Influence of surveying altitude and solar zenith angle on wheat vegetation indices according to UAV data

The purpose of the study is to assess the influence of the survey altitude and the solar zenith angle on the color and spectral vegetation indices (VI) of wheat obtained from survey data from a UAV in the arid climate of Transbaikalia. Tasks: to perform multi-altitude aerial photography of the experimental field at different positions of the Sun; to calculate the VI and conduct a comparative statistical analysis of the obtained data. The study was carried out at the training and reclamation site of the Buryat State Agricultural Academy named after V.R. Filippov in the suburbs of Ulan-Ude on August 12, 2023. An experimental field with an area of 600 m2 is sown with Selenga wheat variety, bred for the arid conditions of the Republic of Buryatia. The field is divided into 4 strips with different levels of irrigation and rainfed option. Each lane is represented by 3 NPK experiments in triplicate. The assessment of dependencies was carried out within one variant, divided into 20 identical segments in order to maintain sample homogeneity. The survey of a section of the experimental field was carried out at altitudes from 40 to 200 m at solar zenith angles from 36.5 to 74.5° using UAVs equipped with RGB and multispectral cameras. Color and spectral VIs were calculated. The groups of color VIs ExG-ExGR-GLI and ExR-NDI-VARI and spectral VIs NDVI-RVI and SAVI-DVI are mutually interchangeable due to their high pairwise correlation (more than 0.99). The results of the Wilcoxon test generally showed the similarity of the average VI values of GLI, ExG, ExGR, as well as NDVI and SAVI during the period from 14:10 to 17:30, which is optimal for surveying. The average VI values decrease with height and differ significantly, so the surveying altitude should be taken into account when monitoring the condition of crops. It is necessary to develop the scientific foundations of unmanned agricultural monitoring in various climatic conditions.