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

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 (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.

Time Domain Reflectometry: Temperature-dependent Measurements of Soil Dielectric Permittivity

“…9. The majority of tested soils show similar trends that confirm other experimental data (Pepin et al, 1995;. For small and medium moisture values there is a negligible temperature effect and the linear trend lines in Fig.…”

“…If bulk soil dielectric permittivity had a temperature dependence related solely to free water, measurements of soil ε b would show a negative correlation with temperature that would increase with water content. Results from Pepin et al (1995) show this negative correlation, www.intechopen.com but the overall changes are smaller than predicted from a dielectric mixing model that integrated the temperature change in dielectric permittivity of water. The same was observed by Halbertsma et al (1995) who made measurements of sand and silt soil samples.…”

“…The significant fluctuation of measured data, which was obviously correlated with soil temperature, was noticed with the introduction of soil moisture field monitoring systems based on reflectometric meters. Also, it was found (Pepin et al, 1995;Halbertsma et al, 1995) that ε b decreased with temperature increase for sandy, silt and peat soils and it did not change for the tested clay soil. The text below uses two terms that need explanation: (i) soil free water -composed from water particles that rotation in the electric filed is not hampered, and (ii) soil bound watercomposed from water particles so close to solid phase that their rotation in the electric field is hampered by surface charge on the solids.…”

Using neural networks for calibration of time-domain reflectometry measurements

“…Dalton, 1992;Sun et al, 2000;Persson et al, 2000), and temperature (e.g. Pepin et al, 1995;Persson & Berndtsson, 1998). In many cases, the actual physical process that induces the effects on the K a -Q w relationship is not fully understood, although studies have shown that the effects are small in most cases (e.g.…”

Reply [to ‘Comment on “On water vapor transport in field soils’ by Anthony T. Cahill and Marc B. Parlange”]

“…The work of Or and Wraith [1999] is a valuable contribution; it was, however, not available when our paper was published. As stated by CP, the neglect of temperature effects on the TDR readings on soil moisture was based on results presented in one of the few papers on this topic which was available at the time we wrote the paper [Pepin et al, 1995]. We deliberately presented all of our 'raw' data so that others could make use of them in that context, as OW have presented here.…”