## 1 Introduction  
The arid area of China is located in the hinterland of the Eurasian continent, on the northern edge of the Qinghai-Tibet Plateau. The specific geographical location has created unique landforms, vegetation, and soil types in arid areas of China, and is the critical protection area of ecologically fragile sites in China. The alternate distribution of mountains and basins is a fundamental feature of the physical geography of the arid regions in northwestern (NW) China. [PERSON] defined this combination of terrestrial ecosystems as a mountain-basin system (MBS) from ecology [1]. The zone system is connected by energy flow, water flow, life flow, value flow, and culture flow. It is the most essential and precious natural resource and mode of action in the arid zone and thus becomes the supporting system of natural ecology and human society in the region. The ecosystem is fragile due to the particularity of the geographical location and geomorphic structure [2]. The MBS in arid regions is ecologically sensitive, with significant ecological and environmental problems such as vegetation degradation and difficulty in restoration. In recent years, with the vigorous advancement of the construction of national ecological civilization, the ecology of some areas has been highly valued, especially in areas with fragile environments. The resource and environmental carrying capacity assessment and ecological quality comprehensive assessment have become hot issues nationwide in recent years. Therefore, the timely and rapid monitoring of ecological environment changes in ecologically vulnerable areas has become the focus of researchers and relevant government departments. At present, remote sensing technology is widely used in ecological environment research due to its advantages of speed and use in real time. It has become an essential means of ecological environment assessment [3; 4]. In recent years, remote sensing technology has been successfully applied to ecological monitoring, particularly for the evaluation of ecosystems including forests [5], grasslands [6], cities [7], and river basins [8; 9]. It can make up for the deficiency of traditional semi-quantitative ecological evaluation methods and provide an effective research method for regional ecological evaluation, and thus many scholars have studied ecological environment monitoring and assessment based on remote sensing methods [10]. In the selection of an ecological environment remote sensing monitoring index, some studies evaluated the ecological environment by a single index, such as using the net primary productivity (NPP) index to evaluate grassland ecology [11], using the land surface temperature (LST) to assess urban heat islands [12; 13; 14], using green vegetation fraction (GVF) to estimate the ecological status and its changes in the riparian zone [15], or using impervious surface coverage to evaluate the urban ecological environment [16]. Although a single index is easy to understand and easy to calculate, the ecological environment is a complex system in the existing ecosystem, and the ecological environmental quality is controlled by multiple ecological factors [17], leading to difficulties in characterizing the quality of the ecological environment by only a single ecological factor [18]. Therefore, it is necessary to comprehensively evaluate the ecological environment from the perspective of the integrity of the ecosystem and explore the change rules of each ecological factor and its synergistic relationship to establish a comprehensive index that can couple multiple factors [19]. The other type is the comprehensive evaluation model constituted by the fusion of remote sensing data and other types of data, such as the ecological environment index (EI) proposed in the Technical Specifications for Ecological Environmental Status Assessment (HJ/T192-2006) issued by the Ministry of Ecology and Environment of the People's Republic of China.