# Near Real-Time Irrigation Detection at Plot Scale Using Sentinel-1 Data
## 3 Methodology
### \(\sigma^{\circ}\) SAR Backscattering at Grid Scale  
The increase in the SSM values could not only be attributed to an irrigation event but also to rainfall events that are the main contributor in the SSM variation. Then, a good separation between rainfall events and irrigation events should be also performed. Actually, rainfall events and irrigation events are both considered as water supplements and thus may have the same effect on the value of \(\Delta VV_{P}\). For example, the threshold value of \(\Delta VV_{P}\geq 1\)dB (presented in Section 3.2) could be a result of a rainfall event and not only an irrigation event. Therefore, the ambiguity between rainfall and irrigation is the principal factor to be resolved for a good detection of irrigation events. Thus, better detection of irrigation events requires information about rainfall. In this study, information about rainfall has been determined through the \(\sigma^{\circ}\) value obtained at grid scale (\(10\) km \(\times\) 10 km). We assume that if the bare soil plots with low vegetation cover within the spatial extent of \(10\) km \(\times\) 10 km show an increase of the SAR backscattering signal (increase in SSM values) between two consecutive radar acquisitions, a rainfall event probably occurred. This correlation between rainfall and \(\sigma^{0}_{G}\) has been presented by [PERSON] et al. [33] where they compared SSM estimations at bare soil plots in 10 km grid scale to rainfall events at the same scale and concluded that strong consistency exists between rainfall events and SSM values at 10 km grid scale. Moreover, [PERSON] et al. [12] used the SAR signal at grid scale conjointly with the SAR signal at plot scale to map irrigated areas at plot scale. They reported that the use of \(\sigma^{0}_{G}\) has remarkably improved the classification accuracy of irrigated/non-irrigated plots by 15%. The SAR backscattering coefficients at grid scale are obtained by averaging \(\sigma^{\circ}\) values of all bare soil pixels within each grid cell (\(10\) km \(\times\) 10 km). A mask for bare agricultural soil pixels has been determined using first a land-cover map to delineate only agricultural areas (excluding urban, forests... ) and then a threshold applied over the NDVI values obtained from S2 images. For this reason, the land-cover map proposed by [PERSON] et al. [34] is used for the two French sites while the agricultural plots of the SIGPAC data have been used for the Catalonia site. A maximum NDVI value of 0.4 is fixed to extract bare soil pixels with low vegetation cover. Thus, at each SAR date, and for each grid cell a \(\sigma^{0}_{G}\) value is obtained describing the \(\sigma^{\circ}\) backscattered from bare soil pixels of agricultural areas only within each grid cell (\(10\) km \(\times\) 10 km). At a given date, the change in SAR \(\sigma^{\circ}\) at each 10 km cell could be obtained through the difference of \(\sigma^{\circ}\) value at the given date \(t_{i}\) and the \(\sigma^{\circ}\) at the first previous date \(t_{i-1}\)  
\[\Delta VV_{G}=VV_{Gti}-VV_{Gti-1} \tag{2}\]  
where \(VV_{Gti}\) is the \(\sigma^{0}_{G}\) in VV polarization at the present SAR date and \(VV_{Gti-1}\) is the \(\sigma^{0}_{G}\) in VV polarization at the first previous date. Using a threshold value of \(\Delta VV_{G}\), rainfall events could be determined and irrigation events detected in Section 3.2 could be filtered. First, the grid based filters are only applied in the case where \(\Delta VV_{P}\) has a value more than or equal to \(-0.5\) (Section 3.2) (probable irrigation event). We consider that if the \(\Delta VV_{G}\) is greater than 1 dB then a rainfall event occurred and, therefore, there is no chance of irrigation detection regardless of the value of \(\Delta VV_{P}\). This filter helps reduce an important part of the ambiguity between rainfall and irrigation. Next, we consider that if the value of \(\Delta VV_{G}\) is between 0.5 and 1 dB (low rainfall possibility), then a probability of irrigation can exist based on the value of \(\Delta VV_{P}\).