Time domain reflectometry (TDR) technology can be used to measure soil
water content, but due to an incomplete understanding of this technol
ogy, some scientists are still reluctant to adopt it. The objectives o
f this paper are to discuss the basic principles of TDR to measure soi
l water content, to present the equations to convert TDR screen readin
gs to soil water content values, and to describe a custom built TDR sy
stem. Commercial cable testers operate under the assumption the dielec
tric constant of the medium is known. For soils applications the diele
ctric constant varies with soil water content and is unknown. The equa
tions which govern the TDR technology presented in logical order allow
the user to understand the relationships between wave velocity, diele
ctric constant, and soil water content. The custom-built TDR system de
scribed herr uses a commercially available cable tester; a balun trans
former, shielded, two-wire transmission cable; and wave guides constru
cted from stainless steel welding rod. Several of these systems have b
een successfully used for six years.