Laboratory versus Field Calibration of Capacitance Probes

Capacitance probes (CP) based on responses to electromagnetic properties in soil are widely used for monitoring soil volumetric water content (theta(v)), but proper calibration is necessary to obtain reliable results. The objectives of this study were to: (i) compare errors in evaluating theta(v) when using soil-spccific equations as opposed to the manufacturer's default equation; (ii) compare the CP calibration equations obtained in the laboratory and in the field; and (iii) evaluate the accuracy of CP under field conditions using soil-specific equations as opposed to the manufacturer's default equation. Laboratory and field calibrations were conducted by fitting statistical relationships between readings from CP and theta(v) measured from soil cores. The accuracies of the obtained calibration equations were evaluated under field conditions during, two irrigation cycles. In both calibration studies, the manufacturer's default equation consistently overcalculated theta(v) (RMSE > 0.044 m(3) m(-3)), whereas custom CP calibration equations provided accurate theta(v) determinations (RMSE < 0.028 m(3) m(-3)). Even when theta(v) was overestimated, the manufacturer's default equation was able to reproduce theta(v) dynamics during irrigation. Therefore, if users are interested in relative differences in theta(v), they may use the manufacturer's default equation. If absolute values are needed, however, soil-specific calibration is required. The accuracy of CP in monitoring theta(v) under field conditions was slightly better using the laboratory calibration equation (RMSE = 0.019 m(3) m(-3)) rather than the field calibration equation (RMSE = 0.023 m(3) m(-3)). The use of a laboratory calibration is recommended given that it is easily reproducible, facilitates work planning, and minimizes uncertainties.