## 3 Results
### The Impact of Drought on NPP  
#### 3.3.1 Characteristics of Drought  
Based on the NDVI anomaly, the spatial distribution of drought across Mongolia from 2003 to 2018 was identified, and the drought years and their affected area percentage are illustrated in Table 4. On average, drought-affected areas accounted for 47.87% every year. To explore the effects of drought on NPP, drought events with larger affected areas in 2004 (60.01%), 2005 (60.37%), 2006 (66.99%), 2007 (63.42%), 2009 (71.52%), and 2017 (72.44%) were further investigated by performing several statistical analyses. We assessed the effects of extensive droughs on vegetation productivity across the Mongolian rangeland by comparing the spatial patterns of growing season total NPP anomaly and drought severity. The most influential drought occurred between May to September 2017 and affected 1.13 million km\({}^{2}\), covering 72.43% of the country's land areas (Figure 5). During this drought event, severe and extreme drought areas covered approximately 4.71 and 0.06% and moderate and mild drought areas covered 31.33 and 36.32% of land areas of the entire country, respectively. The drought in 2004 mainly occurred in the eastern regions of Mongolia, and the drought areas covered nearly 0.94 million km\({}^{2}\), which accounts for about 60% of the total land area of the country. Most of the country experienced mild (35.5%) and moderate (27.4%) drought during the 2009 drought events. The drought from May to September in 2005 mainly affected the central and southeast regions. In 2006, drought was predominant in the southwest and the eastern regions. The country experienced mild (20.95%), moderate (25.04%), and severe (17%) drought during the year 2007. Our result was consistent with previous findings [61;  
Figure 4: (**a**) Spatial distribution of mean growing season NPP across Mongolia from 2003 to 2018. (**b**) The spatial distribution of the trend in growing season NPP during the study period. The white-colored areas on land are non-vegetated pixels. \begin{table}
\begin{tabular}{c c c c c} \hline
**Variable** & **Nonsignificant Increase (\%)** & **Significant Increase (\%, \(\mathbf{p<0.05}\))** & **Nonsignificant Decrease (\%)** & **Significant Decrease (\%, \(\mathbf{p<0.05}\))** \\ \hline NPP & 56.37 & 7.23 & 46.63 & 2.60 \\ Precipitation & 51.15 & 0.07 & 48.85 & 1.39 \\ Temperature & 99.88 & 20.60 & 0.12 & - \\ Net radiation & 9.13 & - & 90.86 & 59.14 \\ \hline \end{tabular}
\end{table} 
Table 3: Statistic summary of the percentages of pixels showing different trends in growing season NPP and climatic variables across Mongolia during 2003–2018. that the NDVI anomaly can also be used for long-term drought monitoring in arid and semiarid regions. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Year** & **Normal** & **Mild** & **Moderate** & **Severe** & **Extreme** \\ \hline
2003 & 58.95 & 26.23 & 13.24 & 1.41 & 0.16 \\
2004 & 39.99 & 33.59 & 20.20 & 6.12 & 0.09 \\
2005 & 31.63 & 32.21 & 29.34 & 6.70 & 0.12 \\
2006 & 33.01 & 40.19 & 21.58 & 5.08 & 0.14 \\
2007 & 36.58 & 20.96 & 25.04 & 17.01 & 0.41 \\
2008 & 45.48 & 31.25 & 18.47 & 4.67 & 0.14 \\
2009 & 28.48 & 35.57 & 27.43 & 8.34 & 0.19 \\
2010 & 49.62 & 32.87 & 15.15 & 2.24 & 0.12 \\
2011 & 66.86 & 25.70 & 6.83 & 0.50 & 0.11 \\
2012 & 72.81 & 22.06 & 4.64 & 0.37 & 0.13 \\
2013 & 73.64 & 20.64 & 5.19 & 0.37 & 0.16 \\
2014 & 71.27 & 21.43 & 6.89 & 0.30 & 0.11 \\
2015 & 58.81 & 28.42 & 11.74 & 0.92 & 0.12 \\
2016 & 74.69 & 14.97 & 7.94 & 2.30 & 0.11 \\
2017 & 27.56 & 36.32 & 31.33 & 4.72 & 0.06 \\
2018 & 64.59 & 21.39 & 11.98 & 1.92 & 0.11 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Drought-affected area in Mongolia over the period 2003–2018. The percentage number (unit: %) indicates the drought-affected area.