The Soil Moisture Active Passive Experiments: Validation of the SMAP Products in Australia

• Published in: IEEE transactions on geoscience and remote sensing 2021-04, Vol.59 (4), p.2922-2939
• Main Authors: Ye, Nan, Walker, Jeffrey P., Wu, Xiaoling, de Jeu, Richard, Gao, Ying, Jackson, Thomas J., Jonard, Francois, Kim, Edward, Merlin, Olivier, Pauwels, Valentijn R. N., Renzullo, Luigi J., Rudiger, Christoph, Sabaghy, Sabah, von Hebel, Christian, Yueh, Simon H., Zhu, Liujun
• Format: Article
• Language: English
• Subjects: Airborne radar; Airborne remote sensing; Aquarius; Australia; Backscattering; Brightness; Brightness temperature; Brightness temperatures; Calibration; Calibration and validation; Catchment area; Catchments; Continental interfaces, environment; Data; Data collection; Datasets; Earth Sciences; Earth sciences & physical geography; Environmental sciences & ecology; field experiment; Hydrology; L-band; Life sciences; Luminance; Mean square error; Microwave frequencies; Microwave observations; Microwave radiometry; Moisture content; Monitoring; Murrumbidgee River catchment; Operational phase; passive microwave remote sensing; Physical, chemical, mathematical & earth Sciences; Physique, chimie, mathématiques & sciences de la terre; Radar; radar remote sensing; Radiometers; remote sensing; River catchments; Root mean squared errors; Root-mean-square errors; Sciences de la terre & géographie physique; Sciences de l’environnement & écologie; Sciences du vivant; Sciences of the Universe; Soil; Soil moisture; soil moisture (SM); Soil Moisture and Ocean Salinity (SMOS); Spaceborne radar; Surface radiation temperature; Surface roughness; Temperature; Vegetation; Vegetation condition; Vegetation water content; Water content
• ISSN: 0196-2892; 1558-0644; 1558-0644
• DOI: 10.1109/TGRS.2020.3007371

The fourth and fifth Soil Moisture Active Passive Experiments (SMAPEx-4 and -5) were conducted at the beginning of the SMAP operational phase, May and September 2015, to: 1) evaluate the SMAP microwave observations and derived soil moisture (SM) products and 2) intercompare with the Soil Moisture and Ocean Salinity (SMOS) and Aquarius missions over the Murrumbidgee River Catchment in the southeast of Australia. Airborne radar and radiometer observations at the same microwave frequencies as SMAP were collected over SMAP footprints/grids concurrent with its overpass. In addition, intensive ground sampling of SM, vegetation water content, and surface roughness was carried out, primarily for validation of airborne SM retrieval over six \sim 3\,\,\text {km} \times 3 km focus areas. In this study, the SMAPEx-4 and -5 data sets were used as independent reference for extensively evaluating the brightness temperature and SM products of SMAP, and intercompared with SMOS and Aquarius under a wide range of SM and vegetation conditions. Importantly, this is the only extensive airborne field campaign that collected data while the SMAP radar was still operational. The SMAP radar, radiometer, and derived SM showed a high agreement with the SMAPEx-4 and -5 data set, with a root-mean-squared error (RMSE) of ~3 K for radiometer brightness temperature, and an RMSE of \sim 0.05~\text{m}^{3}/\text{m}^{3} for the radiometer-only SM product. The SMAP radar backscatter had an RMSE of 3.4 dB, while the retrieved SM had an RMSE of 0.11 m 3 /m 3 when compared with the SMAPEx-4 data set.