Time-Domain Reflectometry

The vital role of water in maintaining the life of the landscape requires the development of techniques to monitor and sustain water supply and its quality. Time-domain reflectometry (TDR) is a recently developed technology for use in soil and landscape processes, which has become widely used in highly diversified applications. Despite the importance of water and ionic solutes in the mass and energy balances of the soil profile, it has been only in the last 20 years that rapid, in situ, nondestructive measurement of soil water content and ionic solute concentration has become possible in the form of TDR. This is an electromagnetic (EM) technique using radar principles at radio frequency where estimates of water content and electrical conductivity of soil can be made separately from the same wave. The effect of the soil and water on the propagation velocity of the EM wave is analyzed to provide a reliable measure of the water content. The bulk electrical conductivity (EC) is determined from an analysis of the rate of decrease in amplitude during the propagation of the wave in soil. The principles of EM wave propagation are also used for the design of soil probes and to indicate the precision of measurements. The strong interaction between water content and electrical conductivity usually hinders the independent and separate determination of these properties in soil. This difficulty is overcome with TDR where both measurements are from the same sampled region and with the same EM wave.