Submitted to: Applied Engineering in Agriculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/5/2013
Publication Date: 10/14/2013
Citation: Casanova, J.J., Schwartz, R.C., Evett, S.R., Anderson, S.K. 2013. Directly coupled vs conventional time domain reflectometry in soils. Applied Engineering in Agriculture. 29(5):771-777.
Interpretive Summary: Measuring the amount of water in soil is important in managing crop irrigation. Current methods of soil water estimation are limited by accuracy, precision, ease of installation and cost. This paper presents the results of lab tests of a new, digital, sensor design.
Technical Abstract: Time domain reflectometry (TDR), a technique for estimation of soil water, measures the travel time of an electromagnetic pulse on electrodes embedded in the soil, but has limited application in commercial agriculture due to costs, labor, and sensing depth. Conventional TDR systems have employed analog signal generators and coaxial cable for supplying the pulse and measuring the reflected waveform; however, transmission through lossy cables and the associated increase in rise times degrades the measured waveform, making interpretation difficult. This paper examines a novel design that uses a digital circuit for directly supplying the step pulse and measuring the reflected waveform, without intervening coaxial cables. Analog and digital TDR circuits using pulse rise times of 200 ps and 150 ps, respectively, were tested in sand, clay loam, and a range of electrolytic solutions in identical cylindrical access-tube geometries. Overall, both systems were capable of producing high-quality waveforms; both were equally sensitive to bulk electrical conductivity, with probe constants of about 6.35 m**1 for both systems; and both exhibited quadratic responses of travel time to water content in clay loam and sand, with greater travel times by about 0.5 ns to about 1.0 ns using the digital circuits, due in part to the faster rise time's effect on waveform interpretation. This work demonstrates the feasibility of using a small, digital TDR system to replace the cumbersome analog TDR systems currently in use.