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Temperature-Dependent Measurement Errors in Time Domain Reflectometry Determinations of Soil Water

Centre for Forest Biology, Dep. of Biology, Univ. of Victoria, Victoria, BC, Canada V8W 2Y2

W. R. Hook

Precision Moisture Instruments Inc., 100-4243 Glanford Ave., Victoria, BC, Canada V8Z 4B9

* Corresponding author ( njl{at}uvvm.uvic.ca).

ABSTRACT

With the recent development of improved time domain reflectometry (TDR) probe design, measurement systems, and calibration procedures, it is now possible to detect and quantify the effect of temperature on the soil apparent dielectric constant (K_a). We investigated measurement errors in K_a associated with soil temperature variations and compared measured changes in K_a with those predicted by a dielectric mixing model. After confirming the accuracy and resolution of our measurement system with a series of measurements on distilled water, we measured changes in K_a with temperature for a range of soil types, including sand, loam, and peat, at soil water contents (_v) ranging from 0.09 to 0.81 m³ m^–3. The measured variation with temperature in the dielectric constant of distilled water (0.322°C^–1) was very close to that reported in the literature (0.356°C^–1). In soils, changes in K_a with temperature were highest at high water contents. For soils near saturation, the overall changes observed in K_a with temperature were lower than those predicted by the dielectric mixing model by 17% for sand, 24% for loam, and 39% for peat. These results suggest that the temperature dependence of the dielectric constant of water in a soil matrix is lower than that of bulk water. Absolute water content errors increased linearly with the size of the water fraction, ranging from 8.75 x 10^–5 m³ m^–3°C^–1 at 0.05 m³ m^–3 soil water content to 1.40 x 10^–3 m³ m^–3°C^–1 at 0.80 m³ m^–3 soil water content. To obtain the highest measurement accuracy, particularly at higher _v, we suggest that a temperature correction of 0.00175_v °C^–1 be employed.

Received for publication August 5, 1993.

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