*3.2. Uncertainty of Satellite Data in MODIS GPP Simulation over Ecosystems in the Arid Region*

3.2.1. Impacts of the Accuracy of the Land Cover Classification on MODIS GPP Simulation

One of the first MODIS products used in the MOD17 algorithm is the Land Cover Product, MOD12Q1. The importance of this product cannot be overstated as the MOD17 algorithm relies heavily on land cover type through use of the BPLUT [15]. Based on the locations of the flux tower sites, we obtained the land cover type of each site from the MCD12Q1 results, and compared them with the actual land cover types. We found that MCD12Q1 misclassified most of the sites downstream of the HRB (Figure 7), which is an artificial oasis ecotone with sparse vegetation in the extremely arid region of China. For example, in our study, the land cover type of the NTZ site is a cropland ecosystem; however, it was classified as grassland in the MCD12Q1 land cover data (Figure 7). In addition, MCD12Q1 also misclassified the forest types at the sites of HHL, SDQ, and HYL as grassland.

**Figure 7.** Misclassification of land cover in the MOD12Q1 products at the downstream HRB, which classified the land cover of forest (i.e., SDQ, HHL, HYL) and cropland (NTZ) as the grassland type in MOD12Q1 data. (**a**) SDQ, (**b**) HHL, (**c**) HYL, and (**d**) NTZ.
