# Estimation of Global Vegetation Productivity from Global LAnd Surface Satellite Data
## 3 Results
### Validation of Estimated GLASS GPP  
#### 3.2.1 Seasonal Variation in GPP  
For various vegetated land cover types, we compared our GPP estimates with eight-day MODIS GPP product and FLUXNET GPP, as shown in Figure 8. With the application of temporal infilling of cloud-contaminated pixels and a consistent forcing meteorology in MODIS revised GPP product (MOD17 C55), we found that the time series of MOD17 C55 GPP was more continuous than the MODIS standard GPP product (MOD17 C05). But, the time series of estimated GLASS GPP in 2004, 2008 and 2012 are clearly more continuous with smoother trajectories than MODIS GPP (both MOD17 C05 GPP and MOD17 C55 GPP), and they agree well with the variations in FLUXNET GPP. More specifically, the time series of estimated GLASS GPP in deciduous broadleaved forests (DBF) are shown in Figure 8a. It can be clearly seen that estimated GLASS GPP agrees better with FLUXNET GPP than MODIS GPP does and is more continuous, especially during the growing season. At US-Ha1 and De-Hai, estimated GLASS GPP matches much better with the FLUXNET GPP and reflects the peak of the growing season, but MODIS GPP is somewhat underestimated at the peak of the growing season, especially MOD17 C05 GPP in 2004. FLUXNET GPP and estimated GLASS GPP could be as high as 14 g C\(\cdot\)m\({}^{-2}\cdot\)d\({}^{-1}\) in the peak of the growing season, but MODIS GPP is much lower, only approximately 8 g C\(\cdot\)m\({}^{-2}\cdot\)d\({}^{-1}\). In evergreen broadleaved forests (EBF), estimated GLASS GPP is obviously more continuous and generally showed better agreement with the FLUXNET GPP, as shown in Figure 8b. FLUXNET GPP showed little seasonal variation at the GF-Guy site, which is located in South America and near the equatorial region. But, at this site, MODIS GPP varied significantly, especially the MOD17 C05 GPP, and was underestimated in the first half of 2004, 2008 and 2012. In 2008 and 2012, MODIS GPP was underestimated most of the time and failed to catch the changes in FLUXNET GPP. Figure 8c shows the GPP time series in evergreen needle-leaf forests (ENF). Good agreement was achieved between estimated GLASS GPP and FLUXNET GPP, in terms of the absolute values and seasonal trends. Overall, the time series of estimated GLASS GPP was more continuous than MODIS GPP. But at DE-Tha, estimated and MODIS GPP were a little underestimated. In 2008, the estimated growing season was delayed a little compared to the FLUXNET growing season. Figure 8d demonstrates the GPP time series in croplands (CRO) in 2004, 2008 and 2012. Both MODIS GPP and estimated GLASS GPP significantly underestimated the FLUXNET GPP. Particularly during the growing seasons, estimated GLASS GPP and MODIS GPP were much lower than the FLUXNET GPP. The FLUXNET GPP could be as high as 16 g C\(\cdot\)m\({}^{-2}\cdot\)d\({}^{-1}\) at the peak of the growing season, but our estimated GLASS GPP was only approximately 8 g C\(\cdot\)m\({}^{-2}\cdot\)d\({}^{-1}\). The MODIS GPP was much less, just about 6 g C\(\cdot\)m\({}^{-2}\cdot\)d\({}^{-1}\). But in the fallow season, estimated GLASS GPP and MODIS GPP were higher than the FLUXNET GPP. FLUXNET GPP started to increase rapidly in early June, but the estimated GLASS GPP and MODIS GPP started to increase in March. Figure 7: Variation in global total GPP and NPP for all vegetated land cover types: (**a**) variation in global GPP; (**b**) variation in global NPP. In grasslands (GRA), as shown in Figure 8e, peak profiles of FLUXNET GPP appeared in May and July at the DE-Gri and AT-Neu sites. MODIS GPP and estimated GLASS GPP did not reflect these  
Figure 8: Seasonal variation in the estimated GLASS GPP, FLUXNET GPP, MOD17 C05 GPP and MOD17 C55 GPP for several sites with different vegetation types. high values. But the matches between the estimated GLASS GPP, MODIS GPP and FLUXNET GPP were very good at other times. The time series for the estimated GLASS GPP, MODIS GPP and FLUXNET GPP in mixed forests (MF) are displayed in Figure 8f.