ASSESSING TEMPORAL VARIATIONS IN SOIL-WATER COMPOSITION WITH TIME-DOMAIN REFLECTOMETRY

Time domain reflectometry (TDR) can be used to study temporal variatio ns in volumetric soil water content (theta) and bulk soil electrical c onductivity (sigma(a)), The variations in sigma(a) are associated with changes in theta and the soil water composition, Laboratory and field experiments were conducted to verify if TDR can be used to monitor th e temporal variation in the soil water composition between solution sa mpling occasions, Effects of cable length and temperature on the sigma (a) measurement were evaluated, Including the series resistance of the cable and connectors in the analysis improves measurements at high el ectrical conductivity levels, The temperature factor of the bulk soil appears to be similar to the temperature factor of soil extracts. Labo ratory experiments showed that the theoretical model giving sigma(a) a s function of theta and the electrical conductivity of the soil soluti on (sigma(w)) combined with the water retention function was capable o f describing sigma(w) measured on soil solution extracted with ceramic cup solution samplers under static water flow conditions. After optim ization of a single parameter, the model was able to describe sigma(w) values of the soil solution obtained in the laboratory, whereas liter ature values were sufficient for held data, Concentrations of a number of solutes in a field data set spanning 3 yr were positively correlat ed with sigma(w). Site-specific regressions between solute concentrati on and sigma(w) combined with automated TDR measurements of sigma(a) a nd theta enable a more meaningful interpretation of the temporal varia tion of the concentration of major solutes present in the soil solutio n between sampling occasions.