## 4 Discussion
[50] highlighted that shadows cast within the crop canopy have a notable influence on the CWSI, and both the morphological characteristics of the canopy and the angle of the sun during the drone flight were identified as significant factors that alter the temperature distribution pattern within the canopy. Moreover, it is not solely the sun's position that affects the CWSI but also the presence of clouds. Cloud cover can lead to considerable variations in the amount of solar radiation reaching the crop canopy [51]. This aspect was duly considered in the design of the current study, which was conducted under clear skies. In this study, apparently, larger canopy sizes were associated with higher values of these VIs, suggesting that more extensive canopies in Pistachio are generally healthier and more vigorous. However, the differences in NDVI values in the maps were less perceptiblecompared to NDRE and GNDVI. Although some similarities in the patterns were exhibited by NDVI, NDRE, and GNDVI, some notable differences were also identified. For instance, NDVI values were consistently high and close to 1 across all canopy crowns, suggesting a strong and uniform presence of healthy vegetation but also the saturation effect already observed by other authors [52], which commonly shows up at high biomass levels [53] and can limit the ability of NDVI to discriminate subtle differences in vegetation among larger canopies. Furthermore, all VIs in June exhibited a strong correlation with yield parameters and a pronounced correlation with the quality parameters that was more or less observed in September, except for NDRE, confirming other authors' findings for other crops, which have also indicated that the VIs can be used to predict yield and quality parameters [54]. On the other hand, regarding nutritional values, a good relationship was observed with NDRE in June, although this association was not observed in September. Nevertheless, CWSI showed a strong correlation with thee nutritional values at both time points, especially CBH and fatty acids, which could help to identify the ideal time for harvesting [55]. However, these correlations were lower in September. In fact, as the season advanced, the connections between all VIs and pistachio yield parameters vanished, except for GNDVI. This observation suggests that the management in June is more critical than in September and, as the growth season progresses, the relationships between VIs and yield may become less pronounced or more complex. The contrasting outcomes observed in the CWSI compared to other VIs throughout the drone flights highlight the divergent behaviors between thermal-based and non-thermal-based indices. This fact exposes the importance of the different bands of the electromagnetic spectrum and emphasizes the critical importance of carefully selecting appropriate indices based on specific research objectives and data collection timing. Indices that provide valuable information at one stage of crop development may not be as informative at another stage and vice versa. This consideration is crucial for researchers utilizing remote sensing technologies in precision agriculture. For instance, based on our study findings, the CWSI appears to be particularly valuable for irrigation assessment and for the quality evaluation of pistachio crops, likely due to its relation to water stress that affects the plants. However, non-thermal VIs are more relevant for predicting yield outcomes, probably because they are more associated with the current status of the crop. GNDVI deserves special recognition for preserving connections relatively consistently over the year. This dynamic nature, where different indices hold relevance at different stages of crop growth, emphasizes the complexity of applying remote sensing to precision agriculture. Nevertheless, thermal sensors are not always readily available due to their higher cost compared to visible light or multispectral-based sensors. Therefore, the correlations observed in this study pertaining to pistachio VIs can be interesting.