Abstract
Time domain reflectometry (TDR) probes are increasingly used for field estimation of soil water content. The objective of this study was to evaluate the accuracy of the multilevel TDR probe under field conditions. For this purpose, eight such TDR probes were installed in small plots that were seeded with beans and sorghum. Data collection from the probes was such that soil moisture readings were automated and logged using a standalone field unit. Neutron probe measurements were used to calibrate the TDR probes. Soil-probe contact and soil compaction were critical to the accuracy of the TDR, especially when a number of TDR probes are combined for a single calibration curve. If each probe is calibrated individually, approximate measurement errors were between 0.005 and 0.015 m³ m-3. However, measurement errors doubled to approximately 0.025 to 0.03 m³ m-3, when TDR probes were combined to yield a single calibration curve.
Highlights
The application of time domain reflectometry (TDR) to measure in situ volumetric water content has been widely accepted
For field estimation of soil water content, TDR and other dielectric techniques are often preferred over neutron probe measurements, due to its automation capabilities and the fact that regulations and special handling requirements that come with the use of radioactive sources are not required
We encountered larger uncertainty in the data than usual, for the 0-30 cm depth interval, which we suggest is caused by local soil water content variations caused by root water uptake
Summary
The application of time domain reflectometry (TDR) to measure in situ volumetric water content has been widely accepted. Since its introduction in the early 1980’s (Topp et al, 1980), TDR is among the most utilized soil moisture measurement techniques in both laboratory and field. Advantages of TDR include its small measurement volume, its relative ease of operation, and its ability to be automated and multiplexed, when compared with neutron probe or gravimetric techniques. For field estimation of soil water content, TDR and other dielectric techniques are often preferred over neutron probe measurements, due to its automation capabilities and the fact that regulations and special handling requirements that come with the use of radioactive sources are not required. Through TDR multiplexing, unattended and frequent water content measurements can be made with many probes sequentially, thereby allowing the measurement of temporal and spatial soil water content changes
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