Submitted to: Transactions of the ASAE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/21/2000
Publication Date: N/A
Citation: Interpretive Summary: Automatic measurement of soil water content and salinity is important in many fields of engineering, agriculture (particularly irrigation control), resource management, and environmental monitoring. Time domain reflectometry (TDR) is increasingly used for these measurements because it can give accurate water contents in a wide range of soils and can be automated. But, computer programs for automatic control of TDR systems ma report inaccurate water contents using currently available methods. This paper describes a computer program, TACQ, that includes new methods and combinations of methods for computing water content from TDR data. The new methods ensure accurate water content determination for a wide range of soils from clay to sand, and for a wide range of soil conditions from air dry to completely wet. It is suitable for low-power, unattended field systems, as well as laboratory systems. The program controls systems of up pto 256 probes. It works with and controls TDR equipment from three manufacturers. It allows user control of the number and type of probes read, their distance from the TDR instrument, and the type of data acquired. The program has been used in an automatic irrigation system, in a remote, solar-powered crop water use measurement system, and in agriculture, resource management, and environmental monitoring work by researchers and engineers in the U.S. and several countries. The program was commercialized through a Cooperative Research and Development Agreement with Dynamax, Inc. of Houston, Texas, and is available for free download on the Internet at http://www.cprl.ars.usda.gov/programs/.
Technical Abstract: Despite the increased use of time domain reflectometry (TDR) for measurement of soil water content and bulk electrical conductivity (BEC), there are few public releases of software for TDR system control. And, even though graphical interpretation of the waveform to find pulse travel times is key to the method, the few published descriptions of computer methods are incomplete. The TACQ program, under development since the 1990s on a wide variety of soils, allows control of multiplexed systems supporting up to 256 TDR probes. Waveform interpretation methods are user-controlled, and allow interpretation using various methods reported in the literature or methods available only in TACQ. The default methods allow automatic interpretation of waveforms from a variety of media including loose, air-dry soil, and wet clay. The present study shows that interpretation methods can have a large effect on reported water contents. The effects, when additive, can result in water content change errors as large as 0.08 m**3/m**3 as the soil wets and dries, and as TDR system temperature changes over a season. Thus, the interpretation methods used should be reported in rigorous studies involving TDR water content determination, calibration, and temperature effects. The TACQ program, and documentation, may be downloaded at http://www.cprl.ars.usda.gov/programs/.