# Semi-Automatic Fractional Snow Cover Monitoring from Near-Surface Remote Sensing in Grassland
## 5 Results
### FSC Algorithm Accuracy Assessment  
A total of 288 processed images were compared with visual estimations. These images are part of the winter of 2017-2018 and 2018-2019. For the first winter, 184 images were processed, and a Root Mean Square Error (RMSE) of 3.43% and 6.66% FSC were obtained, compared with Expert 1 and Expert 2, respectively. The second winter had 104 images, and the RMSEs were 7.01% and 14.71% FSC (Table 3). The estimated FSC was closer to the experts' values, as they both had similar curves (Figure 12). Our pixel-based accuracy assessment showed that the overall accuracy of the images selected for validation was 92%.  
Figure 13 shows the five years of the estimated FSC from our PhenoCam. There was no distinct pattern in FSC in Oakville. Except for the winter of 2017-2018, the majority of winters showed snowfall events at the beginning and at the end that remained for a couple of days, followed by rapid snow melting.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & \multicolumn{3}{c}{**Period**} \\ \hline  & 2017–2019 & 2017–2018 & 2018–2019 \\ \hline No. Images & 288 & 184 & 104 \\ RMSE 1 (\%) & 5.02 & 3.43 & 7.01 \\ RMSE 2 (\%) & 10.32 & 6.66 & 14.71 \\ \hline \hline \end{tabular}
\end{table}
Table 3: RMSE for the FSC algorithm.  
Figure 12: Algorithm-derived FSC estimation comparison with estimated FSC values by two visual image interpretation experts (EXP_1 and EXP_2) over (**a**) winter 2017–2018 and (**b**) winter 2018–2019.  
During the winter of 2015-2016, 203 days were evaluated. The first snow was 1 December, and for 91 days, there was greater than 1% FSC, and 110 days had an FSC of 0. The last snowfall event was on 17 March. The winter of 2016-2017 had 175 images. The first snow was 22 November and the next 102 days had snow present. At the end of the season, two snowfall events occurred on 23 and 27 April. The winter of 2017-2018 had 184 images. This winter was different from the other winters in the study period because it had two snow periods, as was common in other regions (e.g. [69]). The first period was from 3 November to 23 November. Then, the snow melted for 10 days, and on 5 December, Oakville was again covered by snow for 132 consecutive days before it took just three days to melt entirely. The winter of 2018-2019 had one snowfall at the beginning and one at the end of the season. For this winter, a total of 104 images were processed. The first snowfall was 10 October then, it melted, and the next event was after 22 days. The snow melted  
Figure 13: FSC for the winters of (**a**) 2015–2016, (**b**) 2016–2017, (**c**) 2017–2018, (**d**) 2018–2019 and (**e**) 2019–2020 in Oakville. Blank spaces on the graphs related to periods of missing data (the camera did not operate or there were no suitable images).  
on 1 April, but there were two days with snow cover on 12 and 13 April. The winter of 2019-2020 has 190 images. This period had 64 snow-free days and three snowfall events before it was covered with snow for 110 consecutive days. At the end of the season, on 3 April, a snowfall remained on the grassland for seven days. Figures 14 and 15 show the obtained FSC compared with temperature, snowfall, and rainfall for the study period [63].  
Figure 14: FSC, temperature, snowfall, and rainfall for the winters of **(a)** 2015–2016, **(b)** 2016–2017 and **(c)** 2017–2018 in Oakville.