## 4 Results
### Assimilation of Sentinel-1 Derived SSM  
Figure 6 is the same as Figures 3 and A2 but the irrigation retrievals are based on the SSM products derived from S1 data. The statistical metrics are reported in Table 2 for the flooded fields. Figure 7a displays the scatter plots of the 15-day cumulative amounts for the drip irrigated fields and Figure 7b reports the estimated seasonal amounts against observation for all fields. Data with a 6-day revisit time were available for D1 and R1 during 2016-2017 and 2017-2018. By contrast, only one observation every 12 days was available over F2 and F3 in 2015-2016 as already underlined. The results obtained with the S1 products are close to the one based on in situ measurements to some extent: (1) the SSM open loop (without irrigation inputs) does obviously not match the observations while the analyzed SSM is in good agreement with the SSM S1 products for irrigated fields, and to a lesser extent, for the rainfed seasons as well; this means that the algorithm is able to identify at least one irrigation scenario to be able to match the observed SSM; (2) the overall distribution of the irrigation events during the season are properly reproduced; see for instance D1 where almost no irrigation is detected after the farmer decided to stop irrigation apart from some false events detected in early March in 2017-2018 occurring concomitantly with rainfall. This is attributed to an overes \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **3 Days** & **6 Days** & **12 Days** \\ \hline R & 0.77 & 0.74 & 0.65 \\ RMSE (mm/15 days) & 23.6 & 24.8 & 27.1 \\ bias (mm/15 days) & 0.24 & 2.3 & 2.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Statistical metrics for the 15-day cumulative amounts over the drip irrigated fields when discarding data of D2 during the 2017–2018 season, for the three 3-, 6- and 12-day window lengths. Figure 5: Retrieved seasonal sum of irrigation versus observed seasonal sum of all the studied fields and seasons for the three window lengths. timation of the retrieved SSM with regards to in situ data at this time; (3) there are some compensations between some undetected events and some others that are falsely detected leading to acceptable retrieved seasonal amounts; the false detection in early March on D1 2017-2018 (see above) compensates for the underestimation of the retrieved irrigation amounts during the early stage of the season; likewise, false events detected at the end of season on F2 and F3 were compensated by the undetected event in March; (4) finally, only one irrigation is detected for the rainfed field during the 2017-2018 in line with the results obtained by using the in situ data. Figure 6: Time series of the daily estimated irrigation (Irrig\({}_{\rm PF}\)), the actual irrigation (Irrig\({}_{\rm Truth}\)) and measured rainfall. Time series of the assimilated (SSM\({}_{\rm PF}\)), S1–derived (SSM\({}_{\rm obs}\)) and the open loop (SSM\({}_{\rm OL}\)) SSM over the drip-irrigated (D1) and rainfed (R1) field for both seasons 2016–2017 and 2017–2018 for a window length of 6 days and the flood-irrigated fields F2 and F3 during 2015–2016 season at 12-day window length. The overall statistics are only slightly lower than those obtained with the in situ observations. The 15-day cumulative amount for the drip field D1 is retrieved with an R of 0.64 (R = 0.72 when using in situ SSM with a 6-day revisit time); RMSEs are close and bias is even lower. The irrigation retrieval is even better with S1 data for D1 during the season 2016-2017 (R = 0.80 using S1 SSM vs. 0.77). By contrast, the results obtained on F2 and F3 suffered from the 12-day revisit time of the available SSM products in line with the results obtained previously both based on the synthetic experiment and on the in situ SSM using the same revisit time. Overall, the detected events at the start and at the end of the actual irrigated period are in good agreement with the actual irrigation timing and amounts while some false events are detected at the end of the season in May and June.