Evaluation of SMOS, SMAP, ASCAT and Sentinel-1 Soil Moisture Products at Sites in Southwestern France

This study evaluates the accuracy of several recent remote sensing Surface Soil Moisture (SSM) products at sites in southwestern France. The products used are Soil Moisture Active Passive SMAP (level 3: 36 km x 36 km, level 3 enhanced: 9 km x 9 km, and Level 2 SMAP/Sentinel-l: 1 km x lkm), Advanced Scatterometer ASCAT (level 2 with three spatial resolution 25 km x 25 km, 12.5 km x 12.5 km, and 1 km x 1 km), Soil Moisture and Ocean Salinity SMOS (SMOS INRA-CESBIO SMOS-IC, SMOS Near-Real-Time SMOS-NRT, SMOS Centre Aval de Traitement des Donndes SMOS level 3 SMOS-CATDS, 25 km x 25 km) and Sentinel-l(S1) (25 km x 25 km, 9 km x 9 km, and 1 km x 1 km). The accuracy of SSM products was computed using in situ measurements of SSM observed at a depth of 5 cm. In situ measurements were obtained from the SMOSMANIA ThetaProbe (Time Domaine reflectometry) network (7 stations between 1 January 2016 and 30 June 2017) and additional field campaigns (near Montpellier city in France, between 1 January 2017 and 31 May 2017) in southwestern France. For our study sites, results showed that (i) the accuracy of the Level 2 SMAP/Sentinel-1 was lower than that of SMAP-36 km and SMAP-9 km; (ii) the SMAP-36 km and SMAP-9 km products provide more precise SSM estimates than SMOS products (SMOS-IC, SMOS-NRT, and SMOS-CATDS), mainly due to higher sensitivity of SMOS to RFI (Radio Frequency Interference) noise; and (iii) the accuracy of SMAP-36 km and SMAP-9 km products was similar to that of ASCAT (ASCAT-25 km, ASCAT-12.5 km and ASCAT-1 km) and S1 (S1-25 km, S1-9 km, and $ 1-1 km) products. The accuracy of SMAP, Sentinel-1 and ASCAT SSM products calculated using the average of statistics obtained on each site is defined by a bias of about -3.2 vol. %, RMSD (Root Mean Square Difference) about 7.6 vol. %, ubRMSD (unbiased Root Mean Square Difference) about 5.6 vol. %, and R coefficient about 0.57. For SMOS products, the station average bias, RMSD, ubRMSD, and R coefficient were about -10.6 vol. %, 12.7 vol. %, 5.9 vol. %, and 0.49, respectively.