## 3 Results  
### The Spatial Distributions of Annual GPP  
We first analyzed the spatial distributions of annual GPP across the YRB during the period 2000-2018 (Figure 2a). The long-term average of GPP in the YRB was \(1153.5\pm 472.4\) g C m\({}^{-2}\) yr\({}^{-1}\). The total amount of GPP in the YRB was \(2.08\times 10^{15}\) g C yr\({}^{-1}\). The multi-year average GPP of the YRB mainly ranged from 800 to 1800 g C m\({}^{-2}\) yr\({}^{-1}\), and the regions in this interval accounted for 76.24% of the entire YRB (Figure 2b). The high-value areas of GPP (higher than 1500 g C m \({}^{-2}\) yr\({}^{-1}\)) covered 25.77% of the YRB and were primarily situated in the middle and lower reaches. The low GPP (lower than 500 g C m\({}^{-2}\) yr\({}^{-1}\)) areas covered 13.04% of the YRB and were mainly distributed in upper reaches of the YRB (Qinghai Province). The annual average GPP showed a significantly decreasing trend with the increase of latitude (Figure 2c), and the rate of change in GPP varied with increasing latitude. Results show that below 32.125\({}^{\circ}\) N, GPP decreased slightly with the increase in latitude (_Slope_ = \(-47.83\) g C m\({}^{-2}\) yr\({}^{-1}\) degree\({}^{-1}\), \(p<0.01\), \(R^{2}=0.87\)), but above 32.125\({}^{\circ}\), GPP decreased sharply (_Slope_ = \(-419.18\) g C m\({}^{-2}\) yr\({}^{-1}\) degree\({}^{-1}\), \(p<0.01\), \(R^{2}=0.94\)). In general, GPP would be reduced by 107.31 g C m\({}^{-2}\) yr\({}^{-1}\) for every 1 degree increase in latitude in the YRB (\(p<0.01\), \(R^{2}=0.71\)). The annual average GPP exhibited a pattern of initially rising and subsequently declining with the increase in longitude. The increasing trend (_Slope_ = 96.78 g C m\({}^{-2}\) yr\({}^{-1}\) degree\({}^{-1}\), \(p<0.01\), \(R^{2}=0.96\), Figure 2d) was more rapidly than the decreasing trend (_Slope_ = \(-45.28\) g C m\({}^{-2}\) yr\({}^{-1}\) degree\({}^{-1}\), \(p<0.01\), \(R^{2}=0.61\)). The maximum GPP value was 2076 g C m\({}^{-2}\) yr\({}^{-1}\) and was mainly distributed around 110\({}^{\circ}\) E.  
We investigated the variations of the annual GPP in different water resource regions and vegetation types in the YRB, the results of which were shown in Figure 3. Similar to the above results, the annual average GPP of 12 water resource regions first showed an increase and then a decrease with the increase in longitude. In general, the Han River Basin (HJ) had the largest annual average GPP of 1502.3 \(\pm\) 273.1 g C m\({}^{-2}\) yr\({}^{-1}\), followed by the Yibin-Yichang section of the Yangtze River mainstream (UM) and the Poyang Lake Basin (PYL), with an annual average GPP of 1401.5 \(\pm\) 184.0 g C m\({}^{-2}\) yr\({}^{-1}\) and 1382.9 \(\pm\) 304.1 g C m\({}^{-2}\) yr\({}^{-1}\), respectively. The Jinsha River Basin above Shigu (JSJ-1) had  
Figure 2: The spatial variations of annual average GPP in the YRB between 2000 and 2018 (**a**) and its statistical characteristics (**b**). The variations of the long-term average of GPP with the changes in latitude (**c**) and longitude (**d**).  
the lowest average GPP of 321.8 \(\pm\) 328.1 g C m\({}^{-2}\) yr\({}^{-1}\), followed by the Taihu Lake Basin (TL) with 891.7 \(\pm\) 404.8 g C m\({}^{-2}\) yr\({}^{-1}\). In addition, the Jinsha River Basin below Shigu (JSJ-2) had the highest standard deviation of 428.5 g C m\({}^{-2}\) yr\({}^{-1}\) followed by the Taihu Lake Basin (TL) of 404.8 g C m\({}^{-2}\) yr\({}^{-1}\).  
We analyzed the GPP variations in different vegetation types (Figure 3b). Results showed that forests had much higher GPP values, with an average of 1476.9 \(\pm\) 287.3 g C m\({}^{-2}\) yr\({}^{-1}\), followed by savannas and cropland, with GPP averages of 1345.7 \(\pm\) 274.2 g C m\({}^{-2}\) yr\({}^{-1}\) and 1130.4 \(\pm\) 246.2 g C m\({}^{-2}\) yr\({}^{-1}\), respectively. Grassland had much lower GPP values with an average of 469.0 \(\pm\) 354.3 g C m\({}^{-2}\) yr\({}^{-1}\).