# Monitoring Rainfed Alfafa Growth in Semiarid Agrosystems Using Sentinel-2 Imagery
## 2 Material and Methods
### In Situ FVC Measurement  
To estimate the on-field FVC of the experimental parcels, an average of 3-4 sampling points in each parcel, representative of the plot average vegetation density, were visually selected in each. In the most heterogeneous plots, 4-5 sampling points were visually selected, while in the most homogeneous plots, 2-3 points were selected in each. In total, 172 measurements were made inside the eight selected plots in Experiment 1.  
The FVC of each sampling point was measured over a square meter quadrant, using radial images acquired with a 13 megapixel mobile camera from a height of 1 m above the ground. The fieldwork was carried out on three different days in 2017 and three days in 2018, coinciding with different rainfed alfalfa phenological cycle stages. After the data were acquired, the images were cropped at the edge of the sampling plot using the PhotoscapeX imaging software.  
A supervised image classification procedure was applied to measure the area covered by alfalfa in the radial image taken at each sampling point. Taking into account that the spectral separability between the green vegetation and the background soil was high in these images, a simple maximum likelihood algorithm was selected to carry out this classification. This algorithm was trained using several aleatory defined training areas in each image by photointerpretation, which corresponded to the different categories, for classification: alfalfa in the sun, alfalfa in the shade, soil in the sun, soil in the shade, and  
\begin{table}
\begin{tabular}{r c c c c c} \hline \hline
**ID** & **Quality** & **Area (ha)** & **Altitude (m)** & **Orientation** & **Slope (Degrees)** & **E** \\ \hline  
ood & 0.97 & 563.19 & 194 & 2.84 & 2 \\
74 & Good & 1.34 & 534.07 & 171 & 2.77 & 2 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Bardenas Reales rainfed alfalfa plots included in the study from July 2016 to April 2019. A quality label was assigned depending on the soil characteristics of the different plots and following the ranking established by Bardenas Reales. Area (ha), altitude (m), and slope (degrees). The orientations of 0, 90, 180, and 270 represent north-, east-, south-, and west-facing, respectively. The altitude was defined by the centroid of the polygon. The column E (experiments) indicates the plots employed for Experiment 1 and/or Experiment 2 as explained in Section 2. The records in bold represent the parcels that were used in both Experiments 1 and 2.  
unclassified. Training areas represented between 3% and 5% of the total area and were defined specifically for each image (Figure 2).  
The classified accuracy assessment was carried out using validation areas, defined by photointerpretation and representing between 1% and 2% of the total area of each image. The overall accuracy was between 95.9% and 99.47% for all plots, and the kappa coefficient was above 0.94.  
FVC represents the total area of alfalfa, which is the result of the total alfalfa in the shade and alfalfa in the sun in each of the 1 m\({}^{2}\) sampling points, expressed in percentage. Parcel FVC was assessed by the mean of the different individual FVCs inside each parcel.