Calibration equations for Diviner 2000 capacitance measurements of volumetric soil water content in salt affected soils

The capacitance sensor Diviner 2000 was used in an open field to monitor the volumetric soil water content ( ) in the Minqin oasis of Northwest China. The objectives of the work were: i) to evaluate the accuracy of the factory calibration equation; ii) to obtain a soil-specific calibration as a linear and a power equation with both a saline and a tap-water reference ; iii) to obtain a general equation for multiple soil layers. Calibration was done during an irrigation experiment by using water with different salinities. The soil was characterized by different salinities and textures across the plots and soil layers. The calibration was carried out using water having the same salinity and composition as our irrigation water. This procedure would yield a better normalization of the Scaled Frequency (SF) for each irrigation treatment. For the treatments under studying the accuracy of the factory calibration equation was not good (range of the root mean square error, RMSE, 0.048-0.171 cm3 cm-3). The accuracy of the soil-specific field calibration was higher. Therefore we concluded that a soil-specific calibration is necessary. The RMSE shows that there is no difference between the calibration equation with either tap or saline water reference. The linear and power equations, determined with a tap and saline water reference for each treatment and layer gave larger differences in , ??v , at high SF than at low SF values. The ??v values increased with increasing salinity, i.e. from the control to the most saline water irrigation treatment. The linear equation had a slope significantly difference for the layer 0.3-0.6 m that has the higher clay content. The pooling in a single equation of data from multiple layers and/or treatment was satisfactory for the layer of 0.3-0.6 m of the treatments W2 and W5, which had a RMSE of 0.026, with the higher clay content.