Hydrological Sciences–Journal–des Sciences Hydrologiques, 54(1) February 2009

* by F. Bouksila, M. Persson, R. Berndtsson & A. Bahri, Hydrological Sciences Journal 53(1), 253–265.

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DISCUSSION of “Soil water content and salinity determination using different dielectric methods in saline gypsiferous soil”* G. KARGAS & P. KERKIDES

Laboratory of Agricultural Hydraulics, Sector of Water Resources Management, Agricultural University of Athens, 75 Iera Odos, 11855 Athens, Greece kerkides@aua.gr We would like to discuss the findings of Bouksila et al. (2008) and, more specifically, the experimental results presented in their Figure 2. Figure 2 of Bouksila et al. (2008) shows the soil dielectric constant Ka measured with TDR and WET sensors versus volumetric water content θ for various values of pore water electrical conductivity ECp. The authors admit rightly that their results Ka–θ could be described by “an almost linear Ka–θ relationship” and that the reason for this is not fully understood, but it could be related to the properties of gypsum. The experimental procedure which is nicely described in the section “Measurements in gypsiferous soil” indicates rather clearly that the step-by-step wetting of the soil sample from below, using a peristaltic pump, could create a soil column where a rather nonuniform soil water profile would be established, the lower portion of the soil column being wetter than the upper portion. This soil system changes gradually with regard to its soil water content and might evolve into a multi-layer nonuniformly wet system, until finally it reaches its largest θ values, with the completion of the experiment. In this case, where a nonuniformly wet soil is encountered, the problem of the soil dielectric constant regime has been addressed (Topp et al., 1982; Schaap et al., 2003; Robinson et al., 2005), for the case of TDR. Schaap et al. (2003) showed that although the refractive regime:

2

1

1REF

⎟⎟⎟⎟⎟

⎠

⎞

⎜⎜⎜⎜⎜

⎝

⎛

==

∑

∑

=

=N

ii

N

iii

a

L

KLKK (1)

for the Ka estimation seems the most appropriate, nonetheless in the case where a multiplicity of thin layers is encountered, the arithmetic averaging scheme seems to be the most suitable:

∑

∑

=

=== N

ii

N

iii

a

L

KLKK

1

1ARITH (2)

where Li is the length of the individual ith layer with a volumetric water content θi; and Ki is the corresponding dielectric constant value of the ith layer. The terms KREF and KARITH denote the soil dielectric constants calculated according to the refractive and arithmetic averaging schemes, respectively. In this respect, the linearity of the Ka–θ relationship observed by TDR findings might be due to this nonuniformity of the θ profile. For the WET sensor, a similar explanation, which is supported by some independent observations given in Figs 1–3 below (Kargas et al., 2008), might be the answer. Figure 1 shows the case where the Ka values are obtained by the WET sensor when gradually inserting its rods into a water vessel, with the depth of immersion D. In this case the system is a bi-layer system, air over water, in which a gradual change of the length ratio L1/L2, where L1 denotes the extent of air and L2

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Discussion of “Soil water content and salinity determination”

Copyright © 2009 IAHS Press

211

0

10

20

30

40

50

60

70

80

90

0 10 20 30 40 50 60

D (mm)

K

70

WETARITHREF

Fig. 1 Dielectric constant values predicted by the WET sensor together with the calculated ones according to the arithmetic and refractive averaging scheme versus water depth of immersing the probe rods (air over water system).

S L

0

5

10

15

20

25

30

35

0 10 20 30 40 50 60 7

D (mm)

K

0

WETARITHREF

Fig. 2 Dielectric constant values predicted by the WET sensor together with the calculated ones according to the arithmetic and refractive averaging scheme versus depth of immersing the probe rods in saturated sandy loam (air over saturated porous body).

the extent of water, is imposed. Figure 2 shows a similar situation, in which the layer of water is replaced by a saturated sandy loam soil. As one may observe, the Ka values predicted by the WET sensor are better described by the arithmetic averaging scheme than by the refractive one. Figure 3 shows the values of the dielectric constant predicted by the WET sensor against the calculated values of the dielectric constant obtained by the application of the arithmetic and the refractive averaging scheme. The porous material consists of a sandy loam soil where wet over dry portions of various length ratios L1/L2 are established. Similar findings are obtained with other soils tested (not shown here), and also when the sequence of the layering (wet over dry, or dry over wet) is reversed. We believe that our comments support the authors’ work and offer an explanation for their findings. For the WET sensor, which operates at relatively lower frequencies (20 MHz), in cases

G. Kargas & P. Kerkides

Copyright © 2009 IAHS Press

212

SL

0

5

10

15

20

25

30

35

0 5 10 15 20 25 30 35

K

AR

ITH

, RE

F

WETARITHREF

Fig. 3 Estimated values of the dielectric constant according to both the refractive and arithmetic averaging scheme against the one predicted by the WET sensor values for the case of bi-layer system wet (saturated sandy loam) over dry.

where a nonuniformly wet soil profile is encountered, the arithmetic averaging scheme for the Ka prediction is the most suitable. REFERENCES Bouksila, F., Persson, M., Berndtsson, R. & Bahri, A. (2008) Soil water content and salinity determination using different

dielectric methods in saline gypsiferous soil. Hydrol. Sci. J. 53(1), 253–265. Kargas G., Sgoubopoulou A. & Kerkides, P. (2008) Soil moisture and electrical conductivity measurements using the WET

sensor. In: Proc 12th National Conf. of the Hellenic Soil Science Society (HSSS) (Pyrgos, Peloponnesus, 24–26 September 2008), 217–229. Published by HSSS in Greek, abstract in English.

Robinson, D. A., Jones, S. B., Blonquist, J. M. & Friedman, S. P. (2005) A physical derived water content/permittivity calibration model for coarse-textured layered soils. Soil Sci. Soc. Am. J. 69(5), 1372–1377.

Schaap, M. G., Robinson, D. A., Friedman, S. P. & Lazar, A. (2003) Measurement and modeling of the TDR signal propagation through layered dielectric media. Soil Sci. Soc. Am. J. 67, 1113–1121.

Topp, G. C, Davis, J. I. & Annan, A. P. (1982) Electromagnetic determination of soil water content using TDR. I. Applications of wetting fronts and steep gradients. Soil Sci. Soc. Am. J. 46, 672–678.