new developments in the remote estimation of the fraction of absorbed photosynthetically active radiation in crops | 量子荧光|微型光纤光谱仪-ag贵宾会

terrestrial vegetation constitutes a major, if not dominant, element of the interface between the land surface and the atmosphere. quantifying the biophysical properties of terrestrial vegetation and their variation through time is important for a rapid and accurate assessment of the status of the vegetation and its responses to changing environmental conditions. one of such biophysical characteristics is the fraction of absorbed photosynthetically active radiation (fapar). it is one of the main players used in the formulation of production efficiency models (pem). numerous studies [e.g., asrar et al., 1992; sellers, 1985] found that under specified canopy reflectance properties, fapar can be estimated remotely, using the normalized difference vegetation index (ndvi = (rnir rred)/(rnir rred), where rnir and rred are reflectances in the near-infrared (nir) and the red spectral regions, respectively). several pems have been developed that use synoptic ndvi data to estimate net primary production [e.g., running et al., 2004]. nevertheless, ruimy et al. [1994] underscored the fact that the linear relationship between fapar and ndvi is an approximation, and it is only valid during the growing stage. a likely explanation for this is that during the reproductive and senescence stages in crops, the canopy still intercepts the incoming radiation, but the leaves contain less photosynthetic pigments, which leads to a decrease in the ndvi [hatfield et al., 1984; gallo et al., 1985]. therefore, since only the green components of the canopy are used for photosynthesis, fapar needs to be separated into its photosynthetically and non-photosynthetically active components, in order to improve the estimation of vegetation productivity over time [e.g., hall et al., 1992].