# Analysis of Maize Sowing Periods and Cycle Phases Using Sentinel 1&2 Data Synergy
## 3 Results and Discussion
### Identification of Potential Rainfed Fields  
The maize fields within and outside the irrigation districts (respectively called \"in ID\" and \"out ID\" in this section) were compared in terms of NDVI and VH/VV series. This comparison was performed to identify differences between the two groups of fields, in order to potentially calculate separate SP for rainfed and irrigated scenarios. As shown in Figure 3, the 0.25, 0.50 and 0.75 quantiles of the signals from the maize fields were computed every 6 days, to be consistent with the SAR frequency of acquisition. The quantiles series do not show any significant difference between the two groups of fields. Despite the 0.95 and 0.05 quantiles of NDVI showing a different range of values in June and September, this misalignment can be explained considering the differences between the two sets of fields: in fact, less than 14% of maize fields were considered as potentially rainfed. In addition to the numerical difference between the two groups, the potentially-rainfed fields are concentrated in areas at higher elevations and along the rivers, while the irrigated fields are more uniformly distributed over all the pilot area. Figure 3: Time series of NDVI values (interpolation of 15-days revisit time) and VH/VV backscattering signal (6-days revised time). Each boxplot represents the distribution of values of NDVI and VH/VV from all the maize fields. The horizontal blue lines in the boxplots represent the 25%, 50%, and 75% quantiles, respectively (from the ground up). The black lines above and under each boxplot include the values within 95% and 5% quantiles, respectively. The blue dots represent the outlier values. In the two lowest subplots the mean daily values of the series are reported. Time series of mean NDVI and VH/VV were obtained from daily mean values from Figure 3. The NDVI mean signal from fields within the irrigation districts is almost equivalent to the mean signal calculated for the other fields. The VH/VV comparison highlights a slight difference between the two groups from April to May: the mean VH/VV from fields within the irrigation districts is \(-0.6\) dB lower, considering the two months. However, this misalignment can be considered a consequence of the noise induced by the bare soil geometry, since the difference gradually reduces when the VH/VV become more sensitive to the growth of the plants. A further comparison was performed between the two groups of fields, considering the complete series of NDVI and VH/VV among the growing season. Even if the temporal mean signals from the two groups of fields are well aligned, the effect of irrigation could lead to more evident divergences considering the long-term effect during the full growing season. The sum of the mean daily NDVI from April to October was compared for the fields within and outside the ID: a misalignment of \(-0.01\%\) was found between the sums of the two mean NDVI signals. The same analysis shows a misalignment of \(-1.7\%\) from April to October between the sum of mean VH/VV within and outside the ID, and a misalignment of \(-0.4\%\) considering the period from June to October (when the irrigation season is concentrated, and the effect of bare soil is limited). In both cases, the analysis did not point out any appreciable differences between the two groups of fields. The monthly correlations between NDVI and VH/VV were calculated for the two groups of fields, to identify potential impacts of irrigation limited to the areas of the irrigation districts. The analysis was performed for the first part of the maize growing cycle, when the VH/VV is still sensitive to the soil moisture anomalies induced by irrigation since growing plants still do not cover entirely the ground. Table 1 shows the monthly Pearson correlation coefficients (obtained dividing the covariance by the product of the two variables' standard deviations) considering all the signals retrieved every 6 days from all the fields of each group.