VZJ Journal of Natural Resources and Life Sciences Education
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

Published online 3 October 2006
Published in Vadose Zone J 5:1067-1068 (2006)
DOI: 10.2136/vzj2006.0018L
© 2006 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
This Article
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in Web of Science
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via Web of Science (1) Citing Articles via Web of Science (1)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Regalado, C. M.
Right arrow Articles by Becker, R.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Regalado, C. M.
Right arrow Articles by Becker, R.
GeoRef
Right arrow GeoRef Citation
Agricola
Right arrow Articles by Regalado, C. M.
Right arrow Articles by Becker, R.
Related Collections
Right arrow Soil Methods/Instrumentation
Right arrow Water Content
Right arrow Time Domain Reflectometry, TDR

COMMENTS

Comments on "Monitoring Soil Water Content Profiles with a Commercial TDR System: Comparative Field Tests and Laboratory Calibration"

Carlos M. Regaladoa, Axel Rittera and Rolf Beckerb

a Instituto Canario de Investigaciones Agrarias (ICIA), Dep. Suelos y Riegos, Apdo. 60 La Laguna, 38200 Tenerife, Spain
b Imko Micromodultechnik GmbH, Im Stöck 2, D-76275 Ettlingen, Germany
cregalad{at}icia.es


Laurent et al. (2005) proposed an empirical relationship between the permittivity, K, and the pseudo transit time, t2, measured with a TRIME TDR System (Imko GmbH, Ettlingen, Germany). Different probe designs are available for the TRIME System. Among these are access tube cylindrical probes (TRIME-T3), two-rod probes with internal electronics (e.g., TRIME-EZ or TRIME-IT), and simple waveguides without electronics connected to an external TDR instrument (such as the P2 and P2Z probes connected to a handheld TRIME-FM). The P2 probe and TRIME-IT share the same rod geometry and therefore show the same performance. The same holds for the P2Z probe and the TRIME-EZ.

The TRIME System measures the TDR-pulse transit time t1 (ps) relative to an arbitrary reference time. The TRIME coated probes with different geometries all perform differently in that they are characterized by their own specific t1 vs. water content ({theta}) relationship. To reduce small effects of unavoidable tolerances in the electronics, mechanics, and production, a linear transformation was applied, called "basic balancing." This results in a "pseudo transit time" t2, which in fact is a normalized time of the form:

Formula 1[1]
where A is a shift in the zero point and D is some measure for the sensitivity of the probe. Using transformation [1], all of the probe types available with the TRIME System can be brought to similar dynamic ranges for t2 (not t1). The water content is then calculated from a probe type–specific standard calibration, a fifth degree polynomial of the form (Stacheder, 1996):

Formula 2[2]
where the Ci are probe type–specific empirical parameters (Fig. 1).


Figure 1
View larger version (15K):
[in this window]
[in a new window]
  		Fig. 1. Water content ({theta}) vs. pseudo transit time (t2) relationships for different TRIME probe types.

 
Laurent et al. (2005) proposed the following linear relationship between the square root of the permittivity, Formula 2, and the pseudo transit time, t2 (R2 = 0.9886):

Formula 3[3]

This equation was derived by fitting data obtained with the TRIME-T3 access tube probe on a soil sample, referred as "Cylinder container 10l 2003" (Fig. 7 in Laurent et al., 2005). The importance of Eq. [3] stems from the fact that most TDR techniques provide values of the permittivity rather than pseudo transit times. The equation hence allows either recalibration or comparison of moisture data obtained with other TDR systems using measurements carried with the TRIME System.

We derived an alternative K vs. t2 relationship from (i) measurements obtained with a TRIME-T3 access tube probe on reference materials and glass beads (Table 4 in Laurent et al., 2005), (ii) measurements obtained with a TRIME-P2 probe in two-phase mixtures of water and dioxan (Fig. 5.5 in Stacheder, 1996), and (iii) our own measurements with a TRIME-P2 probe in different media of known permittivity (Table 1).


View this table:
[in this window]
[in a new window]
  		Table 1. Permittivity (K) of the different media used in this study.

 
Applying a square root K transformation after Laurent et al. (2005) to the measurements described above produced the following relationship (R2 = 0.8747):

Formula 4[4]

Notice that the data deviate substantially from linear behavior, especially for t2 > 700 (Fig. 2a). Linearity is approached for 100 < t2 < 700, which is the range investigated by Laurent et al. (2005). This explains the different forms of Eq. [3] and [4]. As an alternative to Eq. [4], Fig. 2b presents the logarithmic K transformed vs. t2 data. Linearity is closely achieved in most of the t2 range, which causes us to propose the following K vs. t2 relationship (R2 = 0.9718):

Formula 5[5]


Figure 2
View larger version (14K):
[in this window]
[in a new window]
  		Fig. 2. Permittivity (K) vs. pseudo transit time (t2) relationships after (a) square root and (b) logarithmic transformations of the permittivity.

 
The better performance of the logarithmic transformation can be quantified in terms of the coefficient of efficiency, Ceff (Nash and Sutcliffe, 1970), which compares the variance about the 1:1 line (perfect agreement) with the variance of the observed data. Using Eq. [5] yields a value of Ceff = 0.90 as opposed to Ceff = 0.83 for the square root transformation given by Eq. [4]. In addition, the expected error (root mean square error) in predicting K using Eq. [5] rather than [4] decreased by about 20% (9.5 vs. 11.5).

Our results hence suggest that a logarithmic, rather than a square root, transformation is more appropriate for the relationship between the pseudo transit time and the permittivity with the TRIME TDR System. The transformation appears valid for the entire measurement range of the instrument as opposed to the square root transformed permittivity relationship presented by Laurent et al. (2005).

REFERENCES




This article has been cited by other articles:


Home page
 Vadose Zone JHome page
A. Ritter, C. M. Regalado, and R. Munoz-Carpena
Temporal Common Trends of Topsoil Water Dynamics in a Humid Subtropical Forest Watershed
Vadose Zone J., May 21, 2009; 8(2): 437 - 449.
[Abstract] [Full Text] [PDF]

Home page
 Vadose Zone JHome page
J.-P. Laurent
Response to "Comments on 'Monitoring Soil Water Content Profiles with a TDR Commercial System: Comparative Field Tests and Laboratory Calibration'"
Vadose Zone J., October 3, 2006; 5(4): 1069 - 1070.
[Full Text] [PDF]

This Article
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in Web of Science
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via Web of Science (1) Citing Articles via Web of Science (1)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Regalado, C. M.
Right arrow Articles by Becker, R.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Regalado, C. M.
Right arrow Articles by Becker, R.
GeoRef
Right arrow GeoRef Citation
Agricola
Right arrow Articles by Regalado, C. M.
Right arrow Articles by Becker, R.
Related Collections
Right arrow Soil Methods/Instrumentation
Right arrow Water Content
Right arrow Time Domain Reflectometry, TDR

HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
The SCI Journals Agronomy Journal Crop Science
Journal of Natural Resources
and Life Sciences Education		 Soil Science Society of America Journal
Journal of Plant Registrations Journal of
Environmental Quality		 The Plant Genome