**4. Discussion**

#### *4.1. Driving Analysis of Drought in the Inner Mongolia*

Drought is mainly caused by the imbalance of regional precipitation and evapotranspiration. We found that the change rate of SPEI during the growing season in Inner Mongolia from 2000 to 2018 ranged from −0.4 to 0.75·(10 yr−1), and the area with decreasing SPEI accounted for 20.3% of the total area. The areas where the SPEI increased significantly (*θ*slope is between 0.25 and 0.75·(10 yr−1)) are mainly located in Hulunbuir City, Hinggan League, and Tongliao City. The distribution characteristics of SPEI shown in this study are similar to the results of previous drought monitoring based on long-term series [19,20]. The differences are mainly manifested in the areas such as Hinggan League and Tongliao City, where drought changes increased significantly. Around 2000, the SPEI time series of the Mongolian Plateau showed a significant turning point from increasing to decreasing [34,44]. The results of the Geodetector modeling showed that the SPEI change was driven by four main controlling factors: air temperature, precipitation, DTR, and elevation (Table 4) during the growing season in the study area. Precipitation is a direct factor in drought (*q* = 0.73). Relevant studies have shown that in arid and semi-arid areas, vegetation growth and ecosystem health status depend directly on atmospheric precipitation [45]. The interaction detection results show that the joint effect of precipitation and elevation has the most explanatory power (*q* = 0.87). Conclusions about attribution analysis of drought agreed with a previous study [36]. However, we considered the special climatic background of Inner Mongolia in this study of meteorological drought. The impact of elevation reflects the influence of topography on mass and energy transportation and distribution of temperature and water availability that affect regional ecosystems through altering vegetation species and distribution and the formation and evolution of regional climate [46]. According to the drought trend (Figure 4a) and the spatial distribution of the regression coefficient of SPEI with GWR model factors (Figure 6), it was found that significantly reduced SPEI at a rate of −0.40~−0.25·(10 yr<sup>−</sup>1) occurred in Alxa Left Banner in Alxa League, Dorbod Banner chain Ulanqab City, Darhan Muminggan United Banner in Baotou City, and Wuhai City in the western part of the study area. The drought in Alxa League and Wuhai City was caused by a synergy of hot air temperature, lack of precipitation, high elevation, and high DTR, while the drought in Ulanqab City and Baotou City was mainly caused by hot air temperature, lack of precipitation, and high elevation.

#### *4.2. Variation of Explanatory Power of Factors in Different Elevations*

Due to the large and high terrain environment of the Qinghai-Tibet Plateau in the region of the China-Mongolia Arid and Semiarid Area (CMASA), the lack of water vapor transported over the central and western Inner Mongolia has resulted in scarce precipitation [47]. The central and western part of the study area is dominated by plateaus and mountains. Due to the barrier and uplifting effects of the Great Khingan Range-Yin Mountains-Helan Mountains on water vapor, the eastern and southern piedmont of the mountains are the East Asian monsoon zone (elevation is about 150~500 m) and the west piedmont of the mountains is the non-monsoon zone (the elevation is generally higher than 1000 m).

As an important terrain factor, elevation has a significant impact on the spatial correlation of factors. We found an interesting pattern to speculate the relative importance of environmental and anthropogenic factors in our study area by elevation gradients. As the statistical results of *q*-values of various factors in different elevation intervals (Figure 7a), in the 100–500 m elevation interval, the average precipitation (*X*1) in the growing season is 261.5 mm, and the *q*-value is the smallest. When the precipitation reaches a certain level, the impact of precipitation on SPEI decreases [48]. For the forest area, the ecological water storage is sufficient and the correlation between SPEI and sunshine duration (*X*4) is stronger. In the elevation range of 800~1000 m, the *q*-values of factors such as air temperature (*X*1), precipitation (*X*2), sunshine duration (*X*4), slope (*X*9), POS (*X*11), and LUCC (*X*12) increased significantly, indicating that the change in SPEI was mainly affected by natural factors and some human activities. When the elevation increases to 1000~1300 m, the *q*-values of POS (*X*11) and LUCC (*X*12) reach the maximum of 0.38 and 0.42, respectively, and the *q*-values of other factors show a downward trend. In the area above 1300 m in elevation, the explanatory power of all factors decreases significantly with increasing elevation.

It is worth mentioning that the conversion of land use types is an important influencing factor reflecting human activities, as well as an important explanatory factor for drought changes (Table 3). Between 2000 and 2018, in the 800~1300 m elevation interval, SPEI was significantly enhanced by the land use conversion. According to statistics, the land use types that account for the largest area in this elevation interval are unaltered woodland and unaltered grassland (Table 6), which account for 48.3% and 15.5% of the total area of the region, respectively. The growth rates of SPEI are 0.52·(10 yr−1) and 0.018·(10 yr−1), respectively, indicating that the series of ecological restoration projects such as "closing hills for afforestation and reforestation, retiring grazing and raising grass" implemented by the Chinese government in Inner Mongolia since 2000 have played an important role [49]. In addition, in the conversion of land use from unused land to grassland and from farmland to grassland, SPEI increased by 0.24 and 0.58, respectively, which alleviated the drought conditions in the area to a large extent (Figure 5d). The SPEI of the unchanged farmland increased by 0.57, and the SPEI growth rate was 0.019·(10 yr−1). This may be related to improvement in irrigation. Modern irrigation technology has improved the utilization

rate of water resources and increased the field water holding capacity [50]. At the same time, grassland degradation caused by overgrazing and long-term abandonment of land aggravated the degree of drought [51,52]. For example, the grassland in the 800~1300 m elevation range was converted to unused land; as a result, the SPEI decreased by 0.42, while the SPEI of unaltered unused land decreased by 0.21.


**Table 6.** The area of specific land use conversion in 800~1300 m.

Note: The numbers in parentheses are the percentage of specific land use conversion to the total area (%).
