## 3 Results and Discussion
### VI Performance  
Coefficients of determination were calculated for the expected exponential relationships between ground measured fresh phytomass and all band combinations of the four VIs, as described above. The results indicate clear differences between the VIs and, with the exception of TBVI, only little differences between the variants using one of the six NIR bands for VI calculation. Regardless of the used NIR band, the best relationships are obtained for EVI, followed by TSAVI using the soil-line and NDVI. Coefficients of determination calculated for EVI range between 0.86 (b\({}_{761}\) and b\({}_{953}\)) and 0.88 (b\({}_{802}\), b\({}_{861}\) and b\({}_{899}\)). Lower \(R^{2}\)s can be observed for TSAVI and NDVI but again, MCA b\({}_{761}\) and b\({}_{953}\) (0.80 for TSAVI; 0.71 for NDVI) are less suited than b\({}_{802}\) and b\({}_{899}\) (0.82 for TSAVI; 0.72 for NDVI). Due to the construction of TBVI exclusively from NIR bands only five band combinations with b\({}_{953}\) are possible. Low correlations were obtained when using combinations with b\({}_{761}\) and b\({}_{861}\) (\(R^{2}\) = 0.42 and 0.38, respectively), moderate correlations for the combinations with b\({}_{802}\) and b\({}_{831}\) (\(R^{2}\) = 0.65) and a high correlation (\(R^{2}\) = 0.77) for the combination with b\({}_{899}\) (TBVI\({}_{8999/b953}\) hereafter). The small differences between the results obtained for EVI, TSAVI and NDVI variants can be explained by the low variation of canopy reflectance in the different NIR bands and the corresponding low reflectance in the R band (and B band in the case of EVI). The high correlation of the TBVI\({}_{\text{b899/b953}}\) may be explained by the relatively large difference in reflectance compared to other NIR band combinations. The distance increases with higher amounts of fresh phytomass due to the maximum of NIR reflectance in b\({}_{899}\) observed for all measured spectra and the relatively strong decline of the reflectance in b\({}_{953}\), caused by the respectively higher absolute water contents. However, the highest correlations between the examined VIs and ground-based measurements of fresh phytomass were found for the variants using NIR band b\({}_{899}\). Regardless the mathematical construction of VIs from available broad or narrow band sensors the disadvantage concerning the non-linearity caused by saturation effects, especially in the case of dense vegetation canopies, is still present. Saturation levels are reported in numerous studies using field measurements of biophysical canopy parameters or leaf and canopy radiative transfer model [17; 40; 41]. The studies compared the predictive power and stability of several narrow- and broad-band VIs for estimation of LAI under different environmental conditions (canopy architecture, soil background and illumination geometry). Values for NDVI of 0.90 and 0.75 for TSAVI when using a single soil line are typical for dense vegetation canopies (LAI > 2). The observed saturation effect for the relationship between VIs and fresh phytomass in this study is caused by the strong linear relationship between LAI and fresh phytomass (\(R^{2}\) = 0.88; LAI was measured simultaneously to fresh phytomass). In this study, most NDVI values range in a narrow span between 0.89 and 0.91 when fresh phytomass exceeds 1200 g: m\({}^{-2}\), which holds true for 15 out of 22 samples (Figure 16a). The same effect can be observed for the TSAVI where the same 15 samples range in a span between 0.70 and 0.74 (Figure 16b). These findings indicate that both NDVI and TSAVI are not reliable estimators for fresh phytomass of a dense green vegetation canopy typical for lucerne. In terms of \(R^{2}\) s, the TBVI\({}_{\text{b899/b953}}\) performs better than the NDVI but worse than the TSAVI. The lower \(R^{2}\) compared with TSAVI is caused by a larger scatter in the data. This is probably the result of the small differences in the reflectance in b\({}_{899}\) and b\({}_{953}\).