# Near Real-Time Irrigation Detection at Plot Scale Using Sentinel-1 Data
## 3 Methodology
### Global Overflow for Irrigation Event Detection  
Figure 3 presents a detailed overflow of the proposed tree-based change detection methodology. For a given plot and at a given SAR image acquired at time \(t_{i}\), seven main indicators could be extracted for the plot:  
* \(\Delta VV_{P}\) : Change in SAR signal at plot scale
* \(\Delta VV_{G}\) : Change in SAR signal at grid scale
* \(S\): Smoothed vegetation descriptor \(S\)
* \(SSM_{P}\): SSM value at plot scale
* \(SSM_{G}\): SSM value at plot at grid scale containing this plot
* \(NDVI_{li}\): NDVI value at time \(t_{i}\)
* \(\Delta NDVI\): Vegetation growth indicator  
The chain starts with the \(\Delta VV_{P}\) value where a value less than \(-0.5\) dB is considered as the non-irrigation point. If the value of \(\Delta VV_{P}\) is greater than \(-0.5\) dB, the smoothed vegetation descriptor \(S\) is then checked for being positive and the point is considered as non-irrigation if \(S<0\). If \(S\) is positive, the \(SSM_{P}\) is then checked for the threshold value of \(15\) vol.% and the point is considered as non-irrigation if the \(SSM_{P}<15\) vol.% with \(NDVI_{li}\leq 0.5\) (both conditions should occur simultaneously). When the studied point arrives to pass by all the previous filters, the chance of having a water supplement (irrigation or rainfall) increases, and thus the change of SAR signal at grid scale is required at this stage in order to eliminate the irrigation-rainfall ambiguity. Filters applied at grid scale could be divided into four main cases:  
* **Case i:** If \(\Delta VV_{G}\geq 1\) dB then a rainfall event have occurred and the point is not an irrigation point. * **Case ii:** If \(SSM_{G}>20\) vol.% then a rainfall event probably occurred before and there is low chance to have an irrigation event (humid soil conditions at basin scale). * **Case iii:** If \(0.5\leq\Delta VV_{G}\leq 1\) dB we check the value of \(\Delta VV_{P}\) for two cases:  
* **Case iii:1**: If \(\Delta VV_{P}\leq 0.5\) then no irrigation took place. * **Case iii:2**: If \(\Delta VV_{P}>0.5\) and \(\Delta\ (\Delta VV_{P}-\Delta VV_{G})\geq 1\) then it is considered as irrigation point with high certainty. * **Case iv**: If \(\Delta VV_{G}\leq 0.5\) dB then we check the \(\Delta VV_{P}\) for four different cases: \(\blackdiamond\)**Case iv.1**: \(\Delta VV_{P}\geq 1\) dB then the point is an irrigation point with high certainty. \(\blackdiamond\)**Case iv.2**: \(0.5\leq\Delta VV_{P}<1\) dB then the point is an irrigation point with medium certainty if and only if \(SSM_{P}\geq 20\) vol.% OR \(\Delta\ \geq 1.5\) dB. \(\blackdiamond\)**Case iv.3**: \(0\leq\Delta VV_{P}<0.5\) dB then the point is an irrigation point with low certainty if and only if \(SSM_{P}\geq 20\) vol.% OR \(\Delta\ \geq 2\) dB. \(\blackdiamond\)**Case iv.4**: \(-0.5\leq\Delta VV_{P}<0\) dB then the point is an irrigation point with low certainty if and only if \(SSM_{P}\geq 20\) vol.% AND the previous point at \(t_{i-1}\) is a high certainty irrigation point or a rainfall point (\(\Delta VV_{G}\geq 1\) dB). A certainty indicator of irrigation (high, medium and low) is also associated for each detected irrigation point. The high certainty irrigation points are associated for the points considered as irrigation points from cases iii.2 and iv.1 (significant increase in \(\Delta VV_{P}\)). For the points of case iv.2 a medium certainty is associated, while low certainty is considered for irrigation points detected from cases iv.3 and iv.4. The implementation of the proposed method in a near real-time scenario depends principally on the delivery time of S1 images. S1 images are usually delivered by ESA in a \"fast 24 h\" delivery  
Figure 3: Workflow overview using the tree-based classification for irrigation detection at plot scale. mode. This mode insures that the S1 image is available for download 24 h after the satellite acquisition.