# Analysis of Maize Sowing Periods and Cycle Phases Using Sentinel 1&2 Data Synergy
## 3 Results and Discussion
### Emerging of Plants and Sowing Periods  
In order to identify the periods in which maize starts to grow, the SAR data were analyzed for the period from May to June. The mean and variance of VH/VV over time in each field are shown in Figure 5: each point in the scatterplot represents the VH/VV statistics from May to June, colored according to the mean NDVI calculated for the same period. Fields where maize is sown in the early May (or even in April) show a lower variance of VH/VV, because the growth of plants tends to produce a uniform field coverage. For the same reason, the early sowed fields show a higher mean backscatte  
Figure 5: Comparison of radar backscattering mean and variance during the initial period of maize growth. Each point represents a maize field during the period from May to June, colored according to the mean NDVI for the same period. NDVI, because the growing phase is at a later stage and plants are more developed. The presence of different mean NDVI classes over the May-June period may indicate that maize was sowed in distinct phases. About 25% of maize fields appear to have a mean NDVI lower than 0.3 in the first two months: for these fields, the NDVI was typically higher than 0.3 during April, meaning that winter crops planted on the same plots before maize were probably still cultivated until this time. A high VH/VV variance is typical of scenarios where maize plants break through the soil. The low height of plants corresponds to low NDVI, but also to high variability of ground coverage due to the incipient growing phase, to which VH/VV is sensitive. The normalized backscattering signal (IPR) was filtered to limit the sensitivity of radar data to the variability of bare soil water content, according to the procedure described in Section 2.2.2. The variability of the filtered signal (IPR\({}_{f}\)) is mainly a consequence of geometric changes on the ground surface, which is typically related to the emerging of maize plants during the first part of the season. The emergence time was identified according to the temporal series of IPR\({}_{f}\), checking the time in which the filtered backscattering exceeds the threshold of 0.1. The bar plot in Figure 6 shows the distribution of the emergence periods (EP). Most of the maize reaches the soil surface in the second half of May, especially between 12 th and 18 th of May. The distribution of EP highlights the presence of a second main emergence period, between the last days of May and the beginning of June. It is interesting to observe that maize appears to be sown earlier at low elevations (h < 400 m above the sea level) than at higher elevations. In fact, plants break through the soil between the end of May and mid-June on the fields higher than 500 m, which are characterized by lower temperatures than areas at lower elevations. The presence of two different SP is consistent with the typical agricultural practices of the area, where winter crops are harvested in late spring. These crops may be present  
Figure 6: (**a**) Temporal distribution of emergence periods (i.e., the periods when the plants emerge through the soil). The 6–days revised time refers to the availability of radar data that were used for the analysis. For each day, the hectares where the plants have come out from the ground have been cumulated according to the class of elevation (meters above the sea level). (**b**) Temporal variation of the IPR\({}_{f}\) frequency distribution. After the end of June, no more fields show values < 0.1, showing that the maize has passed the emergence phase. on those fields which are sown later at the lower altitudes, while the time shift at high elevations may depend on the later warming of weather conditions. Despite some areas appears to be sowed in the first week of April, this practice is not widespread over the region.