PP

grassland, and cropland. The ground-based NPP and GPP surfaces were generated by application of the Biome-BGC carbon cycle process model in a spatially-distributed mode. Model inputs of land cover and leaf area index were derived from Landsat data. The MODIS NPP and GPP primary production (NPP) at the 1 km spatial resolution is now being produced operationally based on observations from the interannual variation and long term trends in biosphere behavior (e.g. driven by climate variation or change, Nemani et al., 2003). Validation of these products is an essential step in establishing their utility; however, validation is challenging because of a t 1MODIS (Moderate Resolution Imaging Spectroradiometer) sensor (Justice et al., 2002; Running et al., 2004). The GPPproducts showed no overall bias. They tended to be overestimates at low productivity sites — often because of artificially high values of MODIS FPAR (fraction of photosynthetically active radiation absorbed by the canopy), a critical input to the MODIS GPP algorithm. In contrast, the MODIS products tended to be underestimates in high productivity sites — often a function of relatively low values for vegetation light use efficiency in the MODIS GPP algorithm. A global network of sites where both NPP and GPP are measured and scaled over the local landscape is needed to more comprehensively validate the MODIS NPP and GPP products and to potentially calibrate the MODIS NPP/GPP algorithm parameters