A Coated Helical Transmission Line Time Domain Transmission Sensor for Measuring Water Content in Saline Soils

Soil salinity is a major challenge for soil moisture measurement using time domain reflectometry (TDR) and time domain transmission (TDT) technologies. Both technologies measure the travel time of an electromagnetic pulse propagating in soil, which is highly related to soil wetness. A TDT soil moisture sensor with coated helical transmission line has been developed and tested in nonsaline mineral soils and saline soils to evaluate its performance. For nonsaline mineral soils, a linear calibration equation between volumetric soil water content (θ v ) and output voltage ( V ) from the TDT soil moisture sensor was established, and soil texture showed minor influences on the linear relationship. The RMSE of the measured θ v was smaller than <0.022 m 3 m −3 Under saline conditions, the slope of the θ v – V linear relationship decreased exponentially as soil solution electrical conductivity (EC) increased. When the EC is <6.6 dS m −1 , however, the linear relationship for nonsaline mineral soils can be directly applied to saline soils if the required accuracy of moisture measurement is no less than 0.030 m 3 m −3 For soil solution EC in the range of 6.6 to 20.6 dS m −1 , the linearity of the relationship still holds, but the slope needs to be adjusted according to the EC values. The θ v – V linear relationship held for soil solution EC up to 198.2 dS m −1 The calibration equation for nonsaline mineral soils was also applicable to several natural saline mineral soils, and the RMSE was <0.026 m 3 m −3 Therefore it is possible to develop TDT sensors to measure soil moisture with similar accuracy to that of TDR on nonsaline soils but better performance under saline soil conditions.