# Soil Moisture Monitoring at Kilometer Scale: Assimilation of Sentinel-1 Products in ISBA
## 4 Results
### Assimilation
#### 4.2.2 Regional Comparison  
Particular attention was given to the contrasting summers of 2018 and 2019 in the Montpellier region. Figure 8 shows a comparison of the RZSM simulations (WG5) between July 2019 and July 2018. The difference between the averaged open-loop simulations shows that July 2019 was mostly dryer than July 2018, with certain areas showing a more pronounced difference, especially in the northwest, where some CFs are located (Figure 3). Figure 9 shows the annual evolution from 2018 to 2019 of the mean differences in July between the analyzed WG5 and the OL WG5 simulations. The red (blue) color indicates that the analysis added (removed) more water to the root zone in 2019 than in 2018. The S1 SSM assimilation had little effect on this year-to-year WG5 difference, except for a localized increase in added water in the Ales cropland area sub-domain (box in Figure 9) in the northeast of the domain. In contrast, the assimilation of LAI had a much greater impact on the changes in WG5, with a tendency to increase WG5 more in 2019 in most of the region, especially in the southeastern part. When S1 SSM and LAI were assimilated together (Figure 9c), the impact appeared to be similar to that when LAI was assimilated alone, but it was amplified in the cropland area in the northeast of the domain. Note that Figure 9c corresponds to the difference between Figure 8a,b.  
Figure 7: Monthly (January to December) mean relative difference values of the observed soil moisture at 0.30 m soil depth vs. (**a**) open-loop WG5 simulations and (**b**) analyzed WG5 from the joint assimilation of SSM and LAI for the three SMOSMANIA stations of the Montpellier region. The year of reference to estimate these relative values was 2017. The 2018 and 2019 relative differences are represented by blue dots and red crosses, respectively.  
To further investigate the impact of assimilation, a closer examination of the Ales subdomain, marked by blue rectangles in Figure 9 (4-4.3\({}^{\circ}\)E, 43.95-44.25\({}^{\circ}\)N), was performed and is shown in Figure 10. Figure 10 includes the irrigation zones obtained from [PERSON] et al. [51] at kilometer resolution for this region. Figure 10 shows that the impact of assimilation varied between different analyses and that the assimilation of S1 SSM alone resulted in moderate changes in the July 2019 WG5 analysis in some areas compared to July 2018. The impact of LAI assimilation on WG5 was more pronounced and different when LAI was jointly assimilated together with S1 SSM. Figure 10 also shows that LAI was reduced by the assimilation. Similar results were obtained over the Toulouse and Salamanca regions (see the Supplementary Materials).  
Figure 8: Map over the Montpellier domain of the WG5 difference between the averaged values of July 2019 and July 2018 from (**a**) the open-loop and (**b**) the joint assimilation of SSM and LAI.  
Figure 9: Year-to-year evolution from 2018 to 2019 over the Montpellier domain of the mean differences in July between analyzed and open-loop WG5 when (**a**) S1 SSM was assimilated alone, (**b**) PROBA-V LAI was assimilated alone, and (**c**) S1 SSM and PROBA-V LAI were jointly assimilated. Boxes correspond to the Ales sub-domain area shown in Figures 3 and 10. Figure 9c corresponds to the difference between Figure 8a,b. The red (blue) color indicates that the analysis added (removed) water to the root zone more in 2019 than in 2018 in response to the assimilation.