MEASUREMENT OF VOLUMETRIC WATER-CONTENT BY TDR IN SALINE SOILS

Time-domain reflectometry (TDR) evaluates the bulk dielectric constant , K, of the soil by measuring the travel time of an electromagnetic pu lse through a sensor, and through it estimates the volumetric water co ntent. We show that for saline soils the effects of conductivity and f requency on the travel time cannot be neglected and that, as a result, TDR systematically overestimates the water content in saline soils. S imultaneously the bulk electrical conductivity of soils can be estimat ed by TDR. The equivalent impedance after multiple reflections is rela ted to the bulk electrical conductivity, sigma. This relation differs from sensor to sensor and requires calibration for each individual sen sor. A method is proposed for correcting the volumetric water content in saline soils. First, the bulk electrical conductivity, sigma, is es timated from the equivalent impedance at a specific equivalent distanc e of cable, several times the actual length of the sensor. The zero-sa linity dielectric constant, K-0, of this soil is obtained by correctin g the apparent K as a function of the measured bulk electrical conduct ivity. The volumetric water content is estimated from K-0. The correct ion of K is a function of the equivalent frequency of the electromagne tic pulse. The imaginary part of the dielectric constant is primarily due to ohmic losses. The model, which calculates the velocity of propa gation of the electromagnetic pulse and which takes into consideration the imaginary part, performs reasonably well. An empirical approach b ased on calibration gave slightly better results.