*2.2. MODIS Datasets*

The MODIS data used in this study include MODIS GPP data (MOD17A2H products), FPAR data (MOD15A2H products), and Surface Reflectance data (MOD09A1 products) with Collection 6 at 500 m spatial resolution were downloaded directly from the Oak Ridge National Laboratory Distributed Active Center (ORNL DAAC) website. FPAR is the fraction of photosynthetically active radiation (400–700 nm) absorbed by green vegetation, which is a critical component of the MODIS GPP algorithm. To correct inferior values caused by the effects of clouds and aerosols, we reconstructed the MODIS FPAR time series data with Savizky–Golay filter algorithm [47]. Meanwhile, to validate the performance of the MODIS FPAR data in the study area, we observed the actual FPAR data of cropland and desert grassland sites in HRB using AccuPAR (METER Group, Inc., Pullman, USA) during the growing seasons of vegetation in 2012 [43], and then compared the observations with the MOD15A2H FPAR data at corresponding sites. In addition, we also used the MODIS surface reflectance data to derive vegetation indices, such as the normalized difference vegetation index (NDVI). For the site of NTZ, due to the growing season of cropland (i.e., cantaloupe) is short, the desert or low vegetation land cover was identified in the MOD15A2H product, and thus we calculated the FPAR from NDVI data following the empirical formula of: FPAR = 1.24 × NDVI − 0.168 [48].
