## 3 Results  
### The Anomalies of Meteorological Indicators during Grassland Growth Period  
Firstly, we need to use SPEI, PPT, SM, and LST data to determine drought events in the Xilingol Great Grassland. The drought conditions in the Xilingol grasslands were analyzed based on the annual anomalies of SPEI, PPT, SM, and LST from 2007 to 2015 using the averages of all pixels of the study area. From the results, it can be observed that the SPEI values in 2007, 2009, 2011, and 2014 were all below the annual average (Figure 2). The annual anomalies of PPT and SM showed similar trends, being below the annual average in 2007, 2009, 2010, and 2011. However, the annual anomalies of LST did not show significant changes compared to the average in 2008, 2009, and 2010. Due to the small annual anomaly of SM in 2007 (\(-\)0.008 m\({}^{3}\)/m\({}^{3}\)) and to eliminate the influence of LST anomalies, we ultimately selected 2009 as the drought year.  
Subsequently, we analyzed the monthly anomalies of drought indicators in 2009 by subtracting the monthly value of indicators from the average of each particular month through the 2007-2015 period. As shown in Figure 3, the SPEI anomalies were consistently below the monthly average throughout the year, especially in May, June, and July. Significant decreases in PPT were observed in May, July, and September, resulting in SM anomalies below the monthly average after May. The LST anomalies were only below the monthly average in June, September, and November, while in other months, they were higher than the monthly average.  
Figure 2: Annual anomalies of drought indicators: (**a**) SPEI, (**b**) SM, (**c**) PPT, and (**d**) LST in the Xlingol grassland from 2007 to 2015.  
Next, we further investigated the spatial distribution of anomalies of drought indicators in 2009 (Figure 4). The SPEI anomalies revealed that the entire Xilingol grassland experienced drought in May, June, and July, with the northeastern part being particularly severe. Drought conditions in the northern region eased after August, and by October, only the southern part of the Xilingol grassland remained affected by drought. In May, the entire grassland experienced scarce precipitation, especially in the eastern region. In June, positive PPT anomalies were observed in the western and northern regions, alleviating the drought conditions in those areas. In July, precipitation further decreased in the northeastern part of the Xilingol grassland, intensifying the drought. The occurrence of precipitation in August improved the drought situation. From May to October, most of the Xilingol grassland experienced insufficient soil moisture, particularly in July and August, with severe soil moisture deficits in the northeastern and southern parts. Drought sometimes coincided with high temperatures. As shown in Figure 4, in May, the positive anomalies in LST spatial distribution in most areas of the Xilingol grassland were consistent with the negative anomalies in the other three drought indicators. In August, the positive anomalies in LST spatial distribution in the northeast and southeast of the Xilingol grassland were consistent with the negative anomalies in SPEI and SM.  
Figure 3: Monthly anomalies of (**a**) SPEI, (**b**) SM, (**c**) PPT, and (**d**) LST in the Xilingol grassland for January to December in 2009.