Submitted to: Journal of Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/10/2004
Publication Date: 9/15/2005
Citation: Cosh, M.H., Jackson, T.J., Bindlish, R., Famiglietti, J.S., Ryu, D. 2005. Calibration of an impedance probe for estimation of surface soil water content over large regions. Journal of Hydrology. 311(1-4):15Sept2005, 49-58. Interpretive Summary: Efficient and accurate methods of estimating soil moisture are desireable for hydrologists and soil physicists. However, the calibration of such methods can be cumbersome for large scale experiments or monitoring. This study examines the impact of calibration of one specific handheld soil moisture sensor on soil moisture estimates for two recent field experiments, SMEX02 and SMEX03. These two experiments were designed to support the calibration and validation of satellite soil moisture sensors. This required large scale sampling of soil moisture over diverse surface and soil conditions. Handheld sensor measurements were compared to gravimetric samples of soil moisture for calibration. Improvements were observed in the overall accuracy of the sensor when specific calibration equations were developed for each locale under study. However, the real impact of calibration was a reduction in the bias of the sensor in estimating specific locations.
Technical Abstract: Large region surface soil moisture estimates are important for both hydrologic modeling and remote sensing applications. For soil moisture monitoring, gravimetric soil moisture sampling is reliable; however, it requires a significant effort to gather and process samples. Portable impedance probes serve as a valuable alternative to destructive gravimetric sampling. These probes measure the dielectric properties of the soil-water-air mixture from which we can infer the volumetric soil moisture. As part of recent large-scale experiments in the summers of 2002 and 2003, three different methods for calibrating impedance probes were investigated with the support of coincident gravimetric samples. Field specific calibration improved the accuracy of the probe from greater than ±5% volumetric soil moisture (using the generalized calibration) to less than ±4%. In addition, a significant amount of bias (~2%) was eliminated. It was also determined that field specific calibration removes a bias due to bulk density variations. Based upon these results it was concluded that the generalized calibration is adequate for estimation of diverse conditions. For studies with more stringent accuracy requirements, field specific calibration is necessary because of reduced bias and error.