תפריט נגישות
ניגודיות עדינהניגודיות גבוהה
מונוכרום
הדגשת קישורים
חסימת אנימציה
פונט קריא
סגור
איפוס הגדרות נגישות
להורדת מודול נגישות חינם תפריט נגישותניגודיות עדינהניגודיות גבוההמונוכרוםהדגשת קישוריםחסימת אנימציהפונט קריאסגוראיפוס הגדרות נגישותלהורדת מודול נגישות חינםTime domain reflectometry (TDR) for measuring water content (θTDR) and electrical conductivity of the soil solution (σw) is becoming increasingly popular. Its outstanding advantages are accuracy, speed, reproducibility, good theoretical basis, a well‐defined and selected sampled volume, and the fact that water content and salinity are measured in exactly the same volume. The major practical shortcomings of this technology, attenuation of the energy of the electromagnetic pulses which increases the measurement error, is caused by long cables, large contents of salt or clay, and too long probe rods. Wyseure et al. (1997) strived to set practical limits to the salinity effect on TDR‐determined water content, and suggested a procedure to overcome it. Caution is needed, however, because the theoretical and actual extraction of the soil water dielectrics do not entirely accord (see, for example, the temperature correction in Pepin et al. 1995). It is good to see that several research groups are attempting to overcome these for the benefit of all users, and in particular the recent paper by Wyseure et al. (1997).
The theoretical prediction of bulk soil electrical conductance (σa) effect on θTDR is sound (Hasted 1973) and at present its practical dimensions have to be tested. Wyseure et al. (1997) try to show, for saline soils, that conductance (σa) and frequency effects on travel time of pulses cannot be neglected. They rely on Dalton (1992) and Vanclooster et al. (1993), and assume that when the σw of the pore water exceeds 8 or 10 dS m–1, TDR systematically overestimates θ in saline soils. To compensate for this bias, the authors proposed a method which estimates the dielectric constant (ε) at zero salinity by correcting the apparent ε as a function of bulk resistivity (σa). The physical mechanism for overestimating θ is explained as follows. The polarization losses are caused by a phase lag between the alternating electric field and the dipole orientation of the water molecules, resulting in a dissipation of heat. This phenomenon depends on the frequency. The ohmic losses are due to charged particles moving through the medium (i.e. ions in solution). Finally, the authors suggest a model that, based on the imaginary part of the relation for ε–ohmic losses, calculates the velocity of propagation of the electromagnetic (EM) pulses. I am not happy with the authors’ explanation, and I make the following three comments, two specific and one general.
1 The accuracy of the experimental results does not justify the conclusions reached.
2 The definition of ‘saline soils’ in the context of ε–ohmic losses is vague because the authors do not differentiate clearly between σa and σw.
3 Current theoretical models do not confidently reflect the salinity effect of aqueous solutions on soil ε as determined by TDR.
Time domain reflectometry (TDR) for measuring water content (θTDR) and electrical conductivity of the soil solution (σw) is becoming increasingly popular. Its outstanding advantages are accuracy, speed, reproducibility, good theoretical basis, a well‐defined and selected sampled volume, and the fact that water content and salinity are measured in exactly the same volume. The major practical shortcomings of this technology, attenuation of the energy of the electromagnetic pulses which increases the measurement error, is caused by long cables, large contents of salt or clay, and too long probe rods. Wyseure et al. (1997) strived to set practical limits to the salinity effect on TDR‐determined water content, and suggested a procedure to overcome it. Caution is needed, however, because the theoretical and actual extraction of the soil water dielectrics do not entirely accord (see, for example, the temperature correction in Pepin et al. 1995). It is good to see that several research groups are attempting to overcome these for the benefit of all users, and in particular the recent paper by Wyseure et al. (1997).
The theoretical prediction of bulk soil electrical conductance (σa) effect on θTDR is sound (Hasted 1973) and at present its practical dimensions have to be tested. Wyseure et al. (1997) try to show, for saline soils, that conductance (σa) and frequency effects on travel time of pulses cannot be neglected. They rely on Dalton (1992) and Vanclooster et al. (1993), and assume that when the σw of the pore water exceeds 8 or 10 dS m–1, TDR systematically overestimates θ in saline soils. To compensate for this bias, the authors proposed a method which estimates the dielectric constant (ε) at zero salinity by correcting the apparent ε as a function of bulk resistivity (σa). The physical mechanism for overestimating θ is explained as follows. The polarization losses are caused by a phase lag between the alternating electric field and the dipole orientation of the water molecules, resulting in a dissipation of heat. This phenomenon depends on the frequency. The ohmic losses are due to charged particles moving through the medium (i.e. ions in solution). Finally, the authors suggest a model that, based on the imaginary part of the relation for ε–ohmic losses, calculates the velocity of propagation of the electromagnetic (EM) pulses. I am not happy with the authors’ explanation, and I make the following three comments, two specific and one general.
1 The accuracy of the experimental results does not justify the conclusions reached.
2 The definition of ‘saline soils’ in the context of ε–ohmic losses is vague because the authors do not differentiate clearly between σa and σw.
3 Current theoretical models do not confidently reflect the salinity effect of aqueous solutions on soil ε as determined by TDR.
