# Detecting Cover Crop End-Of-Season Using VEN\(\upmu\)S and Sentinel-2 Satellite Imagery
## 4 Results
### Field-Level Results from VEN\(\mu\)S  
NDVI values in cover crop fields were aggregated at field-level using the defined field polygons in Figure 3. The WIST approach was applied to the cover crop fields using the VEN\(\mu\)S NDVI time-series in 2019 and 2020. Figure 7 shows the VEN\(\mu\)S NDVI time-series, termination dates (red squares), and the recorded harvest/termination operation dates (vertical red lines) in 2019 for six selected fields. Among the chosen fields, #1-37 (Figure 7a) and #5-8 (Figure 7b) were grasses and were moved 2 and 3 times, respectively, in 2019. The termination dates were all captured by VEN\(\mu\)S. Fields #4-25 (Figure 7c) and #5-45 (Figure 7d) were planted with a trtictale and ryegrass cover crop mixture on 4 October 2018, and the crop grew into the 2019 spring. The cover crops were moved on 29 April 2019, and after that, sorghum was planted. Sorghum in field #5-45 (Figure 7d) was harvested twice on 13 July and 11 September 2019. Both harvests were captured by the WIST approach using the VEN\(\mu\)S time-series. The FarmLogic database only has one harvest record for sorghum in field #4-25 (Figure 7c). However,  
Figure 5: Four terminations were detected for an alfalfa pixel (39.0210\({}^{\circ}\) N, 76.8467\({}^{\circ}\) W) in field #5-9 at the BARC using the VEN\(\mu\)S NDVI time-series in 2019. All downward trends were significant, and the detected termination dates were close to the recorded harvest dates (vertical red lines).  
two terminations for sorghum were detected from the VENuS time-series. Compared to field #5-45 (Figure 7d), which had the same cover crop types and same operation dates before the second harvest for sorghum, the second harvest record for sorghum in field #4-25 (Figure 7c) was likely lost in the database. Fields #1-11 (Figure 7e) and #5-2 (Figure 7f) were planted with a cover crop (triticale and ryegrass mixture) in spring and then planted with summer cash crops (corn and soybeans) afterward. The termination dates detected from VENuS agrees with the recorded ryegrass moving dates well. However, both fields show that remote sensing termination dates for summer crops were earlier than the summer crop harvest dates. This is because the WIST approach assumes the termination happens during the most significant decreasing period. The assumption works better for cover crops when they are terminated while still green. Summer cash crops such as corn and soybeans are harvested after drying down, past the end of the productive stage, when most green leaves have turned yellow or brown. The hypotheses for the WIST approach therefore may not work for summer crops. In Figure 7, actual summer crop harvest dates fell within the senescence onset and dormancy date range but closer to the dormancy date, and after estimated date of termination.  
Figure 8 compares 29 harvest/termination events in 2019-2020 across 14 cover crop and alfalfa fields (8 in 2019 and 6 in 2020) at BARC to the termination dates detected from the VENuS time-series. The termination dates varied from early April to late September in 2019. The WIST algorithm accurately detected all termination dates using VENuS observations. The termination dates from remote sensing and harvest/termination dates from operation records are close in the 1:1 line. Uncertainties of the estimated termination dates (vertical bars) are small owing to the frequent VENuS observations. Detailed statistics are shown in Section 4.4 as compared to results from Sentinel-2.