In China's energy structure, coal will still play an irreplaceable role for quite a long time. According to the statistics, by 2030, China's coal consumption will account for about 55% of primary energy consumption. In the short term, China's energy strategy of "coal-based, diversified development" will not be changed (Wang 2015). The Shen Dong Coalfield is rich in resource reserves that account for about 1/4 − 1/3 of China's total coal reserves (Wang 2017). Therefore, the Shen Dong Coalfield serves as the ballast stone of the coal industry and a stepping stone for China's energy safety and security. The region of Shen Dong is located in the transition zone between the Loess Plateau and the Mu Us Desert, with poor natural conditions and a fragile ecological environment (Wang et al. 2008; Zhang et al. 2012); it is also located in the Yellow River Basin, with the low ecological threshold and poor anti disturbance ability. At present, due to coal mining, the underground water level of the Yellow River Basin is declining and the ecological environment is further deteriorating. In 2019, general secretary Xi Jinping inspected the ecological environment and economic development of the Yellow River Basin and delivered important speeches (Xi 2019), which brought more and more attention to the Shen Dong coalfield located in the Yellow River basin. Therefore, coal exploitation and ecological environment protection in Shen Dong coalfield have always been the focus of research.
In the semi-arid northwestern area with wind and sand, soil moisture is a key factor in restricting the growth of vegetation, and limiting the restoration of the ecological environment in subsided areas caused by coal mining (Dougill et al. 1998; Bi et al. 2014). High-intensity mining causes the movement and deformation of rock formations to form subsidence areas and produce cracks, which affects hydrogeology, soil, and vegetation from the bottom to top (Bi et al. 2014; Wang 2017). The first result is the change in soil moisture caused by the formation of subsidence areas and cracks (Zhao 2006). The transports and changes of soil nutrients are closely related to the changes of soil moisture, so changes of soil moisture in subsidence areas will inevitably lead to the transports and changes of nutrients such as nitrogen, phosphorus, potassium and organic matters (Li et al. 2001; Cheng et al. 2014; Liu et al. 2014). The changes of soil moisture also vary the distribution and circulation of the original water and material in the ecosystem, thus affecting the ecological environment. Therefore, it is of great significance to study the change law and influence mechanism of soil water under the condition of coal mining subsidence for vegetation restoration and ecological reconstruction in coal mining subsidence areas.
On the one hand, some scholars believe that ground fractures caused by coal mining are important factors in affecting the changes of soil moisture in subsidence areas (Wei et al. 2006; Wang et al. 2011; Liu et al. 2016; Guo et al. 2019). In addition, the depth, density, width as well as distance from the fractures all influence the changes in soil moisture (Zou et al. 2014; Zhang et al. 2015; Ma and Yang 2019; Wu et al. 2019; Wu et al. 2020). Zhang et al. (2015) tracked and monitored the fractures in a fixed position in Bu Lian ta mine in Shen fu-Dongsheng Coalfield, and then found that the soil water content in the fractured and non-fractured areas within the subsidence area was less than that in the virgin area. Moreover, the soil water content around the ground fractures decreased significantly, with a significant downward trend in the depth of 0–90 cm, ranging from 9.27–15.47%; Wu et al. (2019) took the Fuxin subsidence area as the example to extend their research, finding that with the increase of the distance from the crack, the soil moisture content increased, but the influence was not obvious after 2m; Ma and Yang (2019) took the soil in the development area of ground fractures mined by Ephedra in northern Shanxi as the research object, finding that the moisture content of the soil near stepped ground fissures, except the surface, is higher than that in the non-cracked area at all depths. The above scholars believe that the ground fissures caused by coal mining are important factors in affecting the changes of soil moisture in the subsidence area.
On the other hand, some scholars believe that coal mining subsidence has no or unobvious effect on soil moisture. For example, Wang et al. (2006) studied the soil bulk density, porosity, saturated hydraulic conductivity in the subsided and non-subsided areas of the Bu Lian ta Coal Mine. The study discovered that the soil bulk density and porosity of the top and middle of the collapsed dune did not have significant changes while the soil bulk density at the bottom of the dunes and the lowlands between the dunes was significantly reduced with the significantly increased porosity. Additionally, the saturated hydraulic conductivity within 0-60cm did not change significantly. However, the research results of "ecological restoration technology experiment and demonstration research in Shen Dong mining subsidence area" completed by Inner Mongolia Agricultural University and China University of Mining and Technology in 2007 show that basically, coal mining subsidence does not affect soil nutrients and moisture (Li et al. 2012). The scholars who hold the above views believe that the soil water supply comes from atmospheric precipitation, but not from groundwater. Mining disturbance affects groundwater, but has a limited impact on soil water. Therefore, according to previous studies, there is no unified understanding of whether coal mining causes soil moisture change in subsidence area, which needs to be further studied.
To sum up, the authors believe that from a macro point of view, surface cracking and collapse affect the conditions of soil water infiltration and evaporation, thus resulting in the change of soil water; from a micro point of view, the factors affecting the change of soil texture, bulk density and pore structure will also cause the changes of soil moisture (Zhao et al. 2010). But the main reason for the differences in previous studies is that the influence of coal mining on soil moisture was only studied from the absolute value of soil moisture, while ignoring the characteristics of the soil itself as a heterogeneous continuum. Even at the same time, there will be obvious differences in soil characteristics at different spatial locations, that is, spatial variability. As one of the important parameters of soil physical properties, soil moisture has high spatial variability. At present, scholars in China and other countries mainly focus on the change of absolute value of soil moisture in subsidence area, but there is little research on the spatial variation of soil moisture in aeration zone in subsidence area.
Therefore, the study takes Nalin River No.2 mine in the east of Mu Us Desert as the research object. Through the construction of classical statistics combined with multi-dimensional geo-statistics method, this paper analyzes the variability of soil moisture in the unsaturated zone in the subsidence area from the perspectives of time and space, and discusses the impact of coal mining subsidence on soil water in the aeration zone from the perspective of spatial variability rather than that of absolute soil water changes, so as to provide the scientific basis for ecological restoration in mining areas.