Temperature-Dependent Measurement Errors in Time Domain Reflectometry Determinations of Soil Water
1995
Pepin, S. | Livingston, N. J. | Hook, W. R.
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 (Kₐ). We investigated measurement errors in Kₐ associated with soil temperature variations and compared measured changes in Kₐ 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 Kₐ with temperature for a range of soil types, including sand, loam, and peat, at soil water contents (θᵥ) 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 Kₐ with temperature were highest at high water contents. For soils near saturation, the overall changes observed in Kₐ 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 × 10⁻⁵ m³ m⁻³°C⁻¹ at 0.05 m³ m⁻³ soil water content to 1.40 × 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 θᵥ, we suggest that a temperature correction of 0.00175θᵥ °C⁻¹ be employed.
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