Monitoring the transport of salts and water using time domain reflectometry.

Abstract

An automated method for the simultaneous measurement of water content, soil water pressure, and soil bulk electrical conductivity in soil columns was developed based on time domain reflectometry (TDR), three wire probes and pressure transducers. Soil electrical conductivity was calculated from the attenuated transmitted pulse, and soil moisture from the trace length and the dielectric constant. The above methodology was used for monitoring breakthrough curves at various depths in soil columns and for determining concentration and ionic composition effects on saturated hydraulic conductivity (HC). Concentration measurements at the inflow and outflow ends were made with mini flow-through cells with mini TDR probes. They were installed in horizontal orientation to intercept the solute fronts. Synthetic irrigation water was sequentially applied to study solute transport. Solute transport parameters were estimated by fitting data to a one dimensional convective dispersive equation, and comparing the results to the moment method of analysis. For moisture content and EC measurements, data acquisition at various depths was fully automated using a multiplexer and auto-relay switching system. Resulting retardation factors (R) and dispersion coefficients (D) were found close to each other for the various probes, and agreed with values obtained from the effluent concentration distributions. The constant flux method, used to determine effects of water quality on saturated hydraulic conductivity, resulted in stable values of saturated hydraulic conductivity. This method clearly showed that measurements of pressure be taken at shallow depths where changes in HC are most pronounced, and occur fast after applying different quality water to the soil.