Evaluation And Application Of Satellite-based FAPAR Products Over China

FAPAR,the Fraction of Absorbed Photosynthetically Active Radiation,is the fraction of the incoming solar radiation in the photosynthetically active radiation spectral region that is absorbed by a photosynthetic organism,typically describing the light absorption across an integrated plant canopy.This biophysical variable is directly related to the primary productivity of photosynthesis and reflecting both the ability of plant canopy to absorb and the relative intensity of terrestrial carbon sinks.As one of the important vegetation indices and essential climate variables(ECVs),FAPAR can monitor the dynamic changes of land vegetation under climate change and the intensity changes of terrestrial carbon sinks.At the same time,FAPAR is an important input varnable in many models such as land surface model,climate model,and hydrological model.Therefore,FAPAR has very important significance in the study of climate change and land surface processes.With the use and improvement of satellite remote sensing technology,a variety of FAPAR products have been developed and applied in many different fields.Satellite-based FAPAR products have the advantages of wide coverage,high accuracy,and real-time performance.In order to provide reference for the selection and application of FAPAR product datasets in China clearly,this paper selected MODIS,GEOV1,and QA4ECV FAPAR products,and systematically evaluated these three sets of satellite-based FAPAR products' performances over China.Then the Triple Collocation Error Model was used to calculate the uncertainty of these independent products under different land cover types.Finally,MODIS and GEOV1 FAPAR products were analyzed to monitor the response of biosphere to drought events and then explore the possible causes.The results of the research show that the existing satellite-based FAPAR products have]arge inconsistencies and the overall product errors are relatively large.Among them,(1)The spatial distribution and seasonal variation of the three FAPAR products in China are basically same.The 11-year mean FAPAR distribution shows that the FAPAR in the east is higher than in the west,and it in the south are higher than in the north.The FAPAR reaches the maximum in summer and the minimum in winter;(2)There are differences in the correlation between the three groups of products under different land cover.In spring,in the Northeast forestry area,where forest cover is dominant,there are significant differences among the three groups of products.In the Shandong agricultural region,where cropland is the main focus,the consistency of the three groups of products is better.In summer,the correlations between the three groups of FAPAR products in the forest land and farmland were poor,and the performance was relatively inconsistent;(3)The errors of the three FAPAR products in the spring was relatively small.In the entire China region,the Northeast forestry area and the Shandong agricultural region,MODIS FAPAR has the smallest error,followed by GEOV1 FAPAR,while the QA4ECV FAPAR product has the largest error.In summer,the error of the three groups of products is greater than that of spring.MODIS FAPAR have the smallest error over the entire China region.In the Northeast forestry area,the product with the smallest error is GEOV1 FAPAR,followed by MODIS,while the QA4ECV FAPAR product has large errors.In the Shandong agricultural region,the product with the smallest error is MODIS,while the error of GEOV1 is greater than the other two FAPAR products.Through analyzing the spatial distribution of the correlation between FAPAR and drought index scPDSI,the ability of FAPAR to monitor land surface vegetation,land-based carbon sequestration capacity and the response of land surface ecosystems to drought were assessed.The results of this study show that:(1)MODIS FAPAR can monitor the response of land surface vegetation to drought,and long-term drought is one of the important drivers of the interannual variation of FAPAR.(2)There is a spatial difference in the response of FAPAR to drought.In the north of China,there is a positive correlation between FAPAR and climate drought.In particular,FAPAR in grasslands in central Inner Mongolia,Shandong,and Shanxi has a strong response to drought.In southern China,the correlation between the FAPAR and scPDSI is very low and exists negative correlation in some regions.(3)The reasons for the spatial and temporal differences in the response of FAPAR to drought are discussed.Land cover types and human activities may be parts of the reasons for these spatio-temporal differences.