# Soil Moisture Monitoring at Kilometer Scale: Assimilation of Sentinel-1 Products in ISBA
## 5 Discussion
### Can Geology and Land Use Affect the Assimilation? Land use is a critical factor that can significantly affect the accuracy of satellite product assimilation in land surface models [56]. The accuracy of assimilation results largely depends on the similarity between land surface characteristics, satellite observations, and the land cover database used in LSMs. Several studies have investigated the influence of land use on the assimilation of satellite products in LSMs. For example, [PERSON] et al. [73] found that the accuracy of soil moisture assimilation was higher in cropland than that in forested areas due to the higher sensitivity of the radar signal to soil moisture in cropland. In this study, the ECOCLIMAP-II land cover map was used in the model simulations, which differs slightly from the CGLS-LC100 map shown in Figure 3. The LAI--and indirectly SSM--contributions in this specific case could be related to the percentage of forest present in these areas in the ECOCLIMAP-II database, which is less than 50%, indicating the use of other land uses in the ISBA model. The Montpellier region is characterized as a very contrasted area in terms of land use, as shown in Figure 3. The size of the Ales sub-domain (0.3\({}^{\circ}\)\(\times\) 0.3\({}^{\circ}\)) would correspond to the size of a low-resolution SSM pixel. Figure 10 illustrates the large sub-pixel variability that would be lost at low spatial resolution. Another characteristic of the Montpellier region is the presence of karst areas in the northwest of the domain (Larzac and Cevennes) and flood irrigation for rice production in the southeast of the domain (Camargue). Karstic areas tend to trigger anomalous C-band backscatter signals [74; 75], and irrigation is not represented by the ISBA model version we used. In Section 4.1, it was shown that the OL  
Figure 11: Year-to-year evolution from 2018 to 2019 over the Ales sub-domain of the mean differences in July between analyzed and open-loop LAI vs. the mean differences in July between analyzed and open-loop WG5 between (**a**) S1 SSM assimilated alone, (**b**) PROBA-V LAI assimilated alone, and (**c**) SSM and LAI jointly assimilated. WG2 tends to perform better than the S1 SSM, with better \(R\) and ubRMSD score values. Therefore, we can use OL WG2 as a benchmark for S1 SSM over non-irrigated areas (OL does not represent irrigation). Figure 12 shows S1 SSM vs. OL WG2 \(R\) and ubRMSD maps showing specific problems with the S1 SSM over the karst areas. These maps also show a discrepancy between OL and S1 over the flood-irrigated Camargue area caused by the non-implementation of irrigation in the model version we used. Figure 12c,d also shows the impact of the joint assimilation of S1 SSM and LAI (\"SSM and LAI\" experiment in Table 2) on the \(R\) and ubRMSD score values. Overall, the assimilation had a neutral or negative effect on \(R\) and ubRMSD over the karst area. On the other hand, over the Camargue, the consistency between the S1 SSM and the modeled WG2 was improved by the assimilation, with an increase in \(R\) (up to 0.05) and a decrease in ubRMSD (down to \(-0.003\)). This shows that the assimilation is able to correct the model deficiencies to some extent. To evaluate the difference of the analysis with respect to the OL, with and without assimilation of S1 SSM and PROBA-V LAI, Figure 13 presents the equivalent of Figure 12c,d for the \"SSM\" and \"LAI\" experiments (Figure 13a,b and Figure 13c,d, respectively). As far as the correlation difference is concerned, it seems that Figure 12c is more similar to Figure 13a (\"SSM\") than to Figure 13c (\"LAI\"). The opposite  
Figure 12: S1 SSM vs. OL WG2 over the Montpellier domain: (**a**) correlation coefficient, (**b**) ubRMSE, (**c**) difference between correlation of S1 SSM vs. SSM and LAI analysis WG2 and correlation of S1 SSM vs. OL WG2, and (**d**) difference between ubRMSD of S1 SSM vs. SSM and LAI analysis WG2 and ubRMSD of S1 SSM vs. OL WG2.