*3.2. Identification of Main Control Factors*

The factor detection and ecological detection show that (Table 4) precipitation was the most explanatory factor (*q* = 0.73). From the *q*-values (Table 4), we can see the top four impact factors in decreasing order were MP > Elevation > MAT > DTR, and all passed the significance test (*p* < 0.05). Slope, aspect, LUCC, AOPD, and DTC have lower explanatory power for drought.


**Table 4.** *q* statistics and ecological detector.

SPEI rates of change (*θ*slope) versus factor level for various factors were shown in Figure 5. We can see that *θ*slope shows a decreasing trend (Figure 5a), with increasing factor level for factors MAT (*X*1), MSD (*X*4), DTR (*X*5), and elevation (*X*7). The elevation factor has the greatest influence on *θ*slope when the factor level is low (≤2), while factors MAT and MSD have a larger impact at a high factor level (>4). The elevation increases from the first level (100~500 m) to the fifth level (1500~3500 m), and the *θ*slope decreases from 0.58·(10 yr−1) to −0.02·(10 yr−1). Compared with other factors, the influence of DTR on *<sup>θ</sup>*slope is approximately linear, and *<sup>θ</sup>*slope drops to the lowest value (0.08·(10 yr−1)) in the DTR interval of 90~321.6 km. On the contrary, *θ*slope shows an increasing trend (Figure 5c) with the increasing factor level of MP (*X*2), MWS (*X*3), and Slope (*X*9). Precipitation has the greatest impact on *θ*slope. The amount of precipitation increases from level 1 (61.2~108.2 mm) to level 5 (262.6~305.8 mm), and *<sup>θ</sup>*slope rises from −0.075·(10 yr−1) to 0.53·(10 yr<sup>−</sup>1). The relationship between *<sup>θ</sup>*slope and any of the following factors, DTC (*X*6), aspect (*X*8), AOPD (*X*10), and POS (*X*11), does not show a significant linear trend (Figure 5b). When POS is at the second level (38~47%), *θ*slope reaches a peak value. This may imply that appropriate amount of sand is conducive to the respiration of plant roots, retaining soil moisture, and transportation of nutrients. However, a percentage of sand that is too high can easily cause surface degradation, soil moisture loss, and soil erosion. DTC, aspect, and AOPD have much smaller variation ranges of *θ*slope, which indicates that these factors have little influence on the change of SPEI rate. SPEI over most land cover types (*X*12) increased from 2000 to 2018, and SPEI decreased over only two land covers, in which cases water area was converted to construction area and unused land (Figure 5d). The *θ*slope of the two land covers was −0.09·(10 yr<sup>−</sup>1) and −0.03·(10 yr<sup>−</sup>1), accounting for 0.1% and 11.5% of the study area, respectively. Among all land conversion types, the SPEI of unaltered forest land increased at the fastest rate 0.51·(10 yr<sup>−</sup>1).

Overall, the factors with strong explanatory power in the factor detector have a larger fluctuation range of *θ*slope. The *θ*slope values of factors such as MP, elevation, MAT, and DTR are in four ranges of −0.07~0.55, −0.02~0.58, −0.04~0.53, and −0.09~0.44·(10 yr<sup>−</sup>1), respectively. The change range of SPEI rate influenced by natural factors, such as meteorology and topography in the study area, was larger than that influenced by human factors.

The ranking of the influence by interacting pair of factors was given in Table 5. Only the precipitation ∩ DTC (*X*<sup>2</sup> ∩ *X*7) and wind speed ∩ elevation (*X*<sup>1</sup> ∩ *X*2) pairs of the first 15 interacting pairs showed nonlinear enhancement; the others were dual-factor enhancement. Among them, the influence by the interaction between precipitation and elevation is the strongest, with a *q*-value of 0.870, followed by that between temperature and precipitation; the interaction between wind speed and elevation has the lowest explanatory power, with a *q*-value of 0.686. As expected, precipitation is an important source of water and a crucial driving factor in the process of drought changes. The difference in precipitation between the east and west of Inner Mongolia contributes mainly to the spatial

differentiation of drought condition. Elevation also has a strong explanatory power for drought. It is an important topographical factor for driving drought in Inner Mongolia, and it is also an important factor in combining other factors to form a drought spatial pattern.

**Figure 5.** SPEI's variation trend with the influence of (**a**) MAT (*X*1), MSD (*X*4), DTR (*X*5), Elevation (*X*7); (**b**) DTC (*X*6), Aspect (*X*8), AOPD (*X*10), POS (*X*11); (**c**) MP (*X*2), MWS (*X*3), Slope (*X*9); (**d**) LUCC (*X*12) with their levels. The meanings of the land cover and land use codes for land use conversion types can be found in Figure 2.

**Table 5.** Influence of the interacting pairs of factors.


Note: Only the first 15 combinations are ranked.

#### *3.3. Spatial Difference of Main Control Factors*

The GWR model was used to perform spatial regression analysis on the four main controlling factors, i.e., MAT, MP, Elevation, and DTR, and local adjusted R<sup>2</sup> and Akaike Information Criterion (*AIC*) as the evaluation indexes of the model fitting. Results show that the adjusted R2 of the GWR model is 0.88, and the *AIC* value is −540.58. The action

direction of the factor is reflected by the sign of the coefficient of the fitting equation. A negative coefficient in an area and the absolute value of the coefficient indicate that the area is drought-stricken and the strength of the driving effect, respectively.

The effects of the two meteorological factors, i.e., temperature and precipitation, have significant spatial differences. The overall fluctuation ranges of the two are relatively large, and the regression coefficient intervals are (−1.20, 0.60) and (−1.4, 0.55), respectively. The area in drought driven by temperature accounted for about 70.2%, of which the areas with strong temperature driving (−0.6~−1.20) were mainly located in Alxa Left Banner of Alxa League, Wuhai City, Hanggin Banner and Otog Banner of Ordos City, Urad Rear Banner of Bayannur City, and parts of Xilingol League (Figure 6a). Due to the large temperature difference between the east and west of the study area (up to 12.7 ◦C), the surface vegetation in the western hot area had strong transpiration and respiration, and the dry matter consumption and soil water loss were larger, which further expanded the arid area [41,42]. The difference of the driving results between precipitation and temperature factors is mainly in the semi-arid grasslands (Figure 6b), such as Xilinhot City, West Ujimqin Banner and East Ujimqin Banner in Xilingol League. The average precipitation in the growing season in this region was greater than 150 mm, which was enough for the growth of vegetation such as grassland, shrubs, and other vegetation [43].

**Figure 6.** Distribution of the regression coefficient of SPEI with (**a**) MAT, (**b**) MP, (**c**) Elevation, and (**d**) DTR.

The influence of elevation mainly reflects the effect of the terrain. The GWR regression coefficient interval is (−1.25, 1.2), with the Great Khingan Range-Yin Mountains-Helan Mountains range as the boundary, and the SPEI driving coefficients on both sides are obviously different (Figure 6c). The central and western regions of Inner Mongolia (Alxa Left Banner, Dalate Banner, Zhungeer Banner, etc.) dominated by the Mongolian Plateau are generally higher than 1000 m in elevation, which has a significant effect on drought. The average GWR coefficient is −0.75. The eastern foothills of the Great Khingan Range-Yin Mountains and the southern foothills of Helan Mountains have lower average elevations, ranging from 100 to 500 m, and the average GWR coefficient is 0.5. The DTR factor reflects the water conservation within the basin, and the range of coefficient is the smallest (−0.17, 0.12). Due to low water conservation in Alxa League, Wuhai City, Ordos City, and Erguna in Hulunbuir City, the impact by DTR is shown in Figure 6d: the drought mitigation area driven by DTR is mainly located at the tributary of the Yellow River in Ordos City in the southwest of Inner Mongolia, Tabu River in Ulanqab City, and Dahei River (River inflow area), and the areas where Liaohe River, Songhua River, Nenjiang River, and other rivers adjacent to the Northeast Plain flow through.
