Relationship between Fraction of Photosynthetically Active Radiation and Vegetation Indices,Leaf Area Index of Corn and Soybean

Based on the measured reflectance,photosynthetically active radiation,and leaf area index(LAI),nine common vege-tation indices and two vegetation indices,developed by the authors according to the characteristic of reflectance curves and the vegetation index structures,were used to estimate fraction of photosynthetically active radiation(FPAR) in corn(Zea mays L.) and soybean [Glycine max(L.) Merri.].The relationships between vegetation indices and FPAR differed in different vegetation types.The performances of near infrared and shortwave bands were better than these of visible and near infrared bands in FPAR estima-tion by vegetation indices.Normalized difference vegetation index(NDVI) and ratio vegetation index(RVI) were the best in the vegetation indices calculated by reflectance in visible and near infrared bands for FPAR estimation,with R2 of estimating models 0.82 and 0.81 for corn,both of 0.81 for soybean.Normalized difference shortwave index(NDSI) and ratio shortwave index(RSI) were best in the vegetation indices calculated by near infrared and shortwave bands with almost the same R2 values for NDSI and RSI in corn(0.86) and soybean(0.84).It is suggested that the near infrared and shortwave bands are feasible to be used for FPAR estimation.According to the field experiments,the water content of vegetation canopy had larger effect on FPAR estimation than soil background.The canopy FPAR and LAI showed good logarithm relations in corn and soybean canopy with R2 values of 0.75and 0.70,respectively,indicating that vegetation indices are better than LAI for FPAR estimation.