Possibility of Measuring Water Content of Fine Textured Saline Soils by Time Domain Reflectometry

Soil water content is measured by different methods, such as oven, gamma ray, and time reflectometry methods. Direct methods are time-consuming and lead to disturbance of the soil sample, and some indirect methods have risks of radioactive materials. Among these methods, the TDR method, which is one of the reflectometry methods, is a quick and safe method, and has received attention for this reason and gives acceptable results in soils with normal conditions. However, for saline soils, this method cannot measure water content accurately. The objective of this research was to make TDR sensors that can measure the water content of saline and fine-textured soils. By making these sensors, it would be possible to measure water content of saline soils by a TDR; and if an error is observed, the actual amount of soil water content can be obtained by providing suitable calibration models. For this purpose, a fine-textured soil was obtained from Tikmehdash Research Station and by adding different amounts of salt, soil  samples with electrical conductivity of 20, 35 and 50 dS/m with natural density were prepared. Through trial and error, the type of coating suitable for covering the moisture sensor rod, which is a polymer material, was determined. Then, the length of the coating used in the middle moisture sensor rod was determined for the mentioned soil that had the prescribed salinities and water content. This length was equal to the length of the moisture sensor rod. Then, the coated sensors were tested to measure other soil water contents, and suitable calibration models were presented. Thus, by entering into the models the volumetric moisture content measured by TDR, the moisture content of fine-textured saline soils can be estimated with good accuracy.