# Near Real-Time Irrigation Detection at Plot Scale Using Sentinel-1 Data
## 4 Results
### Results over Tarbes  
The proposed method was applied over irrigated plots in Tarbes, south-west France. Figure 11a represents the \(\sigma_{\rm p}^{0}\) and \(\sigma_{\rm G}^{0}\) in the morning acquisition mode for an irrigated maize plot with the irrigation points detected using the proposed change detection method. While Figure 11a shows the complete series between 01 March and 30 November 2017, Figure 11b represents the period where the irrigation points were detected. However, irrigation dates were not available over this site so a qualitative analysis was performed to show the performance of the method. During the period between 01 March 2017 and the end of June 2017 (Figure 11a), the \(\sigma_{\rm p}^{0}\) and \(\sigma_{\rm G}^{0}\) followed the same behavior for almost all the dates. Following a rainfall event both curves increased and then decreased following a period without rainfall. This consistency could be related to the absence of any additional water supplement on the plot scale. However, between March and mid-May, the NDVI values were low (less than 0.2) indicating bare soil conditions. This also supports the possibility that no irrigation had occurred for this period. The NDVI then started to increase showing a vegetation cycle between May and October. Using the proposed method for detecting irrigation events, four irrigation points were detected over this plot (Figure 11b). In general, all these irrigation points corresponded to an important increase of \(\sigma_{\rm p}^{0}\) accompanied with absence of rainfall events as shown by the GPM data (blue curve) and the \(\sigma_{\rm G}^{0}\)-values at grid scale (pink curve). For example, an important cumulative rainfall of approximately 40 mm three days before the SAR image acquired on 28 June 2017 caused \(\sigma_{\rm G}^{0}\) to increase by 2.5 dB. Six days later, no rainfall events were registered and the \(\sigma_{\rm G}^{0}\) decreased by 1.5 dB. For the same period, \(\sigma_{\rm P}^{0}\) increased by 1 dB. This point was the first irrigation point detected on the plot. Similarly, between 10/07 and 16/07, \(\sigma_{\rm G}^{0}\) decreased by 2.1 dB due to the absence of precipitation while \(\sigma_{\rm P}^{0}\) increased by 1.1 dB indicating that a water supplement could have occurred. The absence of precipitation for 12 days between 22/07 and 04/08 caused \(\sigma_{\rm G}^{0}\) to decrease gradually. However, the SAR image acquired on 04/08 showed an increase of the SAR signal at plot scale which was evidence of an irrigation event occurring. Likewise, between 15/08 and 21/08 the \(\sigma_{\rm P}^{0}\) increased by 1.5 dB indicating the presence of irrigation event along with a stability of low \(\sigma_{\rm G}^{0}\) (\(\sim\)\(-\)11.5 dB) indicating the absence of any rainfall events. After the fourth detected point on 21/08, the NDVI started to decrease. Usually during this phase irrigation events rarely occur. After 02/09, the \(\sigma^{\circ}\) values at both plot and grid scale regained their consistent behavior indicating the absence of irrigation. Finally, it is important to mention that the summer season in Tarbes is humid and encountered several rainfall events. Figure 10: Comparison between accuracy metrics obtained for classification of irrigated and non-irrigated plots using all the detected irrigation events (red bar) and the detected irrigation events between April and September 2018 (blue bar). (**a**) Overall accuracy, (**b**) F-measure of irrigated class, (**c**) F-measure of non-irrigated class and (**d**) weighted F-measure. Figure 12 presents the histogram of the distribution of the number of events detected over irrigated plots in Tarbes for morning acquisition (Figure 12a), evening acquisition (Figure 12b), intersection between morning and evening acquisitions (Figure 12c) and the combination of both acquisitions (Figure 12d).