Design of a new TDR probe to measure water content and electrical conductivity in highly saline soils

The inappropriate irrigation is accelerating the soil salinization in western irrigation districts of China. The amelioration of salinized land must be based on large amount of water content and salinity data. Plastic coated TDR has been designed to measure water content accurately in highly saline soil, but the soil bulk electrical conductivity cannot be measured due to the coated materials. In order to measure the volumetric water content and bulk electrical conductivity in highly saline soils at the same time, a parallel three-wire TDR probe with central rod coated which was used to measure water content and a triangle three-wire TDR probe which was used to measure electrical conductivity were integrated in one probe with four rods and one slide switch. The influence of angle in triangle three-wire TDR probe and the non-working rod on water content or electrical conductivity measurement were fully discussed through HFSS simulation and NaCl solution test. In the soil column experiment, four levels of salinity, 0.2, 0.4, 0.6, and 1.0% were set, the soil water content decreased from 30% in mass through the evaporation and measured by TDR and electronic balance. Then the probe was calibrated by model of Topp and Evett with these data. The results show that probe has the largest EPA (polarization degree index) in angle from 97A degrees to 138A degrees; the non-working rod will enhance the EPA during this angle range and the four-wire probe with angle of 120A degrees is optimal; the PVC is a better insulated material which can improve the effective salinity scope; the Evett model could improve the water content measurement greatly especially in soil with higher salinity. This new four-wire insulated probe can be applied as a beneficial use to monitor the moisture and electrical conductivity in highly saline soils.