Date of Award:

5-2025

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Plants, Soils, and Climate

Committee Chair(s)

Scott B. Jones

Committee

Scott B. Jones

Committee

Bruce Bugbee

Committee

Matt Yost

Committee

Wenyi Sheng

Abstract

Measuring soil water content is crucial for various applications, such as agriculture and environmental monitoring. Scientists use sensors that rely on electromagnetic waves to estimate soil water content by measuring the dielectric of media. However, factors, including the frequency of the sensor's measurements, soil particle size and distribution, and the soil's electrical conductivity, can affect the dielectric measurements and, consequently, the accuracy of the water content estimation. To improve the accuracy of electromagnetic-based sensors, it is important to calibrate them properly and understand their sensing volume to ensure the use of an appropriately sized calibration container. In this study, we determined the sensing area where the electromagnetic fields form around each of nine different electromagnetic-based sensors from various manufacturers. A custom 3 dimensions-printed housing created a seal between each sensor head and the bottom of a clear calibration column in which sensors were tested using two types of media: a standard test sand (ASTM-C778) and a baked clay aggregate. Each of these materials was uniformly packed to the height of each sensor’s end-of-rod fringing distance to fully capture the permittivity of both dry and saturated media, which serves as calibration parameters. We used two simple calibration equations based on arithmetic (linear) and refractive index (curvilinear) averaging theories. Results were dependent on each sensor’s measurement frequency and on other parameters specific to each electromagnetic-based sensor under our proposed 'wet/dry layered' test conditions. The study results highlight some limitations of the method for short-rod time-domain reflectometry sensors, where reflections at the wet/dry layering interface make it difficult to interpret the waveform analysis accurately. This research contributes to the understanding of electromagnetic-based sensor calibration and sampling volume, which can help improve the accuracy of soil water content measurements in various applications.

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