A comprehensive validation of the SMAP Enhanced Level-3 Soil Moisture product using ground measurements over varied climates and landscapes

This study presents a comprehensive validation of the Soil Moisture Active Passive (SMAP) mission Enhanced Level-3 radiometer soil moisture (SM) product (Version 2) over 3 years from April 1, 2015 to March 31, 2018 using extensive ground measurements from sparse networks covering a range of physical and climatological regimes. Using a common spatial resolution, soil moisture retrievals from the descending (6:00 AM) and ascending (6:00 PM) overpasses were assessed based on static conditions such as the climate zone, soil property and land cover. Then validation considering performance with respect to dynamic attributes such as soil wetness, vegetation density and land surface temperature was reported. Given the above six parameters are cross-correlated, the quality of the SMAP enhanced products was further evaluated based on pairwise factors. Overall, higher accuracy was noted over zones where the soil organic carbon is low, the vegetation density is relatively sparse, locations in the temperate and arid climate zones, and the mean LST is high. Results also indicate that the descending (AM) and ascending (PM) products exhibit mean temporal correlation over the ground stations equaling 0.667 and 0.651, and mean unbiased root mean square error (ubRMSE) equaling 0.055 and 0.054 m(3)/m(3) respectively which is close to the ubRMSE requirement of the SMAP mission, 0.04 m(3)/m(3). While the ascending (PM) SM retrievals have been frequently excluded from applications due to its poor performances reported form validation studies, these comparable performances between the two products suggest that the ascending (PM) SM retrievals can be an additional data source. Based on the results here, it can be concluded that there is room for improving the SMAP product especially in areas of the world with very dense vegetation and average to low land surface temperatures.