# Soil Moisture Monitoring at Kilometer Scale: Assimilation of Sentinel-1 Products in ISBA
## 2 Model and Data
### S1 SSM Data  
In this study, SSM products were derived from the Sentinel-1A (S1A) and Sentinel-1B (S1B) C-band (wavelength: \(\sim\)6 cm) backscatters. S1A was launched in April 2014 and S1B was launched two years later in April 2016. Both of them provide images with a spatial resolution of 10 m \(\times\) 10 m. With two satellites in operation, the average sampling time is six days, in both ascending and descending modes over different regions of the globe. The obtained images were used to develop soil moisture maps, using a neural network approach involving inversion of radar signals, S\({}^{2}\)MP (Sentinel-1/Sentinel-2 derived soil Moisture at Plot scale), as described in [10]. The retrieval process was applied to S1 backscatter in VV polarization (vertical transmit and vertical receive), in conjunction with NDVI (normalized difference vegetation index) time series derived from S2. It should be noted that SSM retrievals were not produced over straw cereal crops (mainly wheat and barley) during the spring stem elongation phase (due to the low penetration of the S1 wave in the canopy), forests, vineyards, and orchards. Land cover maps, cropland masks, and cropland and herbaceous vegetation masks at 100 m spatial resolution were derived for each year from the Copernicus Global Land Service dynamic land cover map (CGLS-LC100) (see Section 2.5). These maps were also used to aggregate SSM at 1 km \(\times\) 1 km spatial resolution. First, the SSM values at the 100 m \(\times\) 100 m spatial resolution were estimated using the grid of the land cover map, in order to avoid mixing several crop types within the same 1 km \(\times\) 1 km grid scale in the average value of the S1 radar signal and NDVI. Second,the obtained SSMs at the 100 m scale (only for cropland and herbaceous vegetation pixels) were averaged within each 1 km grid cell to obtain 1 km \(\times\) 1 km SSM estimates. The generated soil moisture maps (in m\({}^{3}\) m\({}^{-3}\)), hereafter referred to as S1 SSM, cover three regions for a time period of three years, from January 2017 to December 2019, with two of them situated in the southern part of France and the other one located in the western part of Spain, as illustrated in Figure 1. The first region encompasses a rectangular area covering the region of Toulouse and measuring 211 \(\times\) 293 km. The second region, Montpellier, has a rectangular area of 112 \(\times\) 154 km. The Salamanca region in Spain covers an area of 246 \(\times\) 319 km. Soil moisture measurement stations, which are presented in Section 2.4, are available within each region. The domain coordinates of Montpellier, Toulouse, and Salamanca are 43.254\({}^{\circ}\)-44.263\({}^{\circ}\)N and 2.987\({}^{\circ}\)-4.379\({}^{\circ}\)E, 43.245\({}^{\circ}\)-45.147\({}^{\circ}\)N and 0.469\({}^{\circ}\)-3.112\({}^{\circ}\)E, and 40.353\({}^{\circ}\)-42.567\({}^{\circ}\)N and 6.790\({}^{\circ}\)-3.924\({}^{\circ}\)W, respectively. Figure 2 shows S1 SSMs for a particular day in early May 2018 at approximately 18:00 UTC for the three regions described above. Note that it was not possible to display SSM for the three regions for the exact same day due to the temporal resolution of S1. The Toulouse region exhibited remarkable moist conditions on 8 May 2018, especially towards the southwest, with SSM values reaching as high as 0.28 m\({}^{3}\)m\({}^{-3}\). Local rainfall observations show that intense precipitation occurred in the morning of 8 May 2018. Conversely, the Montpellier region showed a greater spatial heterogeneity in SSM values on 3 May 2018. In the case of Salamanca, the prevailing conditions were dryer, with SSM values ranging between 0.06 and 0.10 m\({}^{3}\)m\({}^{-3}\). Note that the red and blue points marked on these maps correspond to the locations of SSM in situ measurements that are described in the following subsection.