) = aK(a)/α + b, fits as close as or more closely than the four-parameter polynomial (where the three parameters a, b, and α are determined by fitting water content θ(v) to the dielectric coefficient K(a)). 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 α = 0.5. When α is 0.5, its calibration is equivalent to the linear calibration between θ(v) 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.

Original language | English (US) |
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Pages (from-to) | 2417-2421 |

Number of pages | 5 |

Journal | Water Resources Research |

Volume | 33 |

Issue number | 10 |

DOIs | |

State | Published - Oct 1997 |

### ASJC Scopus subject areas

- Water Science and Technology

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## Cite this

*Water Resources Research*,

*33*(10), 2417-2421. https://doi.org/10.1029/97WR01699