*2.2. Sentinel-2 Data and Derived Vegetation Indices*

The S2 mission, operated by the European Space Agency (ESA), consists of two twin satellites launched in June 2015 and March 2017. These satellites provide multispectral imagery with 13 bands (https://sentinels.copernicus.eu/web/sentinel/user-guides/sentinel-2-msi/resolutions/spatial, accessed on 13 March 2023). In this study, four spectral bands were utilized: blue (B2, centered at 492.4 nm), green (B3, centered at 559.8 nm), red (B4, centered at 664.6 nm), and near-infrared (B8, centered at 832.8 nm) with a spatial resolution of 10 m. In theory, the combined use of both satellites provides an image of the study area every five days. However, in reality, the availability of cloud-free images is much lower. For this study, three cloud-free images of the study area were selected. The first image (Day 1) was obtained on 24 March 2021, when the crop was in the initial stage of stem elongation (GS30 according to Zadocks [58]). On the second date (Day 2), 23 April 2021, the crop was between GS39 (flag leaf ligule just visible) and GS49 (first awns visible).

The final image, taken on 5 June 2021, (Day 3), depicts the crop between complete anthesis and medium milk stage (GS69-75). The satellite data were downloaded from the Copernicus Open Access Hub (https://scihub.copernicus.eu/, accessed on 13 March 2023) in the form of Level 2A products (Bottom-of-Atmosphere reflectance images), which have undergone atmospheric correction [61]. The tile 30 TWN of satellite S2 fully covered the study area.

In this study, the SNAP software was used to calculate the 13 VIs (Table 1) used for wheat or barley (*Hordeum vulgare* L.) grain yield estimation.


**Table 1.** Vegetation indices calculated in this study with their formulae according to the S2 bands used.
