Calibration and Validation of the COSMOS Rover for Surface Soil Moisture Measurement

The COsmic-ray Soil Moisture Observing System (COSMOS) rover may be useful for validating satellite-based estimates of near-surface soil moisture, but the accuracy with which the rover can measure 0- to 5-cm soil moisture has not been previously determined. Our objectives were to calibrate and validate a COSMOS rover for mapping 0- to 5-cm soil moisture at spatial scales suitable for evaluating satellite-based soil moisture estimates. Region-specific calibrations for the COSMOS rover were developed using field-average soil moisture measured using impedance probes and the oven-drying method on volumetric samples. The resulting calibrations were applied to map soil moisture on two dates in a 16- by 10-km region around the Marena, Oklahoma, In Situ Sensor Testbed (MOISST) in north central Oklahoma and one date in a 34- by 14-km region in the Little Washita River watershed in southwestern Oklahoma. The mapped soil moisture patterns were consistent with the regional spatial variability in surface soil texture and with soil wetting by an intervening rainfall. The rover measured field-average soil moisture with a root mean squared difference (RMSD) of 0.03 cm(3) cm(-3) relative to impedance probes. Likewise, the regional-average 0- to 5-cm soil moisture determined by the rover was within +/- 0.03 cm(3) cm(-3) of the best available independent estimates for each region. These results demonstrate that a COSMOS rover can be used effectively for 0- to 5-cm soil moisture mapping and for determining the average soil moisture at spatial scales suitable for satellite calibration and validation.