# Monitoring Rainfed Alfafa Growth in Semiarid Agrosystems Using Sentinel-2 Imagery
## 1 Introduction  
Grain forage legumes are present in approximately 15% of the world's cultivated land, and are one of the most important protein sources both in the human diet (33%) and animal feed [1]. Legumes in symbiosis with soil _Rhizobium_ bacteria fix nitrogen, which naturally minimizes the use of mineral fertilizers, making legumes environmentally sustainable crops [2]. Among grain forage legumes, the perennial species, alfalfa (_Medicago sativa_), which is cultivated in more than 80 countries, accounting for 35 million hectares, is considered one of the most important foraged crops [3]. Compared with grain legumes, alfalfa is considered a drought-tolerant species [4]. As a temperate legume that usually grows in arid and semiarid regions, alfalfa can reach deep soils to obtain water through its well-developed root system [5, 6]. In this context, _Medicago_ species, such as alfalfa, have been promoted in countries, such as the United States, Canada, Australia, and New Zealand, in order to improve and reduce the feeding of herds in rainfed areas [7]. In Spain, several initiatives have been successfully conducted which extend alfalfa cultivation to large areas of arid and semiarid land for grazing use [8]. In 2016, the Governing Board of Bardenas Reales, a semiarid area (41.818 ha) in the north of Spain, aimed to explore the ecological utility of an alfalfa-wheat cropping system and the nutritional value of rainfed alfalfa as a perennial forage crop for seasonal sheep grazing. Thus, 28 plots (-48 ha extension) of this crop were sown in this territory. The global crop yield is estimated to decrease by approximately 70% due to the fact of environmental stresses, with drought as the main environmental stress in agriculture [9]. Alfafla can adapt to water-limited regions, but a water deficit negatively affects the productivity of this crop [10]. Hence, rainfed alfalfa monitoring at different times of its annual cycle is important for evaluating the possibilities of this crop in the Bardenas Reales area. Fractional vegetation cover (FVC) is a useful indicator of crop growth status, representing the ratio of the vertical projected area of vegetation to the total surface area, and it is widely used to indirectly determine crop photosynthesis, transpiration, and water-use efficiency. Traditionally, in situ field inspections are the method used to estimate the FVC of a crop and to analyze its growth [11], which are time consuming and expensive [12]. Remote sensing has proven to be an effective tool for monitoring the growth of crops both spatially and temporally (or for monitoring the growth of crops during cultivation campaigns) [12]. Over the last few years, the availability of high spatial and temporal resolution satellite images has increased, which makes it possible to periodically monitor crop growth [13; 14; 15; 16]. The Sentinel-2A and Sentinel-2B satellites, included in the Europe's Monitoring for Environment and Security (GMES) program [17], were launched on the 23 rd of June 2015 and 7 th of March 2017, respectively. The Sentinel-2 satellites provide global coverage every 5 days in several spectral bands in the visible, red-edge, NIR, and SWIR regions, with resolutions from 10 to 60 m. Several authors have explored the potential of Sentinel-2 imagery for the monitoring of different crops such as alfalfa [3], maize [15; 18], potato [14; 18], and sisal [19]. Thus, sorghum, pearl millet, and cowpea production have been temporally monitored using Sentinel-2 data [20]. [PERSON] et al. (2017) analyzed the evolution of different winter and summer crops (wheat, rapeseed, maize, soybean, and sunflower) using Sentinel-2 imagery [21]. The observation of vegetation has traditionally been carried out through the use of vegetation indices (VIs) derived from multispectral images. These indices relate to a few spectral bands with biophysical properties of vegetation, such as greenness, photosynthetic activity, and water content, while minimizing the soil and atmospheric effects [18; 19; 20; 22; 23].