# Near Real-Time Freeze Detection over Agricultural Plots Using Sentinel-1 Data
## 4 Results
### Detection of Frozen/Unfrozen Plots
#### 4.2.1 Freezing Detection Results over Site A  
Figure 7 shows the detection of frozen episodes over three plots (one for each land cover class) in site A (near Paris, France), using VH and VV polarizations. For the three land cover classes, we observe that the majority of detected freezing episodes across all land cover classes happened over the period that extends from November to February, which corresponds to the coldest months of the year, with temperatures (t) sometimes reaching 0 \({}^{\circ}\)C or lower. There was also one freezing episode that happened mid-April where the temperature dipped below 0 \({}^{\circ}\)C (t = -1.2 \({}^{\circ}\)C), and another potential freezing episode in late March with a temperature close to 0 \({}^{\circ}\)C (t = 2.9 \({}^{\circ}\)C). The majority of the anomalies were detected either before the freezing period (October), or at its end (mid-February onwards) and this was observed for both VH, and VV polarizations. These anomalies are caused by a combination of rainfall events occurring days or even weeks before the detection date of the anomalies, and the vegetation growth period, which starts in February. Irrigation events can equally contribute to the detection of anomalies; however, their effect remains much weaker since irrigation events affect some plots only from April. In addition, Figure 7 shows that the anomalies are more frequent in VV that in VH. Over the entire plots in Site A, the percentages of frozen plots for each Sentinel-1 acquisition date and each land cover class (LC) are presented in Figure 8 for VH and VV polarizations. Figure 8a-c show that the true freezing episodes in VH polarization occurred between the period extending  
Figure 7: Freezing detection over one reference plot by land cover class in site A, for each S1 acquisition date, using VH and VV polarizations. (**a**) VH-LC1, (**b**) VH-LC2, (**c**) VH-LC3, (**d**) VV-LC1, (**e**) VV-LC2, (**f**) VV-LC3. from November to mid-February, and then from late March until mid-April, and for the three land cover classes. The results also show that the detected freezing episodes are strongly correlated with the temperature. Indeed, five of a total of eight dates that were detected as having a freezing episode had temperatures below 0 \({}^{\circ}\)C. Of these five freezing episodes, there are two dates (mid-November and mid-December) where the percentage of frozen plots was below 50% across the three land cover classes. For example, LC1 and LC2 report less than 20% of frozen plots for mid-November. For temperatures between 0 and 3 \({}^{\circ}\)C, only three freezing episodes were detected, with one occurring in early November (t = 0.4 \({}^{\circ}\)C) where the percentage of freezing plots was higher than 75% for LC1 and LC2, and 39% for LC3. For the two remaining freezing episodes, which occurred in late February and late March, the percentage of freezing plots was below 40% for LC1, and LC2, and below 60% for LC3, and the temperature averaged 2.9 \({}^{\circ}\)C. It is worth noting that while the results of LC3 are somewhat inconsistent with the findings for LC1 and LC2, this could be due to the low number of plots corresponding to LC3 (29 plots against 793 plots for LC1 and 179 plots for LC2) in site A, and also due to the lower thresholds used for LC3 in comparison to LC1, and LC2 (Table 1). During the freezing period ascertained through low temperatures (November through February), the anomalies are less frequent. For LC1, LC2, and LC3 in VH polarization (Figure 8a-c), there was only one date (27 January) which had, respectively, \(\sim\)37%, \(\sim\)17%, and 36% of plots falsely detected as frozen. There are four other dates with false detections in LC3 (three in November and one around mid-December); however, the percentage of plots with anomalies is less than 16%. The percentage of plots detected as having anomalies at periods when there was freezing occurrence (temperatures higher than 3 \({}^{\circ}\)C) is in most cases lower than 50% for the three land cover classes (October, and March onwards).