Validation of the SMOS L2 Soil Moisture Data in the REMEDHUS Network (Spain)

The Level 2 soil moisture products from the Soil Moisture and Ocean Salinity (SMOS) mission have been re- leased. The data must be validated under different scenarios of biophysical and climatic conditions. For the current study, the data from January to December 2010 from 20 in situ soil moisture stations from the REMEDHUS soil moisture measurement station network (Spain) were used. A comparison analysis was carried out in terms of the soil moisture content, its spatial variability, and temporal stability. The results show an acceptable level of agreement (R = 0.73, RMSD = 0.069 m³ · m^-3, and bias = 0.053 m³ · m^-3) between the in situ and satellite data. A slight constant underestimation from the SMOS data set was detected. A centered (bias removed) root-mean-square difference was calculated to account for this persistent bias (RMSDc = 0.044 m³ · m^-3). This result is close to the SMOS accuracy objective of 0.04 m³ · m^-3. Two conclusions can be drawn: First, SMOS is close to meet the mission accuracy requirements in REMEDHUS, and second, SMOS is able to detect temporal anomalies and the temporal evolution of ground soil moisture, even though the soil moisture was slightly underestimated. Despite a noticeably reduced spatial variability among the SMOS grid cells, the remotely sensed soil moisture shows a spatial pattern of the soil moisture fields on the area scale, in agreement with the site-specific characteristics of REMEDHUS. No differences were found between the use of ascending and descending orbits. In addition, no differences were detected between the use of time-overpass values of in situ soil moisture and that of the daily average.