## 3 Data Analysis  
### Comparison between S1-SSM and Precipitation  
A first analysis of soil moisture variation according to precipitation events was performed. For a grid cell of the GPM product (0.1\({}^{\circ}\)\(\times\) 0.1\({}^{\circ}\)) over Montpellier (Figure 1), GPM daily cumulative precipitation records were obtained over one year between September 2016 and August 2017. Then, at each available S1-SSM map between 1 September 2016 and 31 August 2017, mean soil moisture values were calculated for the corresponding GPM grid cell. Figure 2 shows the temporal variation in the mean soil moisture values, the daily precipitation records, and the daily air temperature records (minimum and maximum daily air temperature). A global consistency is observed between the S1-SSM values and precipitation records. Figure 2 shows that after a precipitation event, high soil moisture values are estimated, whereas with the absence of precipitation over a period of time, the soil dries due to evaporation, and S1-SSM values generally decrease. For example, an increase in soil moisture values from 16 vol.% on 15 November 2016 to 25 vol.% on 21 November 2016 was observed due to 40-mm precipitation recorded on 20 November 2016. Also, soil moisture values increased from 19 vol.% on 9 December 2016 to 25 vol.% on 15 December 2016 due to 17 mm of precipitation recorded on 15 December 2016. Moreover, the absence of precipitation along with evaporation (maximum temperature between 22 \({}^{\circ}\)C and 25 \({}^{\circ}\)C) between 1 April 2017 and 25 April 2017 caused soil moisture values to drop down below 15 vol.%, followed then by a sharp increase in soil moisture values on 26 April 2017 (24 vol.%) due to rainfall event occurring on 25 April 2017 (13 mm). On the other hand, 18 mm of precipitation on 4 March 2017 did not highly affect the soil moisture values on 9 March 2017 (18 vol.%) due to the five-day difference between the rainfall date and the following S1-SSM date. For the period between May and August, the increase of temperature (and consequently the evaporation) didn't cause a sharp decrease in SSM values. This is probably due to the existence of irrigation activities for summer crops. According to the six-day revisit time of the Sentinel-1 sensor, it is difficult to quantitatively analyze the correlation between the precipitation and the soil moisture, because this correlation also depends on other parameters such as the evaporation rate, the soil texture, and the previous state of soil before rainfall events. For this reason, the correlation coefficient R was only calculated between the average annual soil moisture for each department and the corresponding annual precipitation (Figure 1). A correlation coefficient R of 0.89 was obtained between the mean annual soil moisture at each department and the average annual cumulative precipitation of each department. The temporal analysis of the S1-SSM values along with GPM data was then studied during the period between 21 December 2016 and \(1\) February 2017. Different climatological events were observed during this period: the absence of precipitation for 24 consecutive days in some parts of the region between 27 December 2016 and 25 January 2017 and rain episodes at several dates (Figure 2). The objective here is to analyze the temporal evolution of S1-derived SSM as a function of cumulative rainfall events (existence or absence of precipitation). As mentioned in Section 2.2.1, an S1 soil moisture map can be obtained every six days (during the revisit of the S1 satellite). Thus, a total of eight soil moisture maps acquired between 21 December 2016 and 1 February 2017 were used. For these maps, soil moisture values were classified into four different intervals that were found to best describe the spatiotemporal variation in soil moisture values. To perform the temporal evolution analysis, first, for each S1-SSM map date, one cumulative rainfall map for the six days prior to the S1-SSM map date was computed by summing the 30-minute time interval of the late IMERG-GPM product.