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a Eastern Cereal & Oilseed Research Centre, Agriculture & Agri-Food Canada, 960 Carling Avenue, Ottawa, ON, Canada K1A 0C6
b Terad, Mississauga, ON, Canada
c Sensors and Software Inc., Mississauga, ON, Canada
* Corresponding author (toppc{at}agr.gc.ca).
Received 23 April 2003.
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ABSTRACT |
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 TOP ABSTRACT BACKGROUNDREACTION TO THE FIRST...TDR INSTRUMENTATION SPECIFICALLY...REACHING OUT WITH TDR...TDR COMES OF AGE...IN RETROSPECTREFERENCES |
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Abbreviations: CPN, Campbell Pacific Nuclear • FI, Foundation Instruments • GPR, ground-penetrating radar • GSC, Geological Survey of Canada • IRAMS, instrument for reflectometry analysis of moisture in soil • R&D, research and development • RF, radio frequency • RFP, request for proposal • SPRI, Scott Polar Research Institute • TDR, time-domain reflectometry
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BACKGROUND |
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TOPABSTRACTBACKGROUNDREACTION TO THE FIRST...TDR INSTRUMENTATION SPECIFICALLY...REACHING OUT WITH TDR...TDR COMES OF AGE...IN RETROSPECTREFERENCES |
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There are many cases where collaboration between professionals with different academic backgrounds makes the R&D of complex methodology succeed. There is indeed great value in professionals in different fields interacting and sharing ideas, but more significantly, the personal chemistry of the individuals must be right. There is no formula to define the right personal chemistry between individuals, but fortunately it does occur. Even though these important personal and political aspects can make or break a project, they are rarely mentioned in the scientific literature. In this particular TDR R&D project, the personal chemistry was excellent. We offer no scientific explanation of why, but only some of the ingredients and events which we experienced.
As the core players in the early days of applying TDR for measurement of soil water content, we came together from two directions; Peter Annan and Les Davis, then at the GSC from geophysics, and Clarke Topp of Agriculture Canada from soil physics.
From his days at the University of Wisconsin, Clarke had received inspiration and leadership from Ed Miller and Champ Tanner in the importance of measurement but also in the desire to dig into the physical basis of instrument operation, including building components and/or complete instruments from scratch. So he brought to Agriculture Canada a strong interest in improving methods of measurement in soil and water. He had used 
ray attenuation for laboratory measurement of water content and was incessantly looking for better methods for field measurements.
Peter was a research student at the University of Toronto, in the geophysics department under David Strangway. The department was jointly involved with MIT in NASA's Surface Electrical Properties experiment that was carried to the moon in 1972 as part of the Apollo 17 mission. They developed a system using radio frequencies to determine the near surface electrical properties of the moon. The space program was very exciting for many young minds in the 1960s. It is interesting to note that most of the students involved with this project at the University of Toronto continued to work on high frequency measurements in geological materials, and some remain leaders in ground-penetrating radar (GPR) methodology today. Peter is now one of the foremost scholars on electromagnetic theory.
Les was a student at the Scott Polar Research Institute (SPRI) at the University of Cambridge, working on the challenge of using radio waves to sound temperate glaciers. Most of the other physical scientists at SPRI were involved with airborne sounding of cold polar ice in the Antarctic. Les worked on sounding temperate ice, where ice and water are in equilibrium. The electrical properties of ice and water are very different and the problem is somewhat analogous to using visible light for looking through fog. There were exciting and new discoveries about the properties of glacier ice and many of the world's foremost explorers would visit and give seminars at SPRI, making it an exciting scholastic environment.
Les' first task at the GSC was to evaluate a prototype GPR for sounding geological materials. He read many papers and reports written in the 1960s about the use of radio frequencies to penetrate geological materials. Generally, the literature was not encouraging. It was clear to him that an independent method was required to measure the electrical properties at the same frequencies as for GPR in geological materials in situ. He noted that TDR was being used on coaxial transmission lines filled with soils in the laboratory (Hoekstra and Delaney, 1974). Coaxial transmission lines were not going to be practical for measurements in the field, but Les thought, why not use parallel transmission lines consisting of two parallel rods placed in the soil?
Len Collett, the head of the Electrical Methods Section of the Geophysics and Geochemistry Division at the GSC, kindly let Les test his idea. Len introduced him to Walter Chudobiak, at the Communications Research Centre, Department of Defense, in Ottawa. The center had all sorts of excellent instrumentation and two commercial laboratory TDR instruments. Les spent many happy months playing in a 1-m3 sand box. Different rod lengths and diameters comprising the parallel transmission line were tested in local sandy soil at different water contents. The electronic engineers in the building couldn't believe that Les could call his playing in sand with expensive test equipment work. The TDR method really did seem to offer excellent potential for measuring the radio-signal velocities, and thus the real part of the relative permittivity, in dry and wet sandy soils (Davis and Chudobiak, 1975).
Bill Scott of the GSC and Les tested the TDR along with the GPR in the Mackenzie River delta near Tuktoyaktuk, NWT, Canada. They used GPR to characterize the active layer (seasonal melt zone) in permafrost. At that time, they were already using vertically-oriented TDR rods drilled into the frozen clay till as spot-truthing for their GPR transects. Selectively located rods were used to measure electromagnetic wave propagation velocity from TDR, which allowed the calculation of depth to the melt-frozen interface from the GPR reflection patterns. Testing of both the GPR and TDR continued after Peter Annan joined the GSC in 1974.
It was very clear to Peter and Les that the TDR and GPR offered a practical method for measuring soil water content for agricultural applications. They also reasoned that an independent measure of soil water content would eliminate the need for the labor-intensive TDR spot truthing. They knew little about soils, but Len Collett suggested that they should contact Clarke Topp at Agriculture Canada. This sounded like a good idea and Les met Clarke, naively thinking that he would welcome the concept.
The Meeting of the TDR Trio
Unknown to Les at that time, Clarke had been involved with colleagues in Agriculture Canada who were promising, in his view, excessive possibilities to solve a myriad of agricultural problems using remote sensing. Clarke was not very pleased to meet someone who seemed to him to be yet another person involved with remote sensing. After a cool but polite introduction, Clarke could see that Les was not selling the concept but worse, he was really looking for assistance and cooperation for developing an unheard of method for measuring soil water content. Clarke was interested to hear that the method potentially measured the water content on a volume basis as that was more directly applicable in transport equations. Showing some patience and posing probing questions for Les to ponder, Clarke suggested that he and Les should discuss the concept further. This turned out to be the beginning of a long-lasting friendship and mutually beneficial professional alliance.
A short time later, at a meeting of all three, it was recognized that each of us had unique and vital expertise for the R&D concept to develop. Peter's solid understanding of the principals of electromagnetic theory plus his ability to apply the theory in the real world was essential. Les offered vital practical experience with use of radio-frequency methods. Clarke offered an understanding of soil physics and the physical behavior of water in soil. Clarke was intrigued with some of the early work that Peter and Les had done with GPR and TDR in the field (Annan and Davis, 1976; Davis and Annan, 1977) as was J. S. Clark, a physical chemist and director of Clarke's Institute at Agriculture Canada, who agreed that Clarke should work on testing the TDR concept for measuring soil water content in soils.
Collaboration among Les, Peter, and Clarke commenced. The first problem was to determine how the electrical properties varied with water content in different soils and at different temperatures. We designed coaxial transmission lines 1 m long by 50 mm in diam. with a series of 10-mm-diam. porous ceramic tensiometer cups along two sides for adding and removing water without disturbing the soil. Affectionately named the pig, this configuration allowed both water content changes and repeated TDR measurement on the same soil. Clarke and Les spent many long days that turned into weeks and then months using these lines to measure the TDR signal travel times in different soils. Then measurements were made on other materials like glass beads and vermiculite to determine how robust was the relationship between the relative permittivity and the volumetric water content. At one point during the summer months, when Les and Clarke were taking turns going into the laboratory at nights and on weekends, Clarke, feeling exhausted, said to Les that, "this working day and night is killing me." Les responded that he was just desperately trying to keep up with Clarke. They both laughed, realizing that the excitement of the project results had entrapped them and from then on they tried to enjoy some of the remainder of the summer with their families. Walter Zebchuk, Clarke's technician, was always available with a capable helping hand and additional much appreciated good humor. In spite of the grueling work pace, these were extraordinarily exciting times for the team, especially as we watched how well the data were falling into place.
Clarke and Les spent hours discussing the data, trying to account for differences in their data sets. Peter, while not involved in the day-to-day measurements, was brought in to add ideas and suggestions from a broader perspective. Considerations ranged from the molecular properties of the bound water on the surface of the soil grain, to the difference in the surface area of the different soil types, to the problem of how to make rapid measurement across large areas. We considered numerous questions, some of which have been answered by others since those days, and others that are still waiting for satisfactory answers. Working together inspired each of us to try harder, and this part of the project was one of the best times in our careers.
Although our collaboration was excellent on personal and scientific bases, the progress was not sufficiently evident for others. Very soon it received variable and sometimes minimal support from the management levels in both the GSC and Agriculture Canada. Each of us was allowed to work only part-time on the development of TDR, which meant that much was done on our own time and often in spite of our supervisors, because, as it was determined, TDR did not fit directly the mandate of either department.
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REACTION TO THE FIRST TDR DATA |
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TOPABSTRACTBACKGROUNDREACTION TO THE FIRST...TDR INSTRUMENTATION SPECIFICALLY...REACHING OUT WITH TDR...TDR COMES OF AGE...IN RETROSPECTREFERENCES |
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Our sensing was actually much less remote. We also had great confidence in our data but realized that the soundness of our experiments was not being accepted by our peers. The impact of this experience caused us to add more soils and conditions to our experimental approach. Soil having more than 0.5 g g-1 clay content, wetting sandy loam soil with 0.002 g g-1 NaCl solution, and an organic soil were part of the expansion of our experiment. As our data came together in the late 1970s, a very consistent pattern was emerging for the relationship between relative permittivity and water content and we prepared an extensive paper eventually to be published in Water Resources Research (Topp et al., 1980).
In the 1979 spring meeting of the AGU in Baltimore, Clarke presented the core findings from the Topp et al. (1980) paper to the Hydrology Section. Keeping the presentation within the requested 12 min was a mistake, as this allowed comments from the same team as had criticized the 1977 paper. This time the bold accusation was made that our data were fabricated, as it was well-known that sandy loam and clay soils would not exhibit the similar permittivity vs. water content relationships Clarke had just presented. Needless to say, that was a very deflating experience. Undaunted, we agreed not to be deterred and carried on.
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TDR INSTRUMENTATION SPECIFICALLY FOR SOIL |
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TOPABSTRACTBACKGROUNDREACTION TO THE FIRST...TDR INSTRUMENTATION SPECIFICALLY...REACHING OUT WITH TDR...TDR COMES OF AGE...IN RETROSPECTREFERENCES |
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In 1981, after nearly 2 yr of lobbying by Clarke for support, Agriculture Canada started collaboration with Foundation Instruments (FI) of Ottawa to develop a TDR instrument. The funding was established on the basis of sharing equally between the Canadian government and the company at $275000 (Canadian dollars). Clarke's role was to serve as technical advisor and scientific authority to assure that a working prototype was completed within 2 yr, including field evaluations. The third year of contract was to incorporate needed improvements and develop a marketable version. This brought many new learning experiences for Clarke, as colleagues Les and Peter had moved from the Geological Survey to pursue developments of GPR elsewhere. This parting of the ways was an extremely sad one for the three of us.
Foundation Instruments personnel were composed primarily of electrical engineers with specialties in telecommunication and instrumentation, seemingly appropriate for their task. An early problem was that soil, a lossy or conductive dielectric, was not part of their educational or work experience. As a result, Clarke was expected to provide details of the electrical properties of the soils that might be encountered by this instrument so that the FI engineers could get values for the parameters to complete their design analyses. The details of the soil probe then became simultaneously both an electrical challenge and a soil mechanical problem, adding additional complexity which FI was unable to handle with ease. Clarke's research program had to include soil probe developments in order that the FI instrument would be able to make measurements in soil. Furthermore, FI expected that Clarke would be their source of market survey and assessment for our intended instrument. Market economic analysis was not part of his background! Clarke was, however, able to get the assistance of an agricultural economist from within Agriculture Canada to make limited market potential estimates, which looked very promising. Our experience has been that the R&D aspect of developing a new technology is relatively simple compared with getting the technology manufactured and used by the scientific and engineering community.
As often happens with small Canadian companies, FI lacked adequate venture capital and was unable to complete the instrument development until a year later when they produced the IRAMS (instrument for reflectometry analysis of moisture in soil). The initial field evaluations were to have been a joint involvement of Agriculture Canada (i.e., Clarke's laboratory) and FI. However, the shortage of funds caused the fieldwork to revert largely back on his group which was another cause for delay. This was yet one more challenge as Clarke was not supposed to be working on TDR R&D now that FI had been given a contract. It had been Clarke's hope that at least one person at FI would get sufficient field experience using the instrument so that FI would be better equipped to handle future customer complaints about the IRAMS when marketed. Effectively, Clarke became the troubleshooter for FI with IRAMS customers, particularly if the problem was soil related.
Having been a long-time purchaser of soil instruments, Clarke was a little better equipped to suggest a marketing company for the IRAMS. Soilmoisture Equipment Corp. (Santa Barbara, CA) became the marketing company outside Canada. More detail on the early instrument development and marketing of TDR instruments as experienced by Soilmoisture Equipment Corp. is given by Skaling (1992). The initial experiences with the IRAMS were generally positive. However, IRAMS needed additional development and improvements, which FI was slow to undertake. Very soon after completing the instrumentation development, FI was taken over by a larger company who forced a shutdown of their TDR work. It was Clarke's hope that FI would transfer their know-how to Soilmoisture Equipment Corp. who had been their sales outlet for IRAMS. Unfortunately, a most regrettable scenario occurred when subsequently FI sold the production rights to Campbell Pacific Nuclear (CPN).
Clarke anticipated and advised FI that CPN's potential interest was in removing TDR from the market as there was increasing evidence that TDR was developing into a market force that would offer challenge to neutron devices. This advice was ignored and shortly Clarke was receiving calls from IRAMS users who could not get adequate help from CPN for repairs or operational problems of IRAMS. Personnel at CPN were soon in contact with Clarke seeking his counsel, ostensibly to implement a TDR development program building on the IRAMS background. It became obvious to Clarke that they had no persons with knowledge and expertise in high frequency electromagnetic technology, and significantly had no genuine intention to add such personnel. By Clarke getting involved, even to a very limited extent with CPN, he had allowed himself and TDR technology to be held hostage: an example that one must be very conscious of the politics in the community. This forced other companies such as Soilmoisture Equipment Corp. and Campbell Scientific, both of whom were making independent TDR developments, to avoid consulting with Clarke as he was seen as being involved with a competitor company. In essence, this initial attempt to take government-sponsored research on TDR into the market ended in frustration and a sense of considerable waste of resources.
DEVELOPING THE CAPABILITY TO MEASURE WATER CONTENT PROFILES
When we began our TDR work, we wanted improved capability to spot proof GPR, so it was the capability to measure water content profiles that was most important. Profile measurements were also useful for agriculture, of course. We started in the laboratory in an attempt to refine the capability for profiling. A large cylindrical soil column (0.75-m diam. by 1.3 m high) was packed by hand and a variety of lengths and types of parallel pair probes were inserted. Clarke's soil colleagues did laboratory work on soil samples but few had seen such a large soil sample as a reconstruction of the field. This led to numerous derogatory comments such as, "Were all soil physicists so afraid to go to the field that they had to bring the field into the laboratory?" or "Shouldn't the soil physics lab be in the basement so there won't be such a mess when the soil tank crashes through the two floors below?" In spite of the skeptics, we demonstrated that there were capabilities for TDR profiling of water content up to 1.2-m depths (Topp et al., 1982a, 1982b).
Thanks to Peter's insight, we were encouraged to consider how detrimental were air gaps around TDR rod installations (Annan, 1977; Topp et al., 1982b). One of the more important findings from our soil tank was that averaging along TDR probes was linearly weighted (Topp et al., 1982a). This confirmed again the high potential of TDR for analyzing wetting fronts from rainfall or irrigation and for field water balance studies.
In spite of the comments, the field was not the implied fearsome prospect and our first major long-term installation was initiated in 1979 at three sites having loam, clay loam, and clay textures. A recording rain gauge was added at one site for rainfall comparisons. The TDR probes were vertically installed parallel pairs of lengths to provide water content profiles by soil horizon. Although our intent was to evaluate the capability of TDR for hydrological balance, it was not then achievable. We did not have the capability to collect data sufficiently frequently in time. There were no data loggers for TDR, and our people power was stretched to the limit to collect and process weekly measurements by the methods depicted in Fig. 1. Although little publishable data resulted from these earliest field efforts, we learned a great deal about TDR for field application (Topp and Davis, 1981, 1985).
Vertically oriented TDR rods had three potential problems. First, rods left over winter were subject to frost heave which lifted the rods by up to 10 cm. Second, variably sized air gaps can develop around the upper ends of the rods, resulting in either underestimation of water content or unrepresentative water contents when these openings act as channels for preferential flow. Finally, the rods could cause plains of weakness that resulted in shrinkage cracks passing between the rods. This last phenomenon was not observed when the soil supported vegetation.
Two approaches were adopted to alleviate these problems. From test pits, we installed probes below the surface with a horizontal orientation in the mid-depth of each soil horizon. Rods were also installed at 45° from vertical, requiring installation lengths 1.4 times the length of the intended vertical depth. Although the rods at 45° resulted in better data than from either vertical or horizontal rods, they were difficult to install, and the greater length of rods resulted in increased signal attenuation. The result is that few people, including us, now use this orientation. The comparison of performance of vertical and horizontal installations was presented in Topp (1987) and Topp et al. (1983).
One of the most exciting findings from these field sites, although not judged so by others, was the observation of wetting fronts from rainfall and their stability for more than a week's duration. Prolonged dry periods followed by significant rainfall events are not frequent in Ottawa. August 1980 was relatively dry, and after our measurement of 15 September total rainfalls of 35.9 mm occurred before the next measurement of 23 September. Our weekly readings of 23 and 29 September showed TDR traces from four sets of probes where wetting fronts were distinct and at a depth of 200 mm. In a week they had moved downward only 15 mm but had not redistributed down the profile (Topp et al., 1983).
Later that year, a call came for papers for a special conference on infiltration, sponsored by the American Society of Agricultural Engineers. Clarke was convinced that our wetting front findings and associated water content profiles demonstrated for the first time a measurement technique having high potential to account for infiltration by measuring the change in soil water content. A paper was prepared and presented with enthusiasm for the 1983 conference (Topp et al., 1983). The lack of reaction to our paper, even when solicited from private discussion with personal acquaintances, was extremely disappointing. It seemed that few understood the paper and in fact, one offered that the paper did not pertain to an infiltration conference. How humiliating!
Some years later, Clarke was delighted to discover that for Ian White (Environmental Mechanics Div., CSIRO, Canberra, Australia) and Brent Clothier (Division of Horticulture, Palmerston North, New Zealand) who were in the audience, it was this paper that oriented them toward TDR and formed a large part of their discussion as they returned from North America to Australia. It is noteworthy that Brent Clothier purchased the first IRAMS instrument from outside Canada, and it was Ian White who encouraged Steve Zegelin (also of CSIRO) to visit Ottawa for hands-on learning of TDR in 1985.
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REACHING OUT WITH TDR INFORMATION |
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TOPABSTRACTBACKGROUNDREACTION TO THE FIRST...TDR INSTRUMENTATION SPECIFICALLY...REACHING OUT WITH TDR...TDR COMES OF AGE...IN RETROSPECTREFERENCES |
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Liquid Water in Frozen Soil
Starting in the mid-1970s, Clarke was teaching an undergraduate soil physics course in the Geography Department at Carleton University in Ottawa, where he introduced students to TDR. One of the students, Danny Patterson, used TDR in his Master's thesis investigation of the temperature dependence of the liquidity of water in frozen soil. His investigation showed that the apparent relative permittivity determined by TDR was an excellent measure of the liquid water in frozen soil (Patterson and Smith, 1981). This was a very significant finding among applications for TDR and has been refined considerably since. Regrettably, this collaboration with Carleton University was soured and finally terminated because of overly exuberant and unsubstantiated claims about their findings. It is always unfortunate when a collaborator tries to take advantage of colleagues, resulting in a major loss of time and energy for everybody.
Measuring Electrical Conductivity
From the SSSA 1982 convention in Anaheim, Frank Dalton invited Clarke to accompany him to his facilities in Riverside, CA, to discuss the TDR work that he (U.S. Salinity Lab., Riverside, CA) and Bill Herkelrath (USGS, Menlo Park, CA) were doing. Our three-way discussions were, in Clarke's opinion, mutually supportive and within months Frank and Bill were writing a seminal paper indicating how easily it was to estimate bulk electrical conductivity as well as water content from the TDR waveform (Dalton et al., 1984). This was a very innovative addition to the variety of applications for TDR. From the Riverside labs, TDR was later introduced into Israel when Sam Dasberg did a sabbatical there.
In 1985, Steve Zegelin from CSIRO, Australia, and Moti Yanuka from Israel came together in Clarke's lab in Ottawa. Under arid climates in Israel and Australia, the presence of salt is a significant factor in soil water studies. Therefore, it seemed that our investigations should include more about TDR and electrical conductivity. When we were unable to use the Dalton et al. (1984) approach to estimate the conductivity of solutions, we were challenged to find out why. We went more deeply into the processes accounting for wave propagation and reflections, which resulted in our multiple reflections paper (Yanuka et al., 1988) and our alternative approach to estimating bulk electrical conductivity from the first return reflection (Topp et al., 1988). Our approach using the first return reflection does not give a correct estimate of conductivity, but the Giese and Tiemann (1975) approach, incorporating data after all reflections have occurred, gives good conductivity values and has become the accepted method.
When Gary Kachanoski arrived at the University of Guelph in the mid-1980s, the department had a TDR cable tester but no one was using it. Gary recognized the high potential of TDR and sought Clarke's counsel on how to get started. With little more than encouragement, some balun transformers, and cables from Clarke's lab, Gary and colleagues unleashed a major innovative application in solute transfer experiments using the dual capability of TDR to measure both water content and conductivity (Kachanoski et al., 1992). On hearing Gary's paper on solute transport in 1990, Dick Green (University of Hawaii) speculated with Clarke that the solute transfer application for TDR would eclipse, in significance, the water content advances of the 1980s decade.
Multiplexing and Data Logging to Allow Field Water Balances
Our field installations in the early 1980s were not adequate for achieving water balance analyses. The main limitations were the number of probes or spatial replication and the frequency of measurement with which we could monitor water content. For field data recording, we started with Polaroid photographs of the waveform on the oscilloscope screen of the cable tester (Fig. 1). As implied above, manual graphical analysis of the photograph was used to convert the image detail to relative permittivity and thus to water content (one example in Fig 1b has relative permittivity and water content written on the photo).
It is worth remembering that PC spreadsheets had not yet replaced paper sheets and the hand calculator. Therefore, a reasonable data set required a large number of photographs, so at times during field season, Clarke recruited many volunteers, including his children, to assist in the data assembly. The labor-intensive data gathering of these early years limited the kind and number of experiments that could be undertaken. The 1980s saw more automated data logging, initially with manually controlled analog devices. The advent of the digital cable tester and the general development of digital instrumentation-facilitated TDR data logging from the late 1980s onward. Thus, the frequency-of-measurement limitation was overcome by general instrument developments, whereas the spatial replication was dependent on multiplexing signals of high radio frequency (RF).
Multiple RF switches were not consumer items, which meant those with acceptable impedance and low loss characteristics were costly. These devices were considered to be necessary to reduce unwanted reflections, noise, and maximize the TDR signal amplitude in the transmission lines. Initially, the instrument operator served as a human multiplexer. On panels in the field, we had varying numbers of sockets, each connected by shielded parallel-pair TV antenna cable to a parallel-pair of rods as a probe in the soil (Topp et al., 1983). Multiplexing was accomplished by sequentially plugging the connector from the cable tester into each socket pair. Van Wesenbeeck and Kachanoski (1988) had one of the largest manual multiplex panels where they interrogated 100 TDR probes and theirs was also one of the first studies to show that TDR water content change would agree quantitatively with recorded rainfall.
Bill Herkelrath (USGS, Menlo Park, CA) discovered that multipole wafer rotary switches were effective for switching TDR probes, as the spacing between adjacent wafers resulted in the same impedance as parallel-pair shielded TV antenna cable. Our effort with rotary switches went only as far as manual switching for TDR probes in greenhouse studies. Bill Herkelrath and colleagues, however, used stepping motors to operate the rotary switches and they were successful in monitoring soil water contents in a forested site for a year (Herkelrath et al., 1991).
When Steve Zegelin returned to Australia after time spent both in Ottawa and with Bill Hekelrath in Menlo Park, he was soon to develop an electronically operated multiplexer for TDR along with a PC-based data logging capability (Zegelin et al., 1989, 1992). With this equipment, Steve and his colleagues were able to record the locus and timing of the water uptake from the root zone.
These three approaches, manual, mechanical, and electronic, show TDR development for soil use was pushing against frontiers of electronics to be used effectively for hydrological balance studies. Electronic multiplexers were being developed by several companies and soon the obstacle to effective water balance measurement from TDR was removed.
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TDR COMES OF AGE UNDER SOME SKEPTISM |
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TOPABSTRACTBACKGROUNDREACTION TO THE FIRST...TDR INSTRUMENTATION SPECIFICALLY...REACHING OUT WITH TDR...TDR COMES OF AGE...IN RETROSPECTREFERENCES |
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Later on that day, Clarke was honored to be introduced as the Grandfather of TDR to present his paper (Topp, 1987). This was the first of five papers presented on TDR from four different countries—two from the USA, and one each from Israel, Poland, and Canada. This conference had three indicators of TDR having come of age: (i) Not yet had a single conference been the venue for five papers on TDR. (ii) The dominance of non-U.S. papers at a U.S.-based conference was an indication of how truly international was the development of TDR. (iii) In attendance at the conference were representatives from a large number of companies, including Tektronix, who were closely following the progress of TDR applications. This was the beginning of much wider diversification of TDR applications in earth materials. It was very uplifting for Clarke to experience the burgeoning enthusiasm for the future development of a technique, which continues to offer its challenges and promises for better measurement.
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IN RETROSPECT |
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TOPABSTRACTBACKGROUNDREACTION TO THE FIRST...TDR INSTRUMENTATION SPECIFICALLY...REACHING OUT WITH TDR...TDR COMES OF AGE...IN RETROSPECTREFERENCES |
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The personal chemistry of those working on a project must be right but it is also imperative that all involved work as a team. Each person must adhere to the philosophy that the project will be more successful with a team of experts working together rather than independently. Reliable methods of measurement remain a primary cornerstone of sound science, but in the beginning it was difficult to get sufficient research funding and support for the in-depth foundational research that was required to bring TDR through those early growth stages. A general lack of support for methodology research all too often leads to less-than-ideal research for developing new methods. The TDR experience can serve as an example to encourage perseverance in favor of enhanced methodology for its own sake and not as a side-line to other problems. In spite of the many nontechnical challenges, the early development of the TDR method for measuring water content was one of our most exciting and rewarding projects.
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ACKNOWLEDGMENTS |
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REFERENCES |
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TOPABSTRACTBACKGROUNDREACTION TO THE FIRST...TDR INSTRUMENTATION SPECIFICALLY...REACHING OUT WITH TDR...TDR COMES OF AGE...IN RETROSPECTREFERENCES |
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