# Detecting Cover Crop End-Of-Season Using VEN\(\upmu\)S and Sentinel-2 Satellite Imagery
## 4 Results
### Field-Level Results from Sentinel-2  
Using the same fields, we applied the WIST approach to the Sentinel-2 NDVI time-series. Figure 9 shows the termination dates from Sentinel-2 and the recorded harvest/termination dates. Comparing to results from VENuS in Figure 7, the Sentinel-2 time-series captured most of the terminations such as fields #4-25 (Figure 9c), #5-45 (Figure 9d), #5-2 (Figure 9f). However, fields #1-37 (Figure 9a) and #1-11 (Figure 9e) detected additional termination dates to those shown in Figure 7. For field #1-37 (Figure 9a), undetected clouds (not detected in the cloud mask algorithm) caused lower NDVI values and led to two downtrends and two false terminations. For field #1-11 (Figure 9e), a small peak in late June was caused by the regrowth of grasses before soybeans were planted on 28 June 2019. The decreasing momentum for the small peak was 0.022 from Sentinel-2. However, from the VENuS  
Figure 6: Three terminations were detected for the same alfalfa pixel as in Figure 5 (field #5-9) using the 2019. Sentinel-2 time-series. The first harvest on 7 May 2019, was missed since there were no clear Sentinel-2 observations from 7 May 2019 to 14 June 2019.  
time-series, the momentum of MACD was smaller than 0.01, and the amplitude of NDVI was less than 0.15. This implies that the threshold of termination may be sensor dependent. Field #5-8 (Figure 9b) captured two terminated events but missed the first one on 20 May 2019, due to the lack of cloud-free observations, similar to field #5-9 in Figure 6. The closest observations around the first moving date were 6 May 2019, and 15 June 2019 (lapse of 40 days). Grasses in the 15 June acquisition (25 days after moving) have regrown. This example further demonstrates that frequent observations are critical for detecting cover crop termination.  
Using the Sentinel-2 time-series, 27 harvest/terminations were correctly generated across 14 cover crop fields in 2019 and 2020. Two harvests (~7%) were missing due to the lack of observations. Three terminations (~10%) were falsely detected. Figure 10 shows the detected termination dates from Sentinel-2 compared to the recorded harvest operation dates. Even though Sentinel-2 revisit (5 day) was less frequent than VEN\(\upmu\)S (2 day), it still provided a good estimation of termination dates. Nevertheless, uncertainties (vertical bars) from Sentinel-2 are larger than VEN\(\upmu\)S. The largest uncertainty (20 days) was from field #5-8 due to the large gap (40 days) of cloud-free observations from 8 September 2019 (day 251) to 18 October 2019 (day 291), even though the termination estimate of 28 September 2019, was reasonable (harvest operation on 26 September 2019). Two terminations were not detected due to the lack of clear Sentinel-2 observations, and the downtrend was not significant, as in Figure 9b. Lowering the two thresholds for the MACD momentum and the NDVI amplitude can detect more termination dates; however, this may introduce more false detections. Ground information will be needed to determine appropriate thresholds. More discussion on the topic of thresholds is provided in Section 5.  
Figure 7: Detected termination dates (red squares) from the VEN\(\upmu\)S time-series and the recorded harvest operation (vertical red lines) for six typical fields ((**a**–**f**), field name in the upper left) at the BARC with cover crops.  
Figure 8: Scatter plot of the detected termination dates for fields at the BARC in 2019–2020 using the VEN\(\mathrm{\SIUnitSymbolMicro S}\)NDVI time-series and the recorded harvest/termination operation dates. The vertical bar shows the uncertainty of the remote sensing estimated termination date.  
Figure 9: Detected termination dates (red squares) from the Sentinel-2 time-series and the recorded harvest operation (vertical red lines) for six typical cover crop fields (**(a–f)**, field name in the upper left) in BARC.