# Mapping irrigated Areas Using Sentinel-1 Time Series in Catalonia, Spain
## 3 Methodology
### \(\sigma^{\circ}\) SAR Backscattering at Plot and Grid Scale  
The detection of irrigation activities using SAR data at plot scale requires a good separation of irrigation events from rainfall events. In fact, both events are considered as water supplements and may have the same effect on the radar backscattering signal. In this case, additional information about the rainfall, such as the daily precipitation records, is required to remove the ambiguity between rainfall events and irrigation events. However, rainfall data is normally hard to be obtained freely and accurately. Thus, another indicator is required to separate irrigation events from rainfall events. Following a rainfall event, the soil becomes wet thus causing an increase of the SAR backscattering signal due to the change in the dielectric constant of the soil (25; 26; 30; 43). If all the plots within a limited spatial extent (5 km, 10 km, 20 km, etc.) show an increase in the SAR backscattering signal between two consecutive radar acquisitions, then a rainfall event could have probably occurred. Thus, an indicator of an existing rainfall event could be the increase of the SAR backscattering signal, between two consecutive SAR dates, obtained within a grid scale (5 km, 10 km, 20 km, etc.). This increase of the SAR backscattering signal at a grid cell is mainly due to increase of soil moisture due to a rainfall event. In fact, the relation between rainfall events and soil moisture has been investigated in several studies (44; 45). Most recently, [PERSON] et al. (2014) compared the soil moisture values estimated at plot scale with six days revisit time from S1 data [PERSON] et al. (2014) with GPM (global precipitation mission) rainfall data at 0.1\({}^{\circ}\)\(\times\) 0.1\({}^{\circ}\) grid scale (\(\sim\)10 km \(\times\) 10 km) (27). The study site examined was the Occitanie region of France (\(\sim\)72,000 km\({}^{2}\)). They showed that following a rainfall event occurring couple of days before the radar acquisition, the average soil moisture of plots with low vegetation cover (NDVI \(<\) 0.4) in a grid of \(\sim\)10 km \(\times\) 10 km increases. This increase in soil moisture estimation is directly related to the increase of the radar backscattering signal at grid scale. The increase of the SAR backscattering signal at plot scale accompanied with the stability or decrease of the radar signal at grid scale could be attributed to an irrigation event that took place on the plot. On the other hand, the increase of the SAR backscattering signal at the grid scale could be linked to a rainfall event. Therefore, to remove the ambiguity between rainfall and possible irrigation events a comparison is considered between the SAR backscattering coefficient at each plot and the SAR signal obtained from bare soil plots with low vegetation cover at 10 km \(\times\) 10 km grid cell containing this plot. However, this comparison may not be useful for detecting irrigated plots if the irrigation occur simultaneously for almost all plots existing within the same 10 km grid cell. In this case, an increase of the SAR backscattering signal at grid scale would be due to irrigation that took place on all the plots within the 10 km cell and not rainfall. It is also good to mention that cropland plots could encounter a  
Figure 4: Workflow overview using random forest (RF) and the convolutional neural network (CNN). change in the radar backscattering coefficient due to change in the surface roughness mainly at sowing stage (smooth) or after harvesting (moderate to rough) [48]. Nevertheless, very rough conditions in an agricultural plot are not permanent over the year and may exist only for couple of days. The temporal series of the S1 SAR backscattering coefficients over each agricultural plot was obtained by averaging the \(\sigma^{\circ}\) values of all pixels within the plot at each available date and at both VV and VH polarizations. The average of \(\sigma^{\circ}\) pixels' values reduces the speckle noise.