Laboratory and field evaluation of five soil water sensors

Accurate in situ determination of soil water content is important in many fields of agricultural, environmental, hydrological, and engineering sciences. As numerous soil water content sensors are available on the market today, the knowledge of their performance will aid users in the selection of appropriate sensors. The objectives of this study were to evaluate five soil water sensors in the laboratory and to determine if laboratory calibration is appropriate for the field. In this study, the performances of five sensors, including the Profile Probe(TM) (PP), ThetaProbe(TM), Watermark(TM), Aqua-Tel(TM), and Aquaterr(TM) were compared in the laboratory. The PP and ThetaProbe(TM) were more accurate than the other soil water sensors, reproducing soil water content using factory recommended parameters. However, when PP was installed on a loamy sand in the field, the same soil that was used for the laboratory evaluation, it overestimated field soil water, especially at depth. Another laboratory experiment showed that soil water content readings from the PP were strongly influenced by soil bulk density. The higher the soil bulk density, the greater was the overestimation of soil water content. Two regression parameters, a(0) and a(1) which are used to convert the apparent dielectric constant to volumetric water content, were found to increase linearly with the soil bulk density in the range of 1.2 to 1.6 Mg m(-3). Finally, the PP was calibrated in the field and a good calibration function was obtained with an r(2) of 0.87 and RMSE of 2.7%. The values of a(0) and a(1) obtained in the field were different from factory recommended parameters (a(0) = 2.4 versus 1.6 while a(1) = 12.5 versus 8.4) and were independent of soil depth, bulk density, and texture. As such, individual field calibration will be necessary to obtain precise and accurate measurement of soil water content with this instrument.