Measurement of water content in the soil profile is essential for evaluating soil water dynamics. Capacitance sensors detect the soil permittivity (ε ) which can be converted into volumetric soil water content (θ v ) by equations. The relationship between θ v and ε is usually set by default parameters from the manufacturers, but field calibration would be expected to increase the accuracy of the soil moisture measurement instruments. The purpose of this study is to determine the necessity of a field calibration and to evaluate the efficiency of field calibration for the PR2 capacitance probe (Delta-T Devices Ltd., 2004) for Des Moines Lobe soils in north-central Iowa. In this study, the calibration was conducted by fitting the linear equations between ε measured by a PR2 probe and θ v observed by soil sampling using one-year (2006) and two-year (2006+2007) data for each of 36 locations at 3 depths. The calibrated equations by the one-year data, DeltaTcalibrated-2006 equations, and the two-year data, DeltaT-calibrated-2006+2007 equations, are presented. The predicted soil moisture by the equations were compare with observed soil moisture by four statistical factors, Root mean square error (RMSE), Coefficient of Mass Residual (CMR), Index of agreement (IoA), and Model Efficiency (EF). In predicting the soil moisture in 2006+2007, the results showed that RMSE, CMR, IoA and EF values for DeltaT-default equation were 0.097cm3 cm-3, -0.092, 0.674 and -1.625, which indicated an unsatisfactory performance compared with the RMSE (0.027 cm3 cm-3) from Huang et al. (2004). After field calibration, the statistical factor values were 0.053 cm3 cm-3, 0.033, 0.818, and 0.207 for DeltaT-calibrated-2006 equations, and 0.034 cm3The authors are solely responsible for the content of this technical presentation. The technical presentation does not necessarily reflect the official position of the American Society of Agricultural and Biological Engineers (ASABE), and its printing and distribution does not constitute an endorsement of views which may be expressed. Technical presentations are not subject to the formal peer review process by ASABE editorial committees; therefore, they are not to be presented as refereed publications. Citation of this work should state that it is from an ASABE meeting paper. EXAMPLE: Author's Last Name, Initials. 2008. Title of Presentation. ASABE Paper No. 08----. St. Joseph, Mich.: ASABE. For information about securing permission to reprint or reproduce a technical presentation, please contact ASABE at email@example.com or 269-429-0300 (2950 Niles Road, St. Joseph, MI 49085-9659 USA). cm-3, 0.000, 0.895 and 0.674 for DeltaT-calibrated-2006+2007 equations. The results showed that the field calibration improved the performance of PR2 Probe in soil moisture measurement, and the two-year calibration equations, DeltaT-calibrated-2006+2007 equations, resulted in the best prediction which was comparable to the result from literature (Huang et al., 2004).
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