Performance of Optical Sensors for Soil Water Content Measurement under Salinity of Irrigation Water Conditions

The use of appropriate and reliable soil moisture sensors can greatly help the process of soil moisture monitoring and irrigation scheduling. The purpose of this study is to evaluate optical sensors for measurement of soil moisture under different conditions of irrigation water salinities. These sensors are based on the active proximal sensing techniques in which measure the reflections emitted by the objects in a specific range of the wavelength (300-1100 nm). In this study, four irrigation water salinity treatments (2, 6, 10 and 16 dS.m-1) were applied to the soil columns (or Lysimeters). Each Lysimeter have been equipped with an optical sensor, connected to a data logger and monitoring screen. Simultaneously, gravimetric and volumetric soil moisture of the Lysimeters have been measured to calibrate the readings of these sensors and to estimate the accuracy of calibration equations. The results showed that the highest accuracy of the sensors occurred at the salinity level of 2 dS.m-1, where the coefficients of R2, RMSE and NSE were to around 0.92, 0.02 and 0.92, respectively. In addition, declines in the accuracy of the sensors were observed for the salinities of 6 and 10 dS.m-1, but were in the acceptable range. While, for the salinity level of 16 dS.m-1, the accuracy of the sensors was drastically decreased and the NSE reduced to the unacceptable value of 0.55. According to the results of this study, these optical sensors can be confidently recommended for measurement of soil moisture and irrigation scheduling for salinities of less than 10 dS.m-1.