Comments on "J. Singh et al., Performance assessment of factory and field calibrations for electromagnetic sensors in a loam soil [Agric. Water Manage. 196 (2018) 87-98]

Authors: Schwartz, Robert; Evett, Steven - Steve; Lascano, Robert

Submitted to: Agricultural Water Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/23/2018
Publication Date: 3/22/2018
Citation: Schwartz, R.C., Evett, S.R., Lascano, R.J. 2018. Comments on "J. Singh et al., Performance assessment of factory and field calibrations for electromagnetic sensors in a loam soil [Agric. Water Manage. 196: 87-98]. Agricultural Water Management. 203:236-239.

Interpretive Summary: New and improved soil water sensor technologies are being promoted for the assessment of crop water use and to schedule irrigation. However, the accuracy of some of the sensors is questionable thereby requiring evaluation under field settings where they will be used. In this paper we provide a critique on a recent publication comparing soil water sensors and show that some of the results have the potential to be misleading. Unfortunately, the comparisons are flawed because theory of sensor response to soil water was overlooked. Also, accepted practices in calibrations and field interpretations of soil water were disregarded. In the following paragraphs, we justify this interpretation and point out other concerns with the comparisons made, with the hope that this critique could provide helpful guidance for good practice in the future.

Technical Abstract: The article by Singh et al., (2018) provides a striking example of the wide range of soil water contents reported by several electromagnetic (EM) sensor technologies under field conditions. We commend the authors for taking the initiative to evaluate these sensors in the field and in situations where they could be used to assess crop water use and irrigation schedules. However, some of the results presented in the article have the potential to be misleading, particularly to those looking for reliable assessments of soil water sensors that are currently commercially available. The basis of the Singh et al. (2018) evaluation is readings from a field-calibrated neutron probe (NP) against which the readings from the EM sensors were compared using factory soil water content calibrations. All measurements were taken in the field. Unfortunately, the comparisons are flawed because theory governing the differing response of EM sensor technologies to soil water was overlooked and accepted practices in calibrations and field interpretations of soil water were disregarded. In these sensor evaluations, the authors failed to consider the effect of small scale random variations on measurement uncertainty, bias related to soil water sensing volume, and neutron probe calibration bias which, combined, invalidate the comparisons. In the following paragraphs, we justify these interpretations and point out other concerns with the comparisons made, with the hope that this critique could provide helpful guidance for good practice in the future.