# Analysis of Maize Sowing Periods and Cycle Phases Using Sentinel 1&2 Data Synergy
## 3 Results and Discussion
### Growing Phases  
The analysis performed throughout the growing season showed some relevant results in terms of optical and radar performance in the monitoring of maize growth. The comparison between NDVI and VH/VV shows a good alignment in the remote sensing responses during the initial growing phases, as shown in Figure 3. From May to July the NDVI increases from about 0.2 to the maximum values (0.75-0.9) and the VH/VV backscattering also increases from \(-10\) dB to \(-5\) dB on average. Although the NDVI decreases significantly during the final phase (September-October), there is no evidence of \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{3}{*}{**Month**} & \multicolumn{3}{c}{**Correlation between**} & \multirow{3}{*}{**Comparison of NDVI and Soil Moisture (SM)**} \\ \cline{2-2} \cline{4-6}  & **NDT and VH/VV** & & & & **Comparison of NDVI and Soil Moisture (SM)** \\ \cline{2-6}  & **R** & **R** & **NDVI** & **NDVI** & **SM** & **SM** \\  & **(in ID)** & **(out ID)** & **(in ID)** & **(out ID)** & **(in ID)** & **(out ID)** \\ \hline April & 0.2 & 0.2 & 0.21 & 0.22 & 0.24 & 0.26 \\ May & 0.62 & 0.6 & 0.46 & 0.45 & 0.22 & 0.23 \\ June & 0.64 & 0.66 & 0.71 & 0.68 & 0.2 & 0.2 \\ July & 0.23 & 0.21 & 0.84 & 0.83 & 0.17 & 0.18 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Columns 2–3 Pearson correlation coefficients (R) between NDVI and VH/VV. For each month from April to July, the optical and radar responses were compared, considering all the maize fields and a 6–days frequency. R was calculated for the group of fields within and outside the irrigation districts (ID). Columns 4–7 comparison between mean monthly NDVI [–] and soil moisture [\(\mathrm{Vol_{water}}\)/\(\mathrm{Vol_{soil}}\)] for the fields within and outside the irrigation districts (soil moisture refers to the soil upper layer: 3–5 cm). an analogue decrease in terms of backscattering. Since the NDVI is derived from visible and near infrared frequencies, the strong decrease is driven by changes in the chromatic response of maize leaves. However, the VH/VV does not appear to be affected by an analogous decrease during the same period; in fact, the backscattering signal is sensitive to changes in the ground coverage structure and water content. Since VH/VV shows a low decrease during September and October, we can assume that this response is driven by the decrease of plant water content and the progressive drying of maize leaves. Yet, the signal decreases \"slowly\" because of the presence of dry biomass on the ground inducing volume scattering. This misalignment between NDVI and radar backscattering in the final part of the growing season is particularly meaningful for those algorithms retrieving soil moisture values based on NDVI information: since the plants' height affects the reliability of soil moisture results, these models usually suggest specific ranges of NDVI for which the soil moisture can be used. However, according to the results from Figure 3, any analysis whose reliability is affected by the plant coverage would be more robust taking into account the response of the dry biomass, to which radar is more sensitive than optical measurements. The maize growth was also analyzed in terms of the normalized backscattering signal (IPR). The scatterplots in Figure 4 show the relation between IPR (i.e., the normalized VH/VV backscattering) and NDVI along two main growing phases. In the first growing phase (May-June-July), most of the maize fields appear to be already sowed and the development of plants corresponds to the NDVI changes and the IPR response. NDVI increases are due to the visible and near infrared signals of the land cover. In the first part of the phase, when the NDVI is still lower than 0.25 (bare soil), the variability of IPR is mainly due to changes in the soil moisture and roughness due to the incipient growth of maize plants. The first consequence of maize emerging, before any response in terms of NDVI, is the change of ground roughness, to which the normalized backscattering is very sensitive.