Assessing soil volumetric water content and salinity by Time Domain Reflectometry

This work deals with an investigation aimed to evaluate the possibility of using Time Domain Reflectometry (TDR) for simultaneous measurements of soil volumetric water content (VWC) and salinity. A preliminary test to evaluate the suitability of employing TDR for electrical conductivity measurements was carried out on solutions of different electrolyte concentration. An empirical exponential equation correlated well the amplitude of the reflected signal (VR) to the electrical conductivity of the solution. However, the TDR failed when measuring the transit time of the energy pulse in solutions of concentration higher than 40 meq l-1 (EC > 4 dS m-1). A Four-electrode Probe (4P) was employed to measure directly the bulk electrical conductivity of the soil. These values were compared with those obtained with the TDR technique by applying theoretical equations. The electrical conductivities measured by the Four-electrode Probe were in good agreement with those calculated from TDR trace when applying both Dalton's (?D) and Yanuka's (?Y) equations, and to a lesser extent applying the Topp's (?T) equation. Low values were obtained when Zegelin's (?Z) equation was applied. The best outcome was obtained applying the empirical exponential equation which approached more than others to one to one correlation. Experiments were also carried out to evaluate the effect of salinity on the measured soil volumetric water content. Our results showed that the effect of low salinity levels on water content measurements was negligible for practical purposes. However, because of the strong attenuation of the reflected signal, relatively high salinity levels make difficult to detect the reflection point, and therefore evaluate the actual transit time of the energy pulse. In all these cases, both water content and electrical conductivity cannot be determined by this technique.