## 4 Discussion
### The Quantitative Evaluation of Summer Drought Impacts on Carbon-Water Process  
In NADRs, the difference of average GPP (or ET) between humid and extreme drought summer is near 0.36 g C m\({}^{-2}\) day\({}^{-1}\) (or 0.18 mm day\({}^{-1}\)), and GPP (or ET) reduces by nearly 10% (Figure 6c,d) from a normal to extreme drought summer during 2003 to 2015, which is consistent with the global average value [70]. The quantitative impacts of drought were weakened by the insignificant correlations between GPP (or ET) and SPEI. As Figure 8 shows, the difference of GPP (or ET) between humid and extreme drought summer decreases (0.23 g C m\({}^{-2}\) day\({}^{-1}\) and 0.11 mm day\({}^{-1}\), \(p\) > 0.10), thus only analyzing the insignificant correlated regions of the NADR.  
As for the different biomes, with the increased dryness, the change directions of GPP and ET at grass vegetation decreases and MF regions (Figure 6a-d) agree with the correlation analysis results (Figure 5). However, shrubs and cro  
Figure 8: The average of summer GPP and ET at different dryness levels and biomes. Comparison GPP: (**a,b**) GPP and ET at 0.05 significant correlated levels with drought (\(p\) < 0.05); (**c,d**) GPP and ET at 0.10 significant correlated levels with drought (\(p\) < 0.10); (**e,f**) GPP and ET at insignificant correlated levels with drought (\(p\) > 0.10).  
responses with correlation results (Figure 6a-d). For shrubs, due to the root length and small leaf area, the plants with high water use efficiency could continue the photosynthesis and transpiration process to maintain growth during moderate drought [67; 68]. Therefore, GPP (or ET) increases a little from normal to MD summers (Figure 6a-d). For cropland, radiation (or temperature) is the main limited factor, which improves the photosynthesis and transpiration process, but photosynthesis rate will not increase when temperature reaches a threshold [71].  
Compared with the abrupt shifts during a continuous two years (i.e., a normal summer followed by a dry summer), the persistent or multiple droughts intensify the impacts on the carbon-water process in NADRs with higher change rates (Figure 6c-f). It should be noted that the impacts of abrupt shifts on grassland GPP are a little different due to the different response of steppe types (Figure 6f). However, the transition from a normal summer to an extreme drought summer did not reduce GPP seriously in meadow steppe, temperate steppe and desert steppe, which disagrees with correlation analysis and common knowledge. Fortunately, we noted that the extreme summers were not frequent (1 to 3 times, Figure 4c) and there is a wet winter and warm spring before extreme summer in the grassland of China (Figure 9). It means that the previous wet winter and warm spring weakens the summer drought impacts on vegetation production, which is also verified in the US [72], because the precipitation in winter could be stored in the soil to support plant growth in the spring [73] and warm spring increases ET and carbon uptake to compensate for the GPP reduction during extreme summer drought [72]. Overall, the wet winter and spring weakens the summer drought impacts on GPP in NADR grassland.  
Figure 9: The change of summer GPP and previous winter spring conditions in grasslands of China. (**a**) The grassland types of north China. (**b**) The patterns of GPP reduction in the ED summer are lower than those in the SD summer. (**c**) The spring temperature conditions at (**b**) patterns by spring mean temperature anomaly (warm spring: temperature anomaly \(>0\), cold spring: temperature anomaly \(<0\)). (**d**) The previous winter wet–dry conditions at (**b**) patterns by the winter SPEI values (dry winter: SPEI \(<-1\), wet winter: SPEI \(>1\)).