Soil Water Retention Measurements Using a Combined Tensiometer‐Coiled Time Domain Reflectometry Probe

Abstract

The objective of the presented study was to develop a single probe Watson et al., 1975; Cheng et al., 1975; Arya et al., that can be used to determine soil water retention curves in both 1975; Royer and Vachaud, 1975; Simmons et al., 1979; laboratory and field conditions, by including a coiled time domain Gardner et al., 2001). Disadvantages of field estimation reflectometry (TDR) probe around the porous cup of a standard tensiometer. The combined tensiometer-coiled TDR probe was con-of soil water retention curves are related to the considerstructed by wrapping two copper wires (0.8 mm diam. and 35.5 cm able time effort involved and instrumentation required, long) along a 5-cm long porous cup of a standard tensiometer. The and the relatively small range of soil water matric potendielectric constant of five different soils (Oso Flaco [coarse-loamy, tials that can be measured with a tensiometer (Bruce and mixed Typic Cryorthod-fine-loamy, mixed, mesic Ustollic Haplargid], Luxmoore, 1986). In addition, h and measurements Ottawa sand [F-50-silica sand], Columbia [Coarse-loamy, mixed, sugenerally pertain to different soil volumes, making their peractive, nonacid, thermic Oxyaquic Xerofluvents], Lincoln sandy interpretation difficult and uncertain. loam (sandy, mixed, thermic Typic Ustifluvents), and a washed sand-More recently, TDR has been used in combination SRI30) was measured with the combined tensiometer-coiled TDR with tensiometer and electrical resistance techniques to probe (coil) as a function of the soil water content () and soil water determine in situ soil water retention curves. Baumgartmatric potential (h). The measured dielectric constant (ε coil) as a function of water content was empirically fitted with a third-order poly-ner et al. (1994) and Whalley et al. (1994) combined shalnomial equation, allowing estimation of (h)-curves from the combined low stainless steel electrodes with a porous stainless steel tensiometer-coiled TDR probe measurements, with R 2 values larger material in a standard two parallel probe configuration than 0.98. In addition, the mixing model approach, adapted for the tenfor simultaneous in situ measurement of and h. Prelimsiometer-coiled TDR probe, was successful in explaining the funcinary results showed its functionality and effectiveness, tional form of the coiled TDR data with about 30% of the coiled-TDR but limitations were related to the difference in meaprobe measurement explained by the bulk soil dielectric constant. This sured soil volume between and h measurements of this new TDR development provides in situ soil water retention data from probe. Simultaneous measurement of and h was also simultaneous soil water matric potential and water content measuresuggested by Noborio et al. (1999) using a TDR probe ments within approximately the same small soil volume around the partially embedded in a porous gypsum block. Assumcombined probe, but requires soil specific calibration because of slight desaturation of the porous cup of the tensiometer.

Performance of a multi-level TDR system exposed to tropical field conditions – A time-space comparison with tensiometry

“…Tensiometry and TDR estimates of soil water content, besides being derived from different soil characteristics (soil energy status and dielectric permittivity), also correspond to different soil volumes (Vaz et al, 2002), and in the case of our study both measurements were taken at a distance of about 0.10 m. Nonetheless, because the spatial pattern of soil water content estimated by tensiometry for the different depths were similar ( Fig. 4 and Table 2), rank stability can be assumed.…”

Scattering Parameters Measurements with the Microwave Transmittance Technique Using a Microstrip Patch Antennas, as Non-Invasive Tool for Determination of Soil Moisture

“…As the dielectric constant of the root (42 < εr < 56 (f0 = 1.2 GHz)) is much higher than dry soil (3 < εr < 8) and near of the water (εr ~79) [4], it is possible to hypothesize that it could be a factori influencing root growth, mainly with increasing of the root density, in the region that the measurements were taken. To propose this approach we considered the soil bulk density is low (ρss 0.59 g/cm 2 ); the discussion about application of three phase mixing model [13]; the hydraulic redistribution of water through roots maintain roots turgid, as was discussed by de Willigen and collaborators in his paper [14]; Bao et al, (2018) elaborates on the importance of the water around the root tip, and how it plays an important role in the establishment of root system architecture [15]. In the area analyzed, the root as well the fraction of water could occupy the higher amount of soil porosity and consequently the influence average of S21 (dB) and his standard deviation.…”

A Review of Advances in Dielectric and Electrical Conductivity Measurement in Soils Using Time Domain Reflectometry

“…Or and Wraith (1999a) used a TDR probe as the basis of a matric potential sensor using porous disks with differing pore sizes placed along the probe. Other workers have presented contrasting designs for simultaneous water content and potential measurement using the same probe (Baumgartner et al, 1994; Noborio et al, 1999; Vaz et al, 2002). The relationship between soil strength and water content is complex, and Topp et al (1996) proposed a combined TDR and penetrometer as an instrument for exploring this relationship (Young et al, 2000).…”