# First Vegetation Optical Depth Mapping from Sentinel-1 C-band SAR Data over Crop Fields
## 4 Results  
### VOD Computed using SAR Data in VV Polarization  
Figure 3 shows that, in general, VOD-VV values (VOD-VV\({}_{\rm{A},\rm{av}}\) and VOD\({}_{\rm{D},\rm{av}}\)) in both crop growth cycles have similar temporal dynamics as \(\rm{NDVI}_{\rm{av}}\). For the first crop growth cycle (between September 2017 and September 2018), VOD-VV values of barley, fallow, oat, and wheat, at the early growth stage (before 1 January 2018) are low (about 0.15) and the \(\rm{NDVI}_{\rm{av}}\) values are lower than 0.2 (Figure 3a-d). Later, between 1 January 2018 and 1 May 2018, VOD-VV and \(\rm{NDVI}_{\rm{av}}\) values appear to start increasing at the same time. VOD-VV values reach a maximum values of about 0.50 while \(\rm{NDVI}_{\rm{av}}\) reach values of about 0.8 in the case of barley, oat, and wheat. In the case of fallow, VOD values reach a maximum of about 0.38 while \(\rm{NDVI}_{\rm{av}}\) reaches values of about 0.43. Between 1 May 2018 and 1 November 2018, \(\rm{VOD}_{\rm{A},\rm{av}}\) and \(\rm{VOD}_{\rm{D},\rm{av}}\) start decreasing to reach a minimum of \(\sim\)0.15 while \(\rm{NDVI}_{\rm{av}}\) reaches about 0.2. Similar results were obtained for the second crop growth cycle (between September 2018 and September 2019) where the temporal dynamics of VOD-VV match well to that of \(\rm{NDVI}_{\rm{av}}\). However, for the second crop cycle of fallow, two VOD-VV peaks were observed. The first VOD-VV peak is on 25 February 2019 and the second one is on 1 May 2019. An analysis of plots fallow NDVI profiles shows that some plots have high NDVI between December 2018 and February 2019 while the remaining plots have high NDVI between May 2019 and June 2019.  
In contrast to the other crop types, Rapeseed VOD-VV values from 1 January 2018 to 1 May 2018 in both growth cycles are not consistent with NDVI (Figure 3e). VOD values are found to be almost constant and do not increase with \(\rm{NDVI}_{\rm{av}}\) during the growing period. For instance, in the first crop growth cycle, VOD-VV of Rapeseed increases from around 0.15 to 0.20 while \(\rm{NDVI}_{\rm{av}}\) increases from 0.18 to 0.70.  
Moreover, Figure 3 reveals two interesting phenomena observed in both crop growth cycles:  
* First, VOD-VV\({}_{\rm{A},\rm{av}}\) and VOD-VV\({}_{\rm{D},\rm{av}}\) have very close values except during mainly the periods from 13 May 2018 to 18 June 2018 and from 1 May 2019 to 1 June 2019. During these periods, VOD-VV\({}_{\rm{A},\rm{av}}\) values are lower than those of VOD-VV\({}_{\rm{D},\rm{av}}\). For instance, for wheat at around 31 May 2018 (VOD was calculated with S1 images acquired between 13 May 2018 and 31 May 2018), VOD-VV\({}_{\rm{A},\rm{av}}\) is lower than \(\rm{VOD}_{\rm{D},\rm{av}}\) by 0.18.
* Second, in both crop growth cycles, VOD-VV\({}_{\rm{A},\rm{av}}\) and VOD-VV\({}_{\rm{D},\rm{av}}\) reached a peak earlier than NDVI by about 18 days. For instance, for wheat, VOD-VV\({}_{\rm{A},\rm{av}}\) peaks on 25 April 2018 (VOD calculated with S1 images acquired between 7 April 2018 and 25 April 2018) while the \(\rm{NDVI}_{\rm{av}}\) peaks on 15 May 2018, about 3 weeks later.  
To better analyze the temporal correlation between the retrieved VOD and NDVI values for both growth cycles, the coefficient of determination between VOD-VV values and NDVI\({}_{\text{av}}\) was computed (Figure 4). For barley, fallow, oat, and wheat, medium to good correlations were obtained (R\({}^{2}\) values  
Figure 3: Temporal patterns of VOD values computed at VV polarization (VOD-VV\({}_{A}\)av and VOD-VV\({}_{D}\)av) and NDVI. (**a**) Barley, (**b**) Fallow, (**c**) Oat, (**d**) Wheat, (**e**) Rapeseed. The shaded region represents the mean \(\pm\) standard deviation of VOD values for all plots at a given date.  
about 0.58, 0.39, 0.46, and 0.61, respectively) (Figure 4a-d). For rapeseed, a poor correlation was obtained (R\({}^{2}\) about 0.18).