## 3 Data Analysis
### Comparison between S1-SSM and Precipitation
Then, each S1-derived SSM map was overlaid with the corresponding six days of cumulative GPM rainfall. From the qualitative analysis of the overlaid maps, several arguments can be extracted and discussed. Starting with the initial map on 21 December 2016 (Figure 3a), departments as Tarn, Herault, Lozere, and Aveyron of the eastern and middle parts of the region encountered high soil moisture values (more than 20 vol.%) corresponding to 10 to 20 mm of cumulative precipitation during the past six days, whereas Gers and Haute-Garonne in the western part showed low soil moisture values (20 vol.% at most) corresponding to the absence of precipitation in the area for the same prior period. A map derived six days later on 27 December 2016 (Figure 3b) shows a decrease in soil moisture values in the Herault and Gard regions due to the absence of precipitation between 21 December 2016 and 27 December 2016. Moreover, high soil moisture values were estimated for Tarn due to continuous raining episodes between 21 December 2017 and 27 December 2017, and a slight increase in soil moisture values was observed in western  
Figure 2: Temporal evolution of soil moisture values of Sentinel-1 derived surface soil moisture maps (S1-SSM) with daily precipitation records for a GPM grid cell over Montpellier (\(0.1^{\circ}\times 0.1^{\circ}\)), and daily temperature records (minimum and maximum temperature) at a local station in Montpellier, Occitanie region, France. part due to three mm of precipitation recorded one day prior to the S1-SSM map, according to the GPM data (Figure 3b). A general decrease in the soil moisture values throughout the region was observed between 27 December 2016 (Figure 3b) and 2 January 2017 (Figure 3c), as no precipitation was recorded. A continuous lack of precipitation for 12 days from 27 December 2016 until 8 January 2017 caused soil moisture values to drop to less than 15 vol.% on 8 January 2017 in the whole region, as shown in Figure 3d. Six days later on 14 January 2017 (Figure 3e), soil moisture values remained low at the east, as no precipitation was recorded for an additional six days, whereas a moderate increase in soil moisture values was observed in the middle and western parts (Tarn, Haute-Garonne, and Gers), corresponding to precipitation between 8 January 2017 and 14 January 2017 (a cumulative rainfall increase of five to 20 mm). Twenty-four days with no precipitation recorded in the Herault and Gard region between 27 December 2016 and 20 January 2017 caused the soil moisture values to drop more, with values less than 15 vol.% in this area on 20 January 2017 (Figure 3f). In addition, for the Gers department, low soil moisture values (less than 15 vol.%) were estimated on 20 January 2017 and high cumulative precipitation was recorded between 14 and 20 January 2017 (15 mm) (Figure 3f). This low soil moisture estimates could be linked to the presence of frozen soil in the area. In a recent study using Sentinel-1 SAR data, [PERSON] et al. (2017) showed that a decrease of at least three dB in the radar backscattered signal can be observed over frozen soil conditions. Such a decrease in the SAR signal yields an underestimation of soil moisture in the S1-derived SSM maps. In our case, it seems that the low soil moisture values for this area on 20 January 2017, although with the presence of rainfall, were due to frozen soil conditions. To support our assumption, the temperature curve and precipitation records for a local meteorological station in Auch city located in Gers-Occitanie were analyzed (Figure 4). On 19 January 2017 (S1 acquisition over Gers on 19 January 2017 17:56 UTC), the temperature varied between -8 \({}^{\circ}\)C and 2 \({}^{\circ}\)C with a mean value of -3 \({}^{\circ}\)C throughout the day. Three days before (15 and 16 January), 15 mm of cumulative precipitation was recorded.