# Monitoring Rainfed Alfafa Growth in Semiarid Agrosystems Using Sentinel-2 Imagery
## 3 Results and Discussion
### Temporal Analysis  
Regarding the rainfed alfalfa temporal development, Figure 4A shows the NDVI average value evolution in three different zones of Bardenas Reales, characterized as (I) high, (II) regular, and (III) good quality (Figure 1), and the average values for a total of 28 rainfed alfalfa plots analyzed between July 2016 and April 2019. The rainfed alfalfa crops in the Bardenas Reales were sown in May 2016. During the first year, we checked the in situ crop establishment in plots 195, 199, 221, 257, 11, 59, 63, and 347. We also checked the NDVI values from Sentinel-2, which showed a basal development of the crop (approximately 0.15 in July 2016). The alfalfa crop implantation improved progressively over the following years and, thus, in the 2017 season, the NDVI values reached approximately 0.25-0.35 in the three zones. In the 2018 season, NDVI reached its maximum values of 0.30 for zone I, 0.55 for zone II, and over 0.70 in zone III. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Index**} & \multirow{2}{*}{**Linear Fitting**} & **Polynomial Fitting,** & \multirow{2}{*}{**Exponential Fitting**} \\  & & **Second Order** & \\ \hline NDVI & 0.713 & **0.727** & 0.463 \\ PVI & 0.700 & **0.705** & 0.437 \\ kNDVI & 0.659 & **0.705** & 0.400 \\ SAVI & 0.695 & **0.700** & 0.431 \\ RVI & 0.606 & **0.686** & 0.350 \\ SeLI & 0.680 & 0.680 & 0.570 \\ NDVI\({}_{705}\) & **0.678** & **0.680** & 0.600 \\ MCARI & 0.653 & **0.670** & 0.437 \\ GNDVI & **0.658** & **0.658** & 0.497 \\ SLAVI & 0.614 & **0.652** & 0.345 \\ NDWI & **0.628** & **0.628** & 0.468 \\ MTCI & 0.506 & **0.510** & 0.518 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Statistics obtained with a linear, polynomial of second order, and exponential order fitting for each index. Thirty-eight sample points were used for the FVC calculation in this experiment. The best fitting is represented in bold. Overall, it should be remarked that an important variation in FVC was observed over the different years (Figure 4A). While 2019 was the third year of the alfalfa crop plantation, which is important for better establishing the crop, rain during the spring season was very limited; thus, crop growth remained quite slow (358, 375, 598, and 188 L per m\({}^{2}\) in the 2016, 2017, 2018, and 2019 seasons, respectively). As can be observed, the NDVI values strongly increased in all zones from May to June in 2018, while in the same period of 2019, the NDVI showed an overall decrease due to the drought period in the Bardenas Reales area. It can also be inferred that the alfalfa crops in zone III developed earlier than those in zone II and zone I for all years studied. Thus, for example, in May 2018, the NDVI values were 0.72, 0.58, and 0.40 for zones III, II, and I, respectively. The advanced development in zone III may be related to the higher altitude of the plots in this zone compared to those in zone II and zone I (Table 1), which would contribute to the better establishment of the crop year by year. To evaluate the differences in development between the rainfed plots, the irrigated alfalfa NDVI average values of one irrigated plot (UTMX: 614734; UTMY: 4678413), adjacent to the area of Bardenas Reales, was assessed between January 2017 and May 2019. The NDVI value for the irrigated plot was compared to the average value of the 28 rainfed plots spread along Bardenas Reales (Figure 4B). The NDVI values of the irrigated alfalfa plot were much higher than those observed in the rainfed alfalfa plots as previously expected. It should be remarked that the irrigated alfalfa reached maximum development between February and June. The different NDVI minimum peaks observed in the irrigated alfalfa plots may be the consequence of consecutive harvests during the year (Figure 4B). Conversely, the stability of the rainfed alfalfa NDVI values indicated that there was no harvesting activity in these plots. Contrary to the use of forage production associated with the irrigated alfalfa plots, the rainfed alfalfa plots are intended for in situ sheep feeding.