# Detecting Cover Crop End-Of-Season Using VEN\(\upmu\)S and Sentinel-2 Satellite Imagery
## 4 Results  
### Algorithm Evaluation Using an Alfafla Pixel  
A pixel in the alfalfa field (#5-9) was selected to demonstrate the WIST algorithm. This field was harvested four times during the 2019 growing season, which is a challenging case for traditional remote sensing phenology algorithms. Using the VEN\(\upmu\)S NDVI time-series, the four termination dates were all detected. As shown in Figure 5, the detected termination dates were all close to the recorded harvest dates. In-field photographs taken before and after harvest clearly showed different conditions in the field. VEN\(\upmu\)S observations captured the uptends and the downtrends in the NDVI time-series each time before and after the harvests. The senescence onsets were detected immediately following the peak NDVI dates when the downtrend started. The dormancy dates were close to the dates of the lowest NDVI. The decreasing momentums for four senescent periods ranged from 0.026 to 0.060, well above the threshold value used in the WISE approach (0.010), which means they were all significant downtrends. The uncertainty of the termination estimation ranges from 2-8 days, depending on the number of available cloud-free observations before and after the harvests.  
The first harvest operation at field #5-9 in 2019 happened between two VEN\(\upmu\)S acquisitions on 6 May 2019, and 10 May 2019. The WIST approach estimated the termination date on 8 May 2019, with an uncertainty of 2 days, which agrees with the recorded harvest operation date on 7 May 2019. For the second harvest on 20 June 2019, the downtrend and two cloud-free observations were found on 15 June and 1 July 2019. The estimated termination date was on 23 June, with an uncertainty of 8 days. The higher uncertainty was due to the lack of cloud-free observations around the harvest time. The third harvest was on 27 July. The estimated termination date was on 26 July (between two clear acquisitions on 25 and 27 July) with an uncertainty of 1 day. The harvest date was captured very accurately due to the cloud-free observations 1 day before and after harvest. The fourth harvest happened on 6 September 2019. The estimated termination date was 9 September, with an uncertainty of 6 days estimated from the two cloud-free observations on 3 September and 15 September. For this alfalfa field, the WIST approach captured all harvest dates correctly using the VEN\(\upmu\)S time-series.  
Using the Sentinel-2 NDVI time-series in the same location, WIST detected three terminations (see Figure 6) that were close to the last three harvest dates. However, the first harvest on 7 May 2019, was missed because there was no clear Sentinel-2 observation from 7 May to 14 June 2019. Sentinel-2 NDVI was 0.84 on 6 May 2019, and 0.89 on 15 June 2019. Sentinel-2 did not capture the decreasing trend of NDVI around the first termination. Both momentum of MACD (0.001) and amplitude of NDVI (0.07) are well below the thresholds for a significant downtrend (0.010 for momentum and 0.15 for amplitude). The WIST algorithm produced good results for both VEN\(\mu\)S and Sentinel-2 for harvests that were detected even though VEN\(\mu\)S NDVI values were higher than Sentinel-2. The WIST algorithm depends on the NDVI trend, and thus the temporal consistency is more important than actual values. The alfalfa example also demonstrates that the detection of termination depends on the availability of clear observations.