# Soil Moisture Monitoring at Kilometer Scale: Assimilation of Sentinel-1 Products in ISBA
## 4 Results  
### Validation of S1 SSM  
Validation of the 1 km \(\times\) 1 km S1 SSM products was first carried out by conducting a comparison with in situ SSM observations obtained from the SMOSMANIA, Meteopole-Flux, and REMEDHUS stations located within the study regions, as shown in Figure 1. A time series of daily SSM values at 5 cm depth, at 18:00 UTC, is presented in Figure 5, for the La Grand Combe (LGC) SMOSMANIA station located within the Montpellier domain (see Figure 2 for LGC location within the domain). Figure 5 also shows the S1 SSM values corresponding to the pixel where this in situ station is located, together with the OL simulation. The S1 SSM product matches the seasonal variability of the observed SSM, with better agreement in wetter conditions compared to drier ones. The observed values range between 0.005 and 0.26 m\({}^{3}\) m\({}^{-3}\), while the satellite product varies from 0.07 to 0.20 m\({}^{3}\) m\({}^{-3}\). Notably, S1 SSM represents some of the peak SSM values quite well, especially in November and December 2017. Table 2 shows that S1 SSM tended to perform better than the OL simulation over LGC, for all values. S1 SSM had an ubRMSD of 0.024 m\({}^{3}\) m\({}^{-3}\), against 0.058 m\({}^{3}\) m\({}^{-3}\) for the OL simulation. On the other hand, OL WG2 correlated slightly better with in situ SSM observations than S1 SSM. Figure 5: Hourly time series of surface soil moisture at the LGC station from in situ measurements, S1 SSM, and OL WG2 simulations (red, blue, and orange lines, respectively) at 18:00 UTC. To better assess the accuracy of the S1 SSM product, a seasonal statistical analysis was performed to compare it with the observed SSM at LGC. The scatter plots and correlation coefficients between the two datasets for each season are presented in Figure 6, with winter, spring, and fall showing high correlation coefficient values (\(R\) = 0.92, 0.88, and 0.91, respectively). Overall, a good correlation was observed between the datasets for all seasons, except for summer, for which the lowest correlation coefficient was obtained (\(R\) = 0.62). The statistical distribution of the correlation coefficients between the S1 SSM products and the observed SSM is shown in Figure S1 for all seasons and over the three years, for all SMOSMAIA (Toulouse and Montpellier Regions) and REMEDHUS (Salamanca Region) stations. A seasonal variation of \(R\) can be observed across stations for all regions. A lower \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Comparison** & \(R\) & **ubRMSD (m\({}^{3}\) m\({}^{-3}\))** & **RMSD (m\({}^{3}\) m\({}^{-3}\))** & **MB (m\({}^{3}\) m\({}^{-3}\))** & **Number** \\ \hline OL vs. in situ & 0.87 & 0.043 & 0.165 & 0.159 & 165 \\ S1 SSM vs. in situ & 0.85 & 0.024 & 0.028 & \(-\)0.013 & 163 \\ S1 SSM vs. OL & 0.71 & 0.058 & 0.156 & \(-\)0.145 & 164 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Correlation coefficient (\(R\)), root mean square difference (RMSD), unbiased RMSD (ubRMSD), and mean bias (MB) between OL simulations, S1 SSM products, and in situ observations at the LGC SMOSMANIA station, from 2017 to 2019, at 18:00 UTC. Figure 6: SMOSMANAIA versus S1 SSM products for the LGC station for (**a**) winter (December, January, and February), (**b**) spring (March, April, and May), (**c**) summer (June, July, and August), and (**d**) fall (September, October, and November), at 18:00 UTC. The best linear fit as well as the correlation coefficient between the two datasets are presented in each sub-plot. RMSD, MB, and ubRMSD values are in m\({}^{3}\)m\({}^{-3}\). Freezing cases were sorted out by excluding data for which soil temperature at 5 cm depth was below 4 \({}^{\circ}\)C.  
correlation between the satellite products and the observed SSM was obtained during the summer season, which supports the results presented in Figure 5 for this season. In general, SMOSMANIA stations showed higher correlation coefficients than REMEDHUS, especially in spring and fall. However, one of the SMOSMANIA stations (LHS) showed a negative correlation in summer.