Multiscale Comparison of Eight Satellite Soil Moisture Data Sets Over Two Calibration Sites

Abstract

This paper examines the quality of eight satellite soil moisture products at two typical spatial resolutions, including an intercomparison of SMAP passive, SMOS, JAXA AMSR2, LPRM AMSR2, ESA CCI and the Chinese FY3B soil moisture products at a coarse resolution of ∼0.25°, and the newly released SMAP enhanced passive and JAXA AMSR2 soil moisture products at a medium resolution of ∼0.1°. Insitu measurements from two representative dense networks, i.e., the Little Washita Watershed (LWW) in the United States and the REMEDHUS networks in Spain are used to compare and validate the eight soil moisture products. The results show that the SMAP passive and FY3B products outperform the other products with the lowest unbiased root mean square (ubRMSE) values of 0.027 m3 m−3 and 0.025 m3 m−3 in the LWW and REMEDHUS network regions respectively. SMOS slightly underestimates soil moisture with a dry bias, but it correlates well with insitu data with an average correlation value of 0.77. The JAXA product performs much better at 0.25° than at 0.1o, but both of them underestimate soil moisture (bias>-0.05 m3 m−3) at most time. The SMAP enhanced passive soil moisture well captures the temporal variation of ground measurements with a correlation coefficient larger than 0.8, and is generally superior to the JAXA product. The LPRM shows much larger amplitude and temporal variation than the ground soil moisture with a wet bias larger than 0.09 m3 m−3. The ESA CCI product shows satisfactory performance with acceptable error metrics (ubRMSE<0.045 m3 m−3), revealing the effectiveness of merging active and passive soil moisture products. The good performance of SMAP and FY3B demonstrates the potential in integrating them into the existing long-term ESA CCI product to form a more complete product.