Baker, J. M. and R.J. Luscano. 1989.
The Spatial Sensitivity of Time Domain Reflectometry.
Soil Science, Vol 14, No. 5, pp. 378-384.
(posted October 8, 1996)

With any method for measuring soil water content it is useful to know the spatial sensitivity of the measurement, i.e., what volume of soil is measured and what the distribution of sensitivity is within that volume. We determined this information for the method of time domain reflectometry (TDR), using a laboratory procedure in which the distribution of water surrounding the waveguides was controlled to give different spatial arrangements of water around the waveguides. Stainless steel rods 3.2 mm in diameter and 300 mm long were used as waveguides, spaced 50 mm apart in a matrix of glass tubes. The sensitivity was found to be largely confined to a region with a cross sectional area of approximately 1000 mm^2 (20 mm by 65 mm) surrounding the waveguides, although a limited sensitivity extends much farther, enclosing 3600 mm^2 (60 mm by 80 mm). The width of the region of sensitivity normal to the plane containing the waveguides is approximately 30 mm, indicating that the method should allow excellent depth resolution when the waveguides are installed horizontally, and that it should be possible to place such waveguides within 20 mm of the surface. Finally, there was no discernible variation in sensitivity longitudinally, i.e., along the length of the waveguide. The data indicated that sensitivity ends abruptly at the ends of the waveguides, i.e., changes in water content just beyond the end of the waveguides have no discernible effect on the signal.