Effect of Time of Day and Sky Conditions on Different Vegetation Indices Calculated from Active and Passive Sensors and Images Taken from UAV

Optical sensors have been widely reported to be useful tools assess biomass, nutrition, and water status in several crops. However, the use of these could affected by time day sky conditions. This study aimed evaluate effect conditions (sunny versus overcast) on vegetation indices (VI) calculated from two active (the Crop Circle ACS-470 Greenseeker RT100), passive hyperspectral bidirectional spectrometer HandySpec Field sensor), images taken an unmanned aerial vehicle (UAV). The experimental work was conducted a wheat crop south-west Germany, with eight nitrogen (N) application treatments. sensor measurements were made throughout vegetative growth period different dates 2019 at 9:00, 14:00, 16:00 solar day, sunny overcast only 9:00 h influence indices. For most evaluated, there significant differences between paired measurements, regardless measurement. smallest measurement times found 14:00 h, they observed for vehicle-carried handheld being lower than 2% 4%, respectively, NIR/Red edge ratio, Red inflection point (REIP), index. Differences 5% (indices ratios, NDVI) RT100 (index NDVI). stable over index, REIP used. considerable simple ratios NIR/Green. REIP. In practical terms, results confirm that used measure on-farm any choosing optimized