ӻӿӺ ஼ၟആJ. Jpn. Soc. Soil Phys.
土壌の物理性No. 150, p.5 ∼ 6 (2022)

How to approach research— what research in TDR taught meClarke TOPP

Developing your Own Frame of Reference is Important
When one begins a research assignment early in their career, the choice is usually dependent on a job description written
for the outcome expectations of the employer along with the level of ﬁnancial compensation available. If it appears that aspeciﬁc research project might lead to future possibilities, each person is recommended to consider:− Is this research inspiring to me?
− Do I see additional innovative research possibilities here beyond meeting employer expectations?
− Can I achieve my science aspirations here?
From those considerations you can then develop a sketchy but realistic framework or plan for your personal researchsatisfaction beyond that of what your employer expects.
My research experience in bringing Time Domain Reﬂectometry (TDR) from radio frequency communication technology to soil water content measurement will serve as an example of this process. In 1974 two geophysicists, Les Davisand Peter Annan, with considerable TDR experience, realized that TDR had the potential to provide a reliable measureof water content in soil. The research question they put to me, a soil physicist specializing in soil water, was — how
do we validate that TDR measures soil water content and with what accuracy? Their challenging question offered many
research possibilities, which ﬁtted my hopes perfectly for developing a ﬁeld method for measuring soil water content.
Such a project was also quite acceptable to my employer, Agriculture and Agri-Food Canada (AAFC) Research.
The personal story of this research team is given in Topp et al. (2003). This TDR research project offered numerous
dimensions of research possibilities. In soil there are the effects of soil texture which was expected to limit where it
was possible to use TDR. We showed, however, that a wide range of soil textures did not cause much deviation fromthe observed relationship between apparent dielectric constant and volumetric water content. Developing ﬁeld probes forusing TDR became a major research and development opportunity by which our team learned a great deal of valuable
information about propagation of radio frequency waves in soil.
Do Not Be Afraid of Mistakes
Although mistakes are unavoidable in innovative research, it is advisable to look for the learning opportunities that canbe drawn from a mistaken venture. Good planning is important for avoiding mistakes, but research inevitably involves
risk with uncertain outcomes. Often a seemingly undesirable outcome uncovers or stimulates other initiatives that may
lead to greater research options. In 1980, Ottawa experienced a relatively dry August and the soil proﬁles at our ﬁeld sites
with vertical TDR installations showed quite dry proﬁles in the TDR reading of Sept. 15.
Before the next weekly TDR reading, 35.9 mm of rainfall had occurred. The TDR readings after the rainfall showeddiscernable wetting fronts at the 20 cm depth.
Although this was not replicated research with valid conclusions, our team considered this to be a signiﬁcant ﬁnding
from which to develop subsequent TDR research. When I presented these ﬁndings at the 1983 Advances in Inﬁltration(Topp et al., 1983), I sensed that there was little interest. A colleague at the conference told me that our paper was notsuited to that conference — a kind way of telling me I had made a mistake in presenting unsubstantiated research. Being
convinced that I had blundered an opportunity to promote TDR effectively to hydrologists, I was pleased to learn that Ian

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White, who was in the audience that day, saw the potential in TDR.
Shortly, thereafter he and I arranged for my collaboration with the CSIRO team in Australia which produced importantresearch that broadened the potential for TDR (Topp et al., 2000) and added to the world-wide interest in TDR for
measuring soil water content and electrical conductivity then occurring all round the world.
Colleagues are Your Most Valuable Source of Good Ideas
In the 1980s the acceptance of TDR was much slower than I and my team had expected. To help scientists understand
TDR, I developed the habit of taking a TDR cable tester and a hand held probe to soil conferences. With a ﬂower
pot or a waste can of soil I would invite scientists to my hotel room to show how easily TDR measurements could bemade including the calculation of soil water content. Often scientists who had experienced such simple demonstrations
would invite me to their laboratory to discuss TDR and show others how TDR works. From Frank Dalton I ﬁrst learned
about using TDR to measure soil electrical conductivity. After some productive collaboration, Frank and I both took thatexperience in our own directions for additional research, in my case as given in Topp et al. (2000).
In the early 1990s Ty Ferré and I met and discussed a few things and decided we should keep in touch. Later, theUniversity of Waterloo invited me to serve as external examiner of Ty’s PhD thesis, which I was pleased to do. Ty became
one of my most valued colleagues, as we wrote numerous papers together with Ferré and Topp (2002) being representative.
Concluding Thoughts
Planning for one’s own science goals can provide incentives for improved research, which will bring you much gratiﬁcation especially as you collaborate with colleagues.
Signiﬁcant research results may also encourage your employer to offer increased funding to continue on your chosen
path. Very few researchers can escape some regrettable or unconvincing outcomes but very often more favourable results
occur with deeper investigations of the limiting factors. Sharing one’s research widely and openly encourages a fertile
exchange of ideas, from which arise opportunities for productive research collaboration.

References

Topp, G.C., Davis, J.L. and Chinnick, J.H. (1983): Using TDR water content measurements for inﬁltration studies, pp. 231–240. In
Advances in Inﬁltration: Proc. of the Natl. Conf. on Advances in Inﬁltration, Chicago, IL. 12–13 Dec. 1983. ASAE Publ. 11–83.
Am. Soc. Agric. Engineers, St. Joseph, MI.
Topp, G.C., Zegelin, S.J. and White, I. (2000): Impacts of the real and imaginary components of relative permittivity on time domain
reﬂectometry measurements in soils. Soil Science Society of America Journal, 64(4): 1244–1252,
doi:10.2136/sssaj2000.6441244x
Topp, G.C., Davis, J.L. and Annan, A.P. (2003): The early development of TDR for soil measurements. Vadose Zone Journal, 2:492–499, doi:10.2136/vzj2003.4920
Ferré, P.A. (Ty) and Topp, G.C. (2002): 3.1.3.4 Time domain reﬂectometry. In J.H. Dane and G.C. Topp (ed.), Methods of SoilAnalysis Part 4 Physical Methods, SSSA Book Series No. 5: 434–446. Soil Sci. Soc. Amer., Madison, WI.