March 31, 2011 by Dr. Topp
With water conservation on almost everyone’s mind, we take for granted our ability to measure soil water content easily and inexpensively. However, on a scientific timescale, this technology is fairly new. Before Dr. Clarke Topp and his colleagues perfected the TDR method for measuring water content in soils, soil scientists and growers were limited to single measurements in time by direct sampling or by using a neutron probe.
The TDR concept was initially somewhat hard to sell. Funding agencies were dubious about the project. Other scientists accused Topp and his team of everything from falsifying data to engaging in witchcraft. But the work they ultimately achieved was one of the most successful endeavors in soil physics. According to Google Scholar, the seminal 1980 article by Topp et. al has been cited 2,178 times to date. Topp and his colleagues changed the way that we measure water in soils.
In this seminar, Dr. Topp discusses the development and success of time domain reflectometry as a means to measure volumetric water content in soils. His remarks cover:
Below are some references that Dr. Topp referenced in his discussion:
Topp et al. (1980) – first TDR paper
Topp, G.C., J.L. Davis and A.P. Annan. 1980. Electromagnetic determination of soil water content: Measurements in coaxial transmission lines. Water Resources Research. 16:574-582.
Two on measurement of EC using TDR
Yanuka, M., G.C. Topp, S. Zegelin and W.D. Zebchuk. 1988. Multiple reflection and attenuation of TDR pulses: Theoretical considerations for application to soil water. Water Resour. Res. 24:939-944.
Topp, G.C., M. Yanuka, W.D. Zebchuk and S.J. Zegelin. 1988. The determination of electrical conductivity using TDR: Soil and water experiments in coaxial lines. Water Resour. Res. 24:945-952.
Linearized Topp et al. equation
Topp, G.C. and W.D. Reynolds. 1998. Time domain reflectometry: a seminal technique for measuring mass and energy in soil. Soil & Tillage Res. 47:125-132.
Showing the impact of real and imaginary component of relative permittivity (dielectric constant)
Topp, G.C., S.J. Zegelin and I. White. 2000. Impacts of the real and imaginary components of relative permittivity on TDR measurements in soils. Soil Sci. Soc. Amer. J. 64:1244-1252.
Load cell performance for penetrometer
Adams, B.A. G. St-Amour and G.C. Topp. 2000. Evaluation of a piezoelectric load cell for use on cone penetrometers. J. Agric. Engng Res. 76:205-210.
Topp, Davis & Annan on TDR early days
Topp, G.C., J.L. Davis and A.P. Annan. 2003. The early development of TDR for soil measurements. Vadose Zone Journal 2:492-499.
Topp G.C., D.R. Lapen, M.J. Edwards and G.D. Young. 2003. Laboratory calibration, in-field validation and use of a soil penetrometer measuring cone resistance and water content. Vadose Zone Journal 2:633-641.
Knife blade TDR probe
Lapen, D.R., Topp, G.C., Bugden, J.L. and Pattey, E. 2004. A new TDR probe for evaluating airborne SAR data for soil water content estimates. In Baoji Wang, Quanzhong Huang, Qing Li, Jianhan Lin, Yu Chen, Feng Mei, Qing Wei, and Jieqiang Zhuo. Proceedings of the 2004 CIGR International Conference, (Agricultural Engineering), Bejing, China. Oct. 11-14. (Published on CD-ROM).
Pentrometer and shearlogger as given in the Canadian Soil Measurement Methods
Topp, G.C. and D.R. Lapen. 2007. Field Soil Strength. Ch. 60, In M.R. Carter and E.G. Gregorich, Eds., Soil Sampling and Methods of Analysis, 2nd Ed., Lewis Publ. of CRC Press, Boca Raton, FL, 783-801.