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DIVISION S-1-SOIL PHYSICS

A Dielectric–Water Content Relationship for Sandy Volcanic Soils in New Zealand

^a New Zealand Forest Research Institute Ltd., Private Bag 3020, Rotorua, New Zealand (sponsoring organization)
^b HortResearch, Ltd., Private Bag 11-030, Palmerston North, New Zealand

mark.tomer{at}forestresearch.co.nz

To measure soil water content () by time domain reflectometry (TDR), one must know the relationship between the soil's dielectric constant (K_a) and . Our objective was to determine K_a– calibrations for sandy volcanic soils on the North Island of New Zealand. We collected samples from 24 horizons and 6 soil profiles. The soils were sandy loam and loamy sand textured, with bulk densities between 0.55 and 1.34 Mg m^-3. Samples were air-dried and packed to their field bulk density in plastic boxes. Time domain reflectometry probes (100 mm long) were inserted, and TDR waveforms were recorded and analyzed. Water contents were increased in approximate steps of 0.05 m³ m^-3 volume to the liquid limit, which varied between 0.33 and 0.57 m³ m^-3. Samples were then submerged to obtain water contents as great as 0.646 m³ m^-3. Measured water contents were greater that those predicted by the Topp equation, but the differences varied according to soil texture. Pooled data from 17 of the samples provided a third-order polynomial calibration with an R² of 0.977 and root mean square error (RMSE) of 0.026 m³ m^-3. A third-order mixing model and a linear K_a– expression gave greater mean errors. Although the calibration applied to a range of sandy volcanic soils, there were also two small groups of samples that showed distinct calibrations. These were coarser tephras, with at least 75% of particles >0.125 mm diam. by mass, and with bulk densities >1.0 Mg m^-3.

Abbreviations: RSME, root square mean error • TDR, time domain reflectometry

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