3.2.2. Impacts of Uncertainty of FPAR Data on MODIS GPP Simulation

Figure 8 showed the comparisons of AccuPAR observed FPAR data with the MOD15A2H FPAR data in the corresponding sites in HRB. We found the MODIS FPAR data was overestimated compared to the observations in the growing season of desert grassland sites, as well as the low values of FPAR in the cropland sites, and underestimated in some stages of the high values of FPAR in the cropland sites. The overestimated FPAR impacted the APAR, thus leading to an overestimated GPP. In contrast, the underestimated FPAR would underestimate the GPP. Meanwhile, a good correlation between MODIS FPAR and observed FPAR occurred in cropland sites, while in the desert grassland sites, no significant relationship was found. This revealed that the accuracy of the current MOD15A2H FPAR data in the arid region needs to be further improved, and could also be an important source of uncertainty for the estimation of GPP in desert ecosystems.

**Figure 8.** Validation of MODIS FPAR compared with FPAR measured by AccuPAR. The left plot is the validation at DMZ and SDZ, the right plot is the validation at GBZ and HZZ.
