Comparison of three dielectric moisture sensors for measurement of water in saline sandy soil

The number of sensor types available for measuring soil water content has increased but investigations to compare their performance in saline soils needs clarification. In this study the performance of commercially available, low-cost soil moisture sensors [time domain reflectometry (TDR), PR1 and WET], all measuring changes in the dielectric constant of the soil water, was evaluated under laboratory conditions in a saline sandy soil. The three sensors were also tested in the same sandy soil growing drip irrigated sorghum (Sorghum bicolor L. cv. Moench) in a greenhouse. Plants were irrigated daily with either saline water (ECw: 9.4 dS/m) or fresh water (0.11 dS/m). The volume of irrigation was equivalent to 100% of the pan evaporation. The results showed that measurement accuracy was strongly dependent on the salinity of the soil. The PR1 sensor overestimated volumetric water content (theta) when the salinity level exceeded 4 dS/m [root mean square of the standard error (RMSE) = 0.009 cm(3)/cm(3)]. The WET sensor significantly overestimated theta irrespective of the salinity level (RMSE = 0.014 cm(3)/cm(3)). The TDR sensor estimated theta with more accuracy (RMSE = 0.007 cm(3)/cm(3)) and thus can be considered as more reliable than the other two sensors. The calibrations were strongly affected by the salinity level of the water, so we recommend that calibration equations are modified to take account of salinity.