# Near Real-Time Irrigation Detection at Plot Scale Using Sentinel-1 Data
## 5 Discussion
### Effect of NDVI Optical Filter  
Throughout the study, an optical post-processing filter has been suggested to ameliorate the detection of irrigation events and remove the ambiguity between the increase in the SAR backscattering signal due to irrigation events and soil work. The role of the NDVI filter is to insure that a vegetation growth cycle truly exists within or after the detected irrigation point. As presented in Section 3.6, the NDVI optical filter suggests that if the \(NDVI_{ti}\) (NDVI value at a detected irrigation event) is less than 0.4 and \(\Delta NDVI\leq 0.1\) then the point is a falsely detected irrigation point, and is further eliminated. For \(NDVI_{ti}\) greater than 0.4, the filter was no longer applied because the vegetation cover is already developed (NDVI \(>\) 0.4) and the existence of irrigation event is more probable. To assess the importance of this NDVI optical filter, we obtain first the results over Catalonia without applying the NDVI optical filter. Then we apply the NDVI optical filter using two threshold values on \(NDVI_{ti}\) accompanied with the criterion \(\Delta NDVI\leq 0.1\). The filter was first applied using \(NDVI_{ti}\leq 0.2\) and then applied using \(NDVI_{ti}\leq 0.3\). The results obtained previously with \(NDVI_{ti}\leq 0.4\) were also used in this comparative analysis. For each case, the threshold of \(\Delta NDVI\) was kept equal to 0.1. To present the effect of this NDVI optical filter, we intended to monitor the change in the percentage of plots that did not encounter any irrigation event for both irrigated and non-irrigated plots (0 detected irrigation events). The class with no detected irrigation events was chosen for this analysis since it describes the capability of discriminating between irrigated and non-irrigated plots. Figure 13 presents the evolution of the percentage of the plots that did not register any irrigation event as a function of the value of \(NDVI_{ti}\) used in the optical filter for both morning (Figure 13a) and evening (Figure 13b) acquisition modes. For the morning acquisition mode, Figure 13a shows that when the NDVI filter is not applied only 29% of the non-irrigated plots have no detected irrigation events. When adding the NDVI filter using \(NDVI_{li}\leq 0.2\) (\(\Delta NDVI\leq 0.1\)), this percentage increases to reach 44%. The optical filter applied at \(NDVI_{li}\leq 0.3\) increases the percentage of non-irrigated plots with no detected irrigation events to reach 59%. Finally, using the NDVI optical filter at \(NDVI_{li}\leq 0.4\), 68% of the non-irrigated plots had no detected irrigation events. Beyond 0.4, the filter was no longer applied since vegetation already exists and the occurrence of an irrigation event was more probable than soil work. For irrigated plots in morning acquisition (Figure 13a), the percentage of irrigated plots with no detected irrigation events was 4% when no NDVI filter was added and slightly increased to 12% as the NDVI optical filter with \(NDVI_{li}\leq 0.4\) was used. Therefore, when applying the NDVI filter using \(NDVI_{li}\leq 0.4\) in the morning acquisition mode, the percentage of the non-irrigated plots with no detected irrigation events significantly increased by 39% (68-29%) compared to the percentage obtained when no NDVI filter was used. The percentage of the irrigated plots with no detected irrigation events increased only by 8% (12% minus 4%) compared to that obtained when no NDVI filter was used. For the evening acquisition mode (Figure 13b) the results show that the percentage of non-irrigated plots with no detected events increased from 22% when no filter was applied to reach 35%, 49% and finally 58% when the \(NDVI_{li}\leq 0.2\), \(NDVI_{li}\leq 0.3\) and \(NDVI_{li}\leq 0.4\) were used respectively. For the irrigated plots in evening acquisition, the percentage increased from 3% when no filter was applied to reach 9% when NDVI optical filter was applied at \(NDVI_{li}\leq 0.4\).