Calibration of time domain reflectometry technique using undisturbed soil samples from humid tropical soils of volcanic origin

Time domain reflectrometry (TDR) is used to measure the apparent dielectric number (K-a) in soils. We studied two soil types (Humitropept and Hapludand) of low bulk density (about 0.7 Mg m(-3) at 0.05 m to 0.8 Mg m(-3) at 0.3 m depth) and high organic matter content (about 7% at 0.05 m to 4% at 0.3 m depth). Soils are located in a humid tropical environment (average annual soil water content is 0.51 to 0.58 m(3) m(-3)). For calibration, undisturbed soil blocks, with a TDR probe installed in the center, were saturated and then allowed to dry by evaporation. Volumetric water content was calculated from measured K-a values and from gravimetric measurements. Because we used undisturbed soil samples, our calibration accounts for the natural heterogeneity in soils. We tested the suitability of various calibration functions relating K-a to soil water content for our soils. TDR technique underestimated the actual soil water content by 0.05-0.15 m(3) m(-3), when using the widely applied Topp calibration function. A three-phase mixing model with a geometry parameter, alpha = 0.47, fit our data best. We consider mixing models to be a robust approach for calibration of TDR technique on various soils.