Use of Aerial Imaging to Evaluate Vineyard Canopy Variability
The RVI, NDVI, and PVI were shown to be correlated with VSP canopy density using both field spectroscopy data at the vine scale, and aerial image analysis at the vineyard scale. The ratio based indices (RVI and NDVI) were shown to have similar information contents but the RVI was found to be more linearly related to canopy density over a broad range of values, and thus more desirable for vineyard remote sensing applications. The PVI was found to be ineffective at reducing remote sensing measurement noise and accordingly, showed the poorest correlation with canopy density values. Results from this analysis corroborate with findings from investigators in woodland and forest environments, and provide evidence of the complex nature of vineyard scene reflectance properties. The ratio vegetation index was shown to be linearly related to canopy density at 100%and less than complete canopy cover. It was also shown that the RVI response at less than complete canopy cover was less sensitive to increasing canopy density than that taken at complete cover. This linear response across a broad range of LAI values (1-5) is surprising in light of findings reported by other investigators in agronomic systems. This may suggest that alternate factors other than variation in percent vegetation cover, such as spectral background influences, may be driving the VI to canopy density relationship in VSP vineyard systems. Results reported by investigators in semi-arid woodlands and forest communities’ show that shadow influences on scene reflectance play a significant role in determining the VI to vegetation parameter relationship. VSP vineyards have discontinuous canopies with moderate levels of percent cover. Vine row geometry leads to variation in shadow scene proportions that vary as a function of sun azimuth and zenith angles in relation to row orientation. Moreover, the sensitivity of canopy reflectance on spectral background variation was found to be greatest in canopies with intermediate covers (50-60%cover) because of the prevalence of soil-vegetation spectral interactions at these levels of canopy cover. This evidence may suggest that VSP vineyard scene reflectance more closely approximates that of semi-arid woodland systems as opposed to continuous canopy agronomic systems, due to potential shading effects, moderate percent vegetation cover, and the discontinuous nature of VSP canopies.