Location: Hydrology and Remote Sensing LaboratoryTitle: Evaluation of several calibration procedures for a portable soil moisture sensor Author
Submitted to: Journal of Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/18/2013
Publication Date: 8/19/2013
Publication URL: http://handle.nal.usda.gov/10113/59842
Citation: Rowlandson, T., Berg, A., Bullock, P., Ojo, E.R., McNairn, H., Wiseman, G., Cosh, M.H. 2013. Evaluation of several calibration procedures for a portable soil moisture sensor. Journal of Hydrology. 498:335-344. Interpretive Summary: Soil moisture measurements are a costly and time consuming effort. Handheld instruments which take advantage of the dielectric properties of water and soil are useful to estimte soil moisture quickly and easily, but at the price of accuracy. With appropriate calibration, the instrument accuracy can be improved to meet the expectations of users of the data, which includes the satellite remote sensing community. For a field experiment in southern Manitoba, Canada, a variety of calibration equations are tested for the ability to minimize errors versus a physically collected soil moisture sample. It was found that at the field scale (~800 meters), the soil moisture patterns were uniform enough to generate field specific calibration equations with the desired accuracies for remote sensing purposes. This scale fortunately coincides with the scale of the remote sensing products being collected from the aircraft during the field experiment. Consequently, the ground measurements can be used to validate the remote sensing-based soil moisture algorithms that eventually will be used by space-borne sensors.
Technical Abstract: The calibration and validation of remotely sensed soil moisture products relies upon an accurate source of ground truth data. The primary method of providing this ground truth is to conduct intensive field campaigns with manual surface soil moisture sampling measurements, which utilize gravimetric sampling, soil moisture probes, or both, to estimate the volumetric soil water content. Soil moisture probes eliminate the need for labour-intensive gravimetric sampling. To ensure the accuracy of these probes, several studies have determined these probes need various degrees of localized calibration. This study examines six possible calibration techniques using data collected during a field campaign conducted in 2012, with soil moisture samples being collected over 55 fields in southern Manitoba, as part of the Soil Moisture Active Passive Validation Experiment 2012 (SMAPVEX12). The use of a general equation, applied to all collected data, resulted in the largest error regardless of whether a linear or third order polynomial relationship was established for the calibration of the soil moisture probes. Calibration equations based on soil texture or vegetation land cover reduced the error; however, the individual calibration equations established for each field in the study had the lowest error of all the calibration techniques. Although average bias was low for all of the calibration techniques, the use of the general equation to calibrate individual fields resulted in high biases for some fields.