Performance of soil moisture sensors in gypsiferous and salt-affected soils

Accurate soil moisture measurements are key for optimising field irrigation and for the precise monitoring of water flow in soil profiles or large watersheds. In recent decades, various soil moisture sensors (SMSs) have been widely used, and several studies have been conducted to evaluate the performance of the SMS technologies under different soil and field conditions. The soluble salt content in soil can affect the sensitivity of SMSs because of their influence on soil permittivity. In this field study, we evaluated different types of SMSs in gypsiferous and salt-affected soils located at two agricultural sites in western Iraq. Two WaterScout capacitance sensors, SM100 and SMEC300, were evaluated in addition to the Watermark granular matrix sensor. Soil moisture measurements by the gravimetric method were compared with SMS readings. Bulk electrical conductivity (ECb) ranged between 3.3–8.1 and 5.6–10.3 dS m −1 for sites 1 and 2 respectively, during the experiment. The results showed that site-specific calibration of the sensors significantly increased their accuracy. At Site 1, the RMSE values for the Watermark, SM100, and SMEC300 sensors changed from 3.48, 2.68, and 1.96 before calibration to 1.25, 0.91, and 0.90, respectively. For site 2, the RMSE values changed from 3.78, 3.09, and 2.34, to 1.37, 1.03, and 0.95, respectively. After the site-specific calibration, the capacitance sensors were sufficiently accurate for scientific measurements. The granular matrix sensors should only be used for field irrigation management if a high-accuracy level is not required and only after site-specific calibration. • SMSs estimate soil permittivity as an indicator to soil moisture content. • Salinity influences the accuracy of SMSs due to the impact on dielectric properties. • In situ calibration of SMSs are important for the reliability of the measurements. • Our study focuses on performance evaluation of different SMSs in salt-affected soils.