# Sensitivity of Common Vegetation Indices to the Canopy Structure of Field Crops
## 3 Results  
In the modeled data for typical healthy green vegetation, all indices showed a clear and mostly negative dependence on MTA (Figure 2). The curves were most similar for the lowest LAI value, LAI = 1. At medium LAI values (LAI = 3) of the three LAI levels, the indices showed a smaller dependence on MTA. Also, the curves in Figure 1 become nonlinear and indicate lower dependence on MTA at low MTA values. The latter is especially evident for NDVI (Figure 2a) and WDRVI (Figure 2f). At LAI = 5, the nonlinearity was even larger compared with lower LAI values. The dependence of NDVI and WDRVI on MTA was slightly positive for the smallest MTA values. However, the dependence was very small and for practical purposes, NDVI and WDRVI were insensitive to the variation of MTA between below MTA = 50\({}^{\circ}\) for large LAI values. The other indices, EVI, EVI2, OSAVI and MTVI2, were still highly sensitive to MTA (\(R_{\rm s}>0.6\)) even at LAI = 5. All indices showed a monotonic dependence on LAI when all other inputs in the PROSAIL model (see Section 2.3) were fixed to the values of green healthy vegetation (Figure 2). The dependence, measured as the distance between the lines in Figure 1, generally decreased with LAI and MTA. For NDVI, the lines for LAI = 3 and LAI = 5 nearly intersect at MTA = 15\({}^{\circ}\) indicating a lack of dependence of the index on LAI. This well-known saturation with LAI, however, is not present in EVI, EVI2 and MTVI2, for the LAI intervals investigated here. In the 100,000 simulations (Figures S1-S3) and empirical data, the results visualized in Figure 2 were corroborated: VIs showed generally stronger correlations with MTA than LAI (Table 2). The only exceptions were NDVI and WDRVI, for which the correlation with LAI was stronger than with MTA in both datasets; for NDVI, the correlation with LAI was stronger than with MTA in the field-measured data. However, even the strongest correlations with LAI, (simulated dataset: \(R_{\rm s}=0.71\) and measured dataset: \(R_{\rm s}=0.58\)) for NDVI and WDRVI were weaker than the strongest correlation (simulated dataset: \(R_{\rm s}<-0.81\) and measured dataset: \(R_{\rm s}<-0.71\)) for EVI, EVI2 and MTVI2. The strongest correlations in both the simulated and measured datasets were found between all studied indices and Fcover (Table 2). In the simulated dataset, \(R_{\rm s}\) was consistently above 0.93 with \(R_{\rm s}=0.95\), indicating a near-perfect correlation for NDVI and WRDVI. In the measured data, the values of \(R_{\rm s}\) were somewhat smaller, around 0.8, with the lowest value (\(R_{\rm s}=0.78\)) produced by NDVI and WRDVI. The correlations in the empirical dataset are presented graphically in Figures 3-5. The species-specific MTA values cause the clustering of points into vertical stripes in these figures. The length of each stripe depends on the variation of a VI on parameters other than MTA. The variation of these parameters in the measured dataset is provided in Table 1 in the reference [20]. Despite considerable variation, the strong effect of MTA on EVI, EVI2, OSAVI and MTVI2 is clear from Figure 3b-e. For the other indices (Figure 3a,f) \(R_{\rm s}\) was lower (\(R_{\rm s}\)= \(-0.53\)) and the index values at MTA = 54\({}^{\circ}\) and MTA = 58\({}^{\circ}\) (wheat and oat, respectively) could be as high as for MTA = 18\({}^{\circ}\) and MTA = 27\({}^{\circ}\) (lupin and faba bean, respectively). Figure 2: Relationship between MTA and VIs at different LAI values (low: LAI = 1, medium: LAI = 3 and high: LAI = 5) for (**a**) NDVI, (**b**) EVI, (**c**) EVI2, (**d**) OSAVI, (**e**) MTVI2, (**f**) WDRVI. The correlation between the VIs and LAI were all positive (Figure 4). No obvious saturation at high LAI values (LAI > 3) was visible for any of the indices used in this study. However, the point clouds were divided into two subclouds with a small set of measurements with LAI = 2-3 (corresponding to faba bean and lupin, the two non-cereal crops with the lowest recorded MTAs) with VI values higher than the rest of the data.