The conversion of permittivity as measured by a PR2 capacitance probe into soil moisture values for Des Moines lobe soils in Iowa

Measurement of soil moisture is essential for irrigation scheduling and capacitance sensors have been widely used to monitor soil moisture at different depths. Two approaches for converting permittivity measures using the capacitance probe (PR2, Delta-T Devices) to soil water content are to (a) use the default equation and parameters provided by the manufacturer, and (b) use site specific calibration equations. The objective of this study was to evaluate the performance of the manufacturer's default equation and in-situ calibrated equations for estimating soil water content. Permittivity measurement using the PR2 probe coincided with soil sampling during the growing seasons in 2006, 2007 and 2008 for Des Moines lobe soils in north-central Iowa. The default equation provided by Delta-T Devices for the PR2 probe estimated the soil water content for 3 years with an average root mean square error (RMSE) and index of agreement (IoA) values of 0.097 cm3/cm3 and 0.587, respectively. The default equation was calibrated by a 1-year (2006) and a 2-year (2006 + 2007) data set. The resultant statistics indicate that site specific calibration gives more accurate estimates of soil water content compared to the uncalibrated default equation. Three-year averaged RMSE and IoA values were 0.049 cm3/cm3 and 0.742 for equations calibrated by the 1-year data set, and 0.034 cm3/cm3 and 0.807 for equations calibrated by the 2-year data set. The results from this study indicate that a site specific calibration is necessary for the PR2 probe, and equations calibrated by data from a longer period performed better than data from a shorter period. Where a site-specific field calibration cannot be applied, coefficients are provided for various cropping systems in Des Moines Lobe soils of Iowa based on the results from this study.