Global soil moisture estimates: do SMAP retrievals achieve the accuracy of model reanalysis?

A number of global surface soil moisture (SM) products have been derived from the L-band frequency Soil Moisture Active Passive (SMAP) and the Soil Moisture and Ocean Salinity (SMOS) missions to study the terrestrial water, energy, and carbon cycles. This letter presents the performance of the recently developed 9 km SMAP product (hereafter SMAP-INRAE-BORDEAUX, SMAP-IB9) at the global scale. The product was retrieved from the 9 km dual-channel SMAP radiometric products using the forward model of SMOS INRA-CESBIO (SMOS-IC) and of the official SMOS retrieval algorithms – L-MEB (L-band Microwave Emission of the Biosphere) model. We inter-compared the SMAP-IB9 retrievals with two other products with a similar grid resolution (~10 km): the SMAP Enhanced Level-3 data set (hereafter SMAP-L3) and the land component of the fifth generation of European Reanalysis (ERA5-Land) with the main objective of assessing the difference in accuracy between remotely sensed and model-based SM products. We found that the ERA5-Land and SMAP-IB9 SM products had the overall highest correlations (R = 0.62(±0.16) for ERA5-Land vs. 0.62(±0.15) for SMAP-IB9 and 0.54(±0.15) for SMAP-L3) when compared with in-situ measurements. ERA5-Land showed better performances in the forest areas where SMAP-IB9 and SMAP-L3 still showed high potential in catching the temporal variations of the in-situ measurements, particularly in terms of median correlation values (0.65(±0.17) for SMAP-IB9 vs. 0.69(±0.15) for ERA5-and). Besides, it is interesting to note that, using ISMN as a reference, the difference in R between remote sensing and model-based SM products that existed in the past has decreased to statistically insignificant levels over time. For instance, in the non-forest areas, we found that the latest versions of the SMAP SM products (SMAP-L3 and SMAP-IB9) had relatively comparable performances with ERA5-Land in terms of median ubRMSE (0.06(±0.02) m3/m3 for SMAP-L3 and ERA5-Land) and R (0.60(±0.16) for SMAP-IB9 vs. R (0.60(±0.15) for ERA5-Land), respectively.