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ARS Home » Plains Area » Bushland, Texas » Conservation and Production Research Laboratory » Soil and Water Management Research » Research » Publications at this Location » Publication #374728

Research Project: Precipitation and Irrigation Management to Optimize Profits from Crop Production

Location: Soil and Water Management Research

Title: 4.3.2 Time Domain Reflectometry (TDR)

item Evett, Steven - Steve
item OR, DANI - Eth Zurich
item Schwartz, Robert

Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: 7/29/2020
Publication Date: 10/27/2020
Citation: Evett, S.R., Or, D., Schwartz, R.C. 2020. 4.3.2 Time Domain Reflectometry (TDR). In: Montzka, C., Cosh, M., Nickeson, J., Camacho, F., editors. Good Practices for Satellite Derived Land Product Validation, Land Product Validation Subgroup (WGCV/CEOS), National Aeronautics and Space Administration, Goddard Space Flight Center. Greenbelt, MD: NASA. p. 68-73.

Interpretive Summary: Irrigation management for efficient use of scarce water resources can be greatly aided by use of accurate soil water sensors. USDA ARS scientists developed accurate, low cost sensors known as TDR sensors in cooperation with a commercial partner who now provides them to agricultural producers and equipment suppliers, including irrigation equipment manufacturers. The scientists have written a guide to the best methods of using the sensors and similar TDR sensors for use in agricultural and environmental management, easing the way towards more widespread use of sensors to save water.

Technical Abstract: Time domain reflectometry (TDR) became a useful method for sensing soil volumetric water content and bulk electrical conductivity in the 1980s. Automated TDR systems were described in the 1990s. Commercial systems became available in the late 1980s from a few companies and continued to evolve with TDR instruments, probes, and multiplexers and currently are becoming common in research and some monitoring applications. TDR instruments became smaller as electronic technologies advanced, culminating by 2015 in miniaturized TDR circuits that were small and inexpensive enough to be encased in the plastic head of a TDR sensor where the circuit was directly coupled to the probe electrodes (rods), thus improving the quality of water content and electrical conductivity data. This chapter describes both conventional TDR methods and the newer directly coupled TDR sensors. It provides the theoretical basis for the method, calibration equations and impediments to use of the method. It describes equipment and software currently available as well as some legacy systems still in use. The differences between the TDR method and the so-called quasi-TDR systems are presented as well.