Submitted to: Research Coordination Meeting
Publication Type: Abstract Only
Publication Acceptance Date: 9/8/2014
Publication Date: 9/8/2014
Citation: Evett, S.R., Schwartz, R.C. 2014. New down-hole TDR method for deep profile soil water content and bulk electrical conductivity. Research Coordination Meeting [abstract].
Technical Abstract: Comprehensive irrigation and salinity management both require accurate knowledge of field soil water content and bulk electrical conductivity to depths greater than the root zone depth in agricultural fields. Scientists at the USDA-ARS Conservation & Production Research Laboratory, Bushland, Texas, cooperated with a sensor and irrigation controller manufacturer to develop a new, down-hole device, based on time domain reflectometry (TDR), for continuous, deep monitoring of soil water content and bulk electrical conductivity. The system is called a waveguide on access tube (WOAT) system, and it consists of a 50-mm diameter stainless steel tube with plastic inserts on opposite sides that carry a stainless steel rod that acts as the TDR electrode, while the tube acts as the ground electrode. The tube is installed vertically in the soil with the outside and rods in close contact with the soil. The TDR electrodes are 20-cm long. Plastic inserts are spaced 20-cm apart along the length of the 1-m long WOAT tube, resulting in five sets of two measurements in each 20-cm depth increment along the tube. The TDR circuitry is contained in a movable insert that can be moved up and down the inside of the tube to make contact with each set of electrodes. Once contact is established, a TDR waveform is collected and analyzed to determine the soil dielectric permittivity, soil bulk electrical conductivity (EC) and soil water content for each electrode. Multiple tubes can be used together to obtain measurements below 1-m depth. The presentation will include demonstration of WOAT tube installation, movement of the TDR insert to make contact with electrodes and collect waveforms, and use of the software for data collection. A three-rod TDR sensor probe using the same TDR circuit will also be demonstrated. This probe is meant to be inserted into undisturbed soil for local measurements of permittivity, water content and bulk EC.