# Soil Moisture Monitoring at Kilometer Scale: Assimilation of Sentinel-1 Products in ISBA
## 5 Discussion
### Why Is a Synergy of S1 SSM with LAI Observed in the Assimilation? Previous results based only on the assimilation of low-resolution SSM products have shown that the assimilation of SSM in ISBA has a limited impact on RZSM. The LAI assimilation has a much larger impact. The reason for this is that the sensitivity of SSM to small perturbations in the model soil moisture profile is greatly reduced below the depth of 0.1 m, while the sensitivity of LAI peaks for the 0.2-0.4 m soil layer, i.e., for WG5 [34; 68]. Under dry conditions, RZSM is somewhat decoupled from SSM [69], but not from LAI. Figures 9 and 10 show that the assimilation of S1 SSM alone can induce limited changes in RZSM and in LAI and that these changes vary from place to place. However, the assimilation of SSM products alone is not sufficient to significantly improve ISBA outputs. Errors in the ISBA soil and vegetation parameters, land cover, and topography affect the SSM simulations. The impact of SSM products alone is reduced by the use of the seasonal linear rescaling applied to the assimilated product, which results in the loss of part of the original information contained in the SSM product. Therefore, the assimilation of SSM products must be combined with other relevant satellite-based products, such as LAI, to improve the accuracy of LSM outputs. It was checked that this conclusion is valid for all seasons. The larger impact of the joint assimilation of S1 SSM and LAI products can be explained by (1) the direct link of S1 SSM with soil moisture of the more superficial soil layers, (2) the indirect effect of LAI on SSM and RZSM through its influence on evapotranspiration, (3) the indirect effect of SSM on LAI, and (4) the direct effect of LAI assimilation on RZSM. This finding is consistent with previous studies (e.g., [8; 70]) and confirms the importance of considering these two variables to disentangle the complex land-atmosphere interactions. Most of the areas with positive LAI differences in response to the assimilation (Figure 10g,f) correspond to closed forests, as seen in Figure 3. Over urban areas, these differences are very small. Conversely, other areas showed a decrease in simulated LAI in response to assimilation. Negative LAI changes were mostly observed over agricultural areas. Figure 11 shows the relationship between the LAI differences and WG5 differences displayed in Figure 10 for the three experiments. When SSM was assimilated alone, no clear relationship between the LAI differences and WG5 differences was observed, but increases in WG5 in July 2019 with respect to July 2018 tended to trigger an increase in LAI (\(R=0.22\)). \begin{table}
\begin{tabular}{c c c c c} \hline
**SSM Product, Spatial Resolution, Reference** & **Region, Soil Moisture Network, Period** & **Average \(R\)** &
\begin{tabular}{c} **Average \(\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{ \mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{\mathbf{ \mathbf{   }}}}}}}}}}}}} \mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}}\mathbf{}\mathbf{ }\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{} \mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{} \mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{}}\)** & **Number of** \\ **S1, 1 km, this study** & Toulouse, SMOSMANIA, 2017–2019 & 0.57 & 0.072 & 4 \\ S1, 1 km, this study & Montpellier, SMOSMANIA, 2017–2019 & 0.64 & 0.05 & 3 \\ S1, 1 km, this study & Salamanca, REMEDHUS, 2017–2019 & 0.48 & 0.053 & 19 \\  & Southern France (SMOSMANIA and & 0.59 & 0.056 & 8 \\  & Montpellier), 2016–2017 & 0.48 & 0.043 & 8 \\  & Southern France (SMOSMANIA and & 0.86 & 0.04 & 7 \\  & Southern France (SMOSMANIA and & 0.49 & 0.062 & 8 \\  & Southern France (SMOSMANIA and & 0.57 & 0.053 & 8 \\  & Salamanca, REMEDHUS, 2015–2017 & 0.65 & 0.062 & 6 \\  & Southern France (SMOSMANIA and & 0.69 & 0.047 & 8 \\  & Salamanca, REMEDHUS, 2015–2017 & 0.7 & 0.058 & 6 \\ \hline \end{tabular}
\end{table}