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 0to 5-cm soil moisture has not been previously determined. Our objectives were to calibrate and validate a COSMOS rover for mapping 0to 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 16by 10-km region around the Marena, Oklahoma, In Situ Sensor Testbed (MOISST) in north central Oklahoma and one date in a 34by 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 cm3 cm−3 relative to impedance probes. Likewise, the regional-average 0to 5-cm soil moisture determined by the rover was within ±0.03 cm3 cm−3 of the best available independent estimates for each region. These results demonstrate that a COSMOS rover can be used effectively for 0to 5-cm soil moisture mapping and for determining the average soil moisture at spatial scales suitable for satellite calibration and validation.