| The variation of empirical LAI-VI relationship and the variation of the extinction coefficient(KVI) derived from Beer's law were analyzed by using PROSAIL model and canopy spectrum data across different canopy geometry varieties(erective varieties,middle varieties and horizontal varieties).Reflectance of blue(B) at 450 nm,red(R) at 670 nm,near infrared(NIR) at 830 nm were chosen to calculate 3 kinds of VIs(Normalized Difference Vegetation Index-NDVI,Ratio Vegetation Index-RVI,Enhanced Vegetation Index-EVI).The results showed that the relationships between VIs and LAI were affected by canopy geometries,and they should be developed separately for different geometry types.However,the influencing degrees of canopy geometries were different among NDVI,RVI and EVI.In particular,the EVI-LAI relationship could be improved significantly(R2 increased over 30%) when canopy geometries were considered.The difference of NDVI-LAI relationship among different wheat geometries was slim and was displayed only when LAI was lower than 3.The RVI-LAI relationship was more sensitive than NDVI-LAI relationship.When the canopy geometry was erective,the estimated model of RVI and LAI could be changed from exponential form to linear form,and the value of extinction coefficient(KVI) would be reduced.It was also showed that the sequence of KVI was KNDVI>KEVI>KRVI.Therefore,the influence of canopy geometry structures should not be ignored in studying the relationship between LAI and VIs for different crop geometry varieties. |