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, α=0.47, fit our data best. We consider mixing models to be a robust approach for calibration of TDR technique on various soils.