# Response of Grassland Vegetation Growth to Drought in Inner Mongolia of China from 2002 to 2020
## 3 Results
### Divergent Grassland Vegetation Growth Response to Two Drought Events  
Figures 8 and 9 show the spatial distributions of SA for NDVI, EVI, GOSIF, and GPP in June, July, August, and September 2007, as well as June and July 2017. Spatially, both NDVI and EVI indicated a widespread decline (SA \(<-0.5\)) in these months, particularly in the east-central regions of Inner Mongolia (SA \(<-2\)) (Figure 8a-h). Specifically, NDVI decreased by 17.39%, 11.38%, 10.80%, and 6.18% in June (\(-0.047\)), July (\(-0.039\)), August (\(-0.036\)), and September (\(-0.016\)), respectively, relative to the multi-year average (Figure 9a,b). Similarly, EVI declined by 18.33%, 13.97%, 11.24%, and 4.98% in June, July, August, and September, confirming a widespread vegetation browning phenomenon following drought from June to September 2007. Compared to NDVI and EVI, GOSIF indicated a similar spatial distribution pattern but displayed more drastic changes from June to September (Figure 8i-l). GOSIF decreased by 34.37%, 30.42%, 29.88%, and 14.12% in June (\(-0.015\) Wm\({}^{-2}\) um\({}^{-1}\) sr\({}^{-1}\)), July (\(-0.024\) Wm\({}^{-2}\) um\({}^{-1}\) sr\({}^{-1}\)), August (\(-0.023\) Wm\({}^{-2}\) um\({}^{-1}\) sr\({}^{-1}\)), and September (\(-0.005\) Wm  
Figure 7: Spatial pattern of SPEI-3 (0.5\({}^{\circ}\) spatial resolution) (**a,b**) and SA\({}_{\text{CCI-SM}}\) (0.25\({}^{\circ}\) spatial resolution) (**c,d**) in June and July 2017. \(\mu\)m\({}^{-1}\) sr\({}^{-1}\)), respectively, relative to the multi-year average (2002-2020) (Figure 9e,f). The spatial distributions of SA for GPP indicated a net loss in carbon sequestration relative to the multi-year average because of the drought events. As a result, GPP decreased by 26.59%, 26.51%, 25.41%, and 10.28% in June (\(-\)10.93 gcm\({}^{-2}\)), July (\(-\)17.58 gcm\({}^{-2}\)), August (\(-\)16.23 gcm\({}^{-2}\)), and September (\(-\)3.48 gcm\({}^{-2}\)), respectively, relative to the multi-year average (2002-2020) (Figure 9g,h). SM decreased by 11.06%, 7.52%, 21.01%, and 6.91% in June, July, August, and September, respectively, relative to the multi-year average (2002-2020) (Figure 9i,j). Figure 8: _Cont._  
Figure 8: The spatial distributions of SA for NDVI (\(\mathbf{a}-\mathbf{d}\)), EVI (\(\mathbf{e}-\mathbf{h}\)), GOSIF (\(\mathbf{i}-\mathbf{I}\)), and GPP (\(\mathbf{m}-\mathbf{p}\)) in June, July, August, and September 2007. In June-July 2017, NDVI, EVI, GOSIF, and GPP revealed a significant inhibition of grassland vegetation growth in east-central Inner Mongolia (SA \(<-0.5\)) (Figure 10). NDVI decreased by 7.31% and 5.22% in June (\(-0.022\)) and July (\(-0.019\)), respectively (Figure 11a,b). Similarly, EVI exhibited reductions of 9.05% and 6.36% in June (\(-0.017\)) and July (\(-0.015\)), respectively (Figure 11c,d). Under the influence of drought stress, GOSIF decreased by 10.20% and 17.01% in June (\(-0.006\) Wm\({}^{-2}\) um\({}^{-1}\) sr\({}^{-1}\)) and July (\(-0.015\) Wm\({}^{-2}\) um\({}^{-1}\) sr\({}^{-1}\)), respectively, signifying a clear inhibition of grassland vegetation growth (Figure 11e,f). GPP decreased by 8.33% and 14.91% in June (\(-4.001\) gcm\({}^{-2}\)) and July (\(-10.893\) gcm\({}^{-2}\)), respectively (Figure 11g,h). SM decreased by 9.06% and 7.52% in June and July, respectively (Fig.11i and 11j). Notably, the decrease in GPP in June was less pronounced than in July. This differential response was particularly evident in GOSIF, while NDVI and EVI failed to capture this variability. Therefore, GOSIF emerges as a superior indicator for assessing the impact of drought events on grassland photosynthesis. Figure 9: Temporal variation of NDVI (**a**,**b**), EVI (**c**,**d**), GOSIF (**e**,**f**), GPP (**g**,**h**), and SM (**i**,**j**) during the June, July, August, and September drought in 2007. \(\Delta\) is the different between values of monthly NDVI, EVI, GOSIF, and GPP in May, June, July, August, September, and October 2007 and multi-year monthly average values of NDVI, EVI, GOSIF, GPP, and SM from 2002 to 2020.