ICT International

Time Domain Reflectometry (TDR) for soil moisture measurement

Time Domain Reflectometry (TDR) calculates volumetric soil moisture content by measuring dielectric properties. This short piece sets out to explain the fundamental science behind the measurement.

TDR is still used in the Soil Moisture Equipment Handi-Trase range of soil moisture sensors.

This piece is an excerpt from ICT227 Soil Moisture Measurement Instrumentation, which was published in the 1990s by ICT International as an application note.

TDR determines the apparent dielectric (Ka) of the soil matrix and this is empirically related to the volumetric soil moisture content. The method is quick, independent of soil type, non-destructive, suited for surface and profile measurements, and allows repeatable in situ measurement. The TDR is a portable unit that can be carried allowing point soil moisture measurements or linked to a multiplexer to measure an array of buried waveguides. The moisture content determined by the TDR is the average moisture along the length of the waveguides. Therefore, to measure at depth of 20 cm, waveguides are placed in the soil horizontally at that depth. If 30 cm waveguides are placed vertically into the soil, the moisture content determined by the TDR will be the integrated moisture content from the soil surface to a depth of 30 cm.