# Monitoring of Sugarcane Harvest in Brazil Based on Optical and SAR Data
## 5 Discussion
### Analysis of the Time Series
Thus, it is most probably that the harvest, falling on the early dates of a high-coherence period, recorded by the optical algorithm took place several days earlier--within the period between optical images (Figure 14, Coherence VV). That allows postponing the harvest date by several days but not more than by five days. One of the reasons for erroneous algorithm functioning is non-harvest field operations (plantation burning, plowing, etc. ); those operations are detected as harvest. Early September is characterized by the period of dense clouds resulting in a false response of the optical algorithm (Figure 18). The algorithm, relying on the SAR data, determines correctly the harvest in October but determines it falsely in August. Apparently, the coherence decrease in August is caused  
Figure 16: Example of false response of the optical algorithm due to cloudiness in June (cid = 226). Actual harvest date (red lines): 12 June 2018; harvest date according to the optical algorithm (blue lines): 2 June 2018; harvest date according to the SAR algorithm (green line): 23 June 2018. Figure 17: (a,b) Example of the cloudiness effect during the harvest period (June). by the field works. Pay attention to the slow coherence growth during March-July that corresponds to the slow _NDVI_ decrease. The following group of problems is connected with the erros in field boundaries and inclusion of the nonagricultural lands into its area. Such inclusions change the behavioural pattern of the _NDVI_ and coherence time series, making it impossible to record the fact of harvest. For the spatially heterogeneous cell shown in Figures 19 and 20, the harvest date is determined according to the optical data for 12 June 2018. According to the SAR data, the harvest took place within the period up to 11 June 2018 when the coherence growth begins. However, that date is omitted by the SAR algorithm due to insufficient coherence increase. The latter is caused by the fact that the parcel includes the nonagricultural lands (Figure 20). Nevertheless, the observed coherence jump violates the behavioral pattern of the time series and results in the harvest date omission. The false response of the algorithm occurs in July. It appears that further coherence drop in late July-early August is connected with the post-harvest operations. Figure 19: Example of the harvest date omission by the SAR algorithm due to nonagricultural lands within the parcel (cid = 40). Actual harvest date (red lines): 12 June 2018; harvest date according to the optical algorithm (blue lines): 12 June 2018; harvest date according to the SAR algorithm (green line): 5 July 2018. For the cell represented in Figures 21 and 22, a part of the sugarcane parcel is covered with nonagricultural vegetation and has low coherence within the whole season. The first March harvest date was omitted since it did not cause the coherence jump. High coherence appears when most of the parcel is harvested. That happens in June-July. The response of the optical algorithm in September is not confirmed by low coherence values within that period. Figure 21: Example of the harvest date omission by the SAR algorithm due to nonagricultural lands within the parcel (cid = 163). Actual harvest dates (red lines): 14 March 2018, 12 June 2018, 2 July 2018, and 7 July 2018; harvest dates according to the optical algorithm (blue lines): 7 July 2018 and 15 September 2018; harvest date according to the SAR algorithm (green line): 5 July 2018. Figure 20: Example of the sugarcane field fragment with the included nonagricultural lands (cid = 40) in natural colours (**a**) and _NDVI_ (**b**). Spatial heterogeneity can be the result of a digitization error or inaccuracy in combining several cadastral parcels into a single agricultural field. Incorrect delineation of the field boundaries (Figure 23) has resulted in the fact that the field includes a road and a part of a neighbouring field with other harvest dates.