Published online 28 June 2005
Published in Soil Sci Soc Am J 69:1356-1357 (2005)
DOI: 10.2136/sssaj2005.0086le
© 2005 Soil Science Society of America
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Comments and Letters to the Editor
Response to "Comments on ‘Field Calibration of Water Content Reflectometers’"
David G. Chandlera and
Mark Seyfried*,b
a Department of Plants, Soils, and Biometeorology Utah State University 4820 Old Main Hill Logan, UT 84322-4820
b USDA-ARS 800 Park Blvd. Boise, ID 83712
david.chandler{at}usu.edu
In Dr. Nadler's critique of our paper (Chandler et al., 2004), he makes several points critical of the research approach. It seems that his concerns are founded in a misunderstanding of the intent of the paper, which on review, could be open to interpretation. To clarify, the paper was intended for the measurement needs of the plant-soil-water and hydrologic research community, many of whom have purchased and installed this popular sensor for long-term monitoring purposes in nonagricultural settings. Its intent was to demonstrate that improvements in water content (WC) measurements made by water content reflectometers (WCRs) are possible through field calibration. We used data collected at an experimental rangeland watershed in Idaho for this purpose. Our paper is not an endorsement of this particular sensor, which is indeed influenced by both WC and electrical conductivity (EC), as are all electromagnetic WC measurement methods, albeit to varying extent.
Below we respond to the points made by Dr. Nadler, beginning with his interpretation of the paper (Points 1–3), followed by some comments (i–iv). Finally, we address four specific issues, identified by Dr. Nadler as A-D, as he requested in his letter.
- Correct, we acknowledge that the tradeoff between measurement precision and instrumentation expense has resulted in widespread use of sensors such as the WCR. In the end, reliable and standardized distributed WC measurements may be of more utility to field investigations than a few very precise measurements.
- Nearly correct. We propose using collocated time domain reflectometry (TDR) measurements as standards to develop field calibrations of these sensors. This is the main point of the paper.
- A good idea, but not mentioned in the paper.
In response to Dr. Nadler's four comments we offer the following.
- The calibration strategy is straightforward. To remove the individual sensor bias and improve the slope of the sensor response as affected by soil properties, calibrations were derived from simple regression analysis between measurements made by 22 WCRs and corresponding TDR waveguides.
- Our intent was not to bridge the frequency gap between TDR and the WCR. In Fig. 2 of Chandler et al. (2004), it is shown that even for low salinity soils, WC values obtained by factory calibrations of WCR period vary considerably from those obtained from standard TDR measurements.
- Although these calibrations did not include all the factors that may influence the WCR response in an agricultural setting, they were made in two soils, over a range of temperatures typical of a (vegetated) mountain rangeland site in the western USA.
- After listing his intended points, Dr. Nadler uses excerpts from two related papers that share common authors, Chandler et al. (2004) and Seyfried and Murdock (2001), as the basis of an argument that, due to the sensitivity of the CS-615 to EC and the dynamic nature of soil EC in the field, "the CS-615 is no better than two bare metallic rods of rigid configuration." Based on this premise he proposes testing electrical resistance measurement as a means to measure water content for these soils. We are not inclined to do so, but encourage his efforts along these lines.
As to the specific issues raised by Dr. Nadler, we respond as follows.
- The linear regressions are simple to use. We clarify that the WCR does not operate at the same frequency as real TDR and identify this as a limitation for the sensor. However, it should also be noted that once calibrated, sensors with the electronics adjacent to the waveguides, such as the WCR avoid the signal transmission loss and noise encountered with TDR systems multiplexed to long cable lengths and deployed in field conditions with widely variable surface temperature and moisture conditions. As clarified above, our paper is not an "effort to provide growers with an accurate yet inexpensive tool for monitoring ... WC," nor did the intended audience consist mainly of ranch managers, extension agents, or technicians. Considering the availability of TDR in many research institutions, we do not view equipment access as an insurmountable obstacle.
- We see no contradiction here. We use TDR as a standard technique and merely state the accepted limitations of TDR.
- We do not avoid the use of TDR. We identify the cost of TDR as a limiting constraint for many field studies. It was hoped that the WCR would suffice in our medium-textured soils with low EC. The papers cited describe important advances in the use of TDR for highly saline soils, but do not address the issue of cost.
- We do not follow the logic in this point. As in (iv), we invite Dr. Nadler to test his own hypotheses. We reiterate: The main purpose of our paper was to demonstrate that improvements in WC measurements made by WCRs are possible through field calibration to TDR, which is widely accepted as a standard technique. In fact, including soil temperature in the calibration did not improve the calibration of WCR period to TDR WC in this case and as such, soil temperature was not presented.
In conclusion, the proposed calibration approach has been demonstrated only for low-EC soils, where WC can be expected to dominate the sensor response.
NOTES
* mseyfrie{at}nwrc.ars.usda.gov 
REFERENCES
- Chandler, D.G., M. Seyfried, M. Murdock, and J.P. McNamara. 2004. Field calibration of water content reflectometers. Soil Sci. Soc. Am. J. 68:1501–1507.[Abstract/Free Full Text]
- Seyfried, M.S., and M.D. Murdock. 2001. Response of a new soil water sensor to variable soil, water content, and temperature. Soil Sci. Soc. Am. J. 65:28–34.[Abstract/Free Full Text]
