SALINE CLAYEY SOIL MOISTURE MEASUREMENT USING TIME-DOMAIN REFLECTOMETRY
Laboratory experiments were conducted to explore the effects of bound water and soil electrical conductivity(EC) on soil moisture determination using time-domain reflectometry(TDR) in saline clayey soil. The elevated soil EC increases both the rise time of the reflected signal and the total time delay, resulting in overestimation of soil moisture. The bound water in clayey soil reduces the time delay because of its lower dielectric constant, resulting in underestimation of soil water content. There is a turning point moisture, at which, the effect of bound water is balanced by the effect of soil electrical conductivity. Below this point bound water effect dominates and TDR underestimates soil moisture. Above this point the effect of soil EC dominates and TDR overestimates soil moisture. This turning point moisture decreases as the soil salinity increases. The linear calibration (T/Tair vs. volumetric water where T/Tair is the ratio of propagation time in soil to that in air over the same distance) is no longer valid for saline clayey soil. A logarithm calibration has been developed and this relation includes soil EC as a parameter. It is also possible to estimate the amount of bound water in a soil sample from the turning point moisture.
Sun, Z.J.*, & G.D. Young
E.S.I. Environmental Sensors Inc.
Victoria, British Columbia, Canada.