# Near Real-Time Freeze Detection over Agricultural Plots Using Sentinel-1 Data
## 5 Discussion
### Frozen Mapping Qualitative Analysis  
Even if Figure 15 shows that the global mapping of frozen surfaces is coherent with the temperatures, some plots are detected as unfrozen with surrounding severely frozen plots. On 27 December 2018, 19% of plots in Site A (Figure 15a) were detected as unfrozen, while the temperatures showed favorable freezing and severe freezing conditions. On this date, 43% of the plots were detected as being moderately frozen, and 38% as severely freezing. While it is not uncommon for some plots to actually be unfrozen at a temperature of \(-2.7^{\circ}\)C, the question pertaining to the relevance of mapping unfrozen plots around severely frozen plots arises. For all the plots detected as unfrozen, the radar signal on December 27 has decreased significantly but did not cross the calculated thresholds in order to be detected as frozen. In fact, 50% of the plots detected as unfrozen in LC1 registered a drop in the radar signal between 2.8 and 3.48 dB (at most 0.7 dB from the chosen freezing threshold, which was set at 3.48 dB). Moreover, 34% of the plots detected as unfrozen in LC1 registered a drop in the radar signal between 2.3 and 2.8 dB (at most 1.2 dB from the selected threshold). As for LC1, the same patterns also appeared for LC2, and LC3. For LC2, and LC3, 50% of the plots detected as unfrozen were only 0.7-0.8 dB below the chosen detection threshold values, and 34% were 1.3-1.4 dB below the thresholds. Therefore, only 16% of the plots mapped as unfrozen on 17 December 2018 have a weak signal decrease. These plots, which correspond to only 3% of the total number of plots within site A, could be falsely detected as unfrozen, most probably due to variations in soil roughness (plots tilted a few days before the mapping date, which causes an increase in soil roughness and consequently of the radar signal). Other factors, such as the soil type or the presence of hedges, can also explain in part these behaviors. The vegetation effects are less probable to cause any misdetections due to the low vegetation growth at this time period.  
On 22 January 2019, 12% of the plots within site B (Figure 15c) were detected as being unfrozen, while the temperatures were well below \(-5\ ^{\circ}\)C, which suggests severe freezing conditions. On this date, 26% of the plots were detected as being moderately frozen and 62% as being severely frozen. Overall, 50% of the plots in LC1 which were detected as unfrozen showed a drop in the radar signal between 2.5 and 3.48 dB. However, since the calculated threshold for LC1 was 3.48 dB, these plots could not be classified as frozen (the radar signal drop was at most 1 dB below the threshold). Moreover, 34% of the plots detected as unfrozen in class LC1 registered a decrease in the radar signal between 1.7 and 2.5 dB (at most 1.8 dB below the selected threshold). For LC2 and LC3, 50% of the plots detected as unfrozen were 0.7 dB below the calculated thresholds, and 34% were below the calculated thresholds by 1.2 to 1.3 dB. As such, only 50% of the plots which were detected as unfrozen on 22 January 2019 had a radar signal decrease below the detection threshold of less than 1 dB forLC1, and of less than 0.7 dB for LC2 and LC3, which corresponds to around 6% of the total number of plots within site B. These weak drops in the radar signal for certain plots could be explained, as in site A, by an increase in the soil roughness several days before the acquisition of 22 January 2019. Finally, the results presented in Figure 15b,d, do not show any aberrant detections, as we only detected unfrozen to mild-to-moderately frozen plots, which is consistent with temperatures around 0 \({}^{\circ}\)C.  
Figure 15: Maps of the frozen/unfrozen plots over site A (**a**, **b**) and site B (**c**, **d**): (**a**) 27 December 2018; (**b**) 25 February 2019; (**c**) 22 January 2019; (**d**)25 January 2019.