In our work we evaluated and inter-compared the performances of two SMAP SM retrievals against ECMWF modeled SM (global scale) and ground-based measurements (local scale) following three main rules: i) using a long period (i.e., April 2015–December 2017) of SM retrievals of each SMAP product as much as possible (until November 2019, MT-DCA was only updated to the end of 2017); ii) strictly using the same number of pixels (and thus of dates) between the two SMAP SM retrievals; iii) doing same data filtering, i.e., not recommended by the SMAP retrievals quality flag, SM values outside the range 0–0.6 m

^{3}/m

^{3} [

46], and temporal series of SMAP data pairs (i.e., number of observations available for validation) lower than one month (~31) were filtered out [

51]. Four metrics, which are widely used in the soil moisture community [

45,

47], were used to compare the SMAP SM retrievals with the reference data (i.e., modeled ECMWF and in situ measurements): (Pearson) correlation coefficient (R; Equation 1) to assess the performance of SMAP retrievals to capture the seasonal variations of the reference SMs, bias (m

^{3}/m

^{3}; Equation 2) to measure the wetness or dryness of the SMAP SM compared to the reference SMs, root mean square difference (RMSD; m

^{3}/m

^{3}; Equation 3), and the

ubRMSD (m

^{3}/m

^{3}; Equation 4) [

18]. It should be noted that R and

ubRMSD were considered as first-order criteria in comparison to Bias and RMSD, as the reference soil moisture datasets do not represent the value as “observed” by the SMAP measurement, considering the different “sampling” depths of simulated, retrieved and in-situ SM data.

where

n is the number of SM data pairs, SM

_{est} is SMAP SM estimates (i.e., either SPL3SMP_E or MT-DCA SM), SM

_{ref} is the reference SM (i.e., either ECMWF SM or ISMN in situ SM), and overbar represents the temporal mean of the entire time series. It should be noted that when comparing retrieved SM with in situ observations, the time series of satellite-based retrievals of SM were extracted from the original SMAP pixels covering each site (based on latitude and longitude of the site) to obtain values corresponding to the field measurements, and only the in situ data matching with the instantaneous overpass of the SMAP observation within a time window of 1 hour were selected. In our analysis, as done by Al-Yaari et al. (2019) [

29], we accounted only for SM retrievals where p-value < 0.05 and R > 0.4 when comparing to in situ data sets (this condition only applies to the local scale evaluation). As correlation coefficients (R) cannot be simply averaged, we computed the median of R. In our analysis, in addition to the median skill metrics, we considered the spatial standard deviation.