iction System (EPS); (2) The data of agricultural water consumption, industrial water consumption, domestic water consumption, and ecological water consumption are mainly derived from Bulletin of Water Resources Development of each province and supplemented by the China Environmental Statistics Yearbook; and (3) The remaining data, such as total grain output, per capita energy consumption, and investment in landscaping, are mainly from the China Regional Economic Statistical Yearbook, and the missing data are supplemented by the Statistical Bulletin of National Economic and Social Development and the corresponding statistical Yearbook published by local governments.

Dynamic Evolution of the W-E-F-E Nexus
Comprehensive evaluation value is an overall quantitative evaluation of things or objects restricted by a variety of factors. The comprehensive evaluation value of each subsystem of water, energy, food, and ecology reflects its development status. As shown in Figure 1, the development of the four subsystems presents a dynamic evolution trend of alienation. First of all, the average comprehensive evaluation value of the energy system in Northwest China is the highest, which is about 0.7. In 2007-2008 especially, the comprehensive evaluation value of the energy system achieved rapid growth, reaching12.9% and peaking at 0.79 in 2008. This may be related to the investment and construction of energy industry infrastructure by the Chinese government's economic stimulus plan under the background of economic crisis. The high comprehensive evaluation value of the energy system is closely related to the natural energy advantages in Northwest China and the policy support of the country. On the one hand, the northwest region has abundant energy reserves, which is suitable for the development of the energy industry. On the other hand, the national western development strategy and the "One Belt and One Road" initiative have ensured the energy industry in the northwest region received greater support in investment and construction, so the development level is also higher.
Secondly, it can be seen that the comprehensive evaluation values of water and food systems in Northwest China are relatively close, at about 0.5. Due to the drought and water shortage in Northwest China, production and living water are limited. The comprehensive evaluation value of the water resources system shows a fluctuating upward trend, with an average annual growth rate of only 1.3%. However, it still indicates that the intensive utilization of water resources in Northwest China is improving. In addition, the development of the grain system in Northwest China is relatively stable, and its comprehensive evaluation value fluctuates slightly around 0.52. Food production cannot be separated from water. The natural conditions in Northwest China determine that the improvement of local agricultural production potential cannot avoid the rigid constraint of water resources. It is worth noting that in recent years, the development level of water resources system in Northwest China shows a trend of overtaking the grain system, which indicates that local awareness of water resources' protection and rational utilization has been strengthened, which is conducive to further developing the potential of food production in Northwest China and ensuring regional food security. Finally, among the four subsystems, the development of the ecosystem in Northwest China is the worst. Except for in 2002, the comprehensive evaluation value of the ecosystem is always lower than 0.3, which is related to the local climate, terrain, and other natural conditions. However, the fragile ecological environment in Northwest China is also related to human's insufficient attention on ecological work and unreasonable development activities. Insufficient investment in ecological construction and excessive discharge of pollutants from production and living all make it impossible to effectively improve the ecological environment in Northwest China.
Based on the calculation of the comprehensive evaluation value of water, energy, food, and the ecosystem, this paper investigates the coupling and coordination status of all subsystems (as shown in Figure 2). As can be seen from the figure, the W-E-F-E nexus in Northwest China showed a fluctuating downward trend from 0.39 in 2002 to 0.29 in 2012, with a decrease rate of 25.6%. Among them, in 2003-2008 and 2009-2012, there was a V-shaped trend of first rising and then declining. This is because during the above period, a series of policies on water resources, energy resources, agricultural production, and ecological environment were issued in Northwest China, resulting in significant fluctuations in the W-E-F-E nexus in the short term. Each time the W-E-F-E nexus experienced a relatively large decline, there would be a relatively slow recovery process, which may be related to the local concept of development of natural resources first, protection later. Since 2012, the central government has paid more attention to coordinated development and ecological and environmental protection, and put forward a series of related policies. For example, the Measures for Examining the Implementation of the Strictest Water Resources Management System was officially released in 2013, and the Air Pollution Prevention and Control Act was revised in 2015 and implemented in 2016. The efficient exploitation and utilization of resources, the improvement and restoration of ecology, and the coordinated development of economic development and ecological protection will be promoted to a new height. This is reflected in the fact that the W-E-F-E nexus continued to increase from 2012 to 2017, rising from 0.29 in 2012 to 0.38 in 2017, with an average annual growth rate of 5.5%, and the growth rate from 2016 to 2017 was particularly obvious, reaching 12.8%.

Regional Heterogeneity of W-E-F-E Nexus
This paper calculates the W-E-F-E nexus of prefecture-level cities, regions, and counties directly under the jurisdiction of autonomous regions in Northwest China and analyzes its spatial heterogeneity. This paper has selected an observation point of every 5 years since 2002 and visualized the W-E-F-E nexus in Northwest China in 2002China in, 2007China in, 2012, and 2017 by using GeoDA spatial data statistical software (as shown in Figure 3). The darker the shadow color in the figure is, the higher the CCD of the composite system in this area is, while the white color shows areas with missing data.
Through data analysis, it is found that the majority of the W-E-F-E nexus between different cities or regions in the five northwest provinces from 2002 to 2012 is lower than 0.5. It indicates that the development of the W-E-F-E nexus in Northwest China is in a state of imbalance or near imbalance, and most cities or regions have shown a decline to varying degrees, with the number of cities or regions with W-E-F-E nexus misalignment (coupling coordination score less than 0.4 1 ) nearly doubling from 18 in 2002 to 34 in 2012. Among these cities or regions, the W-E-F-E nexus in Yulin of Shaanxi Province, Wuwei of Gansu Province, and Zhongwei of Ningxia Province decreased most significantly, by 79.9, 58.1, and 52.2%, respectively. In many cases, economic development comes at the expense of the ecological environment, and the ecological cost of development is high. The waste of water resources, pollution, and destruction of the ecological environment restricts local food production, which in turn has a negative impact on the production of energy and gradually forms a vicious circle. In terms of spatial dimension, the W-E-F-E nexus is relatively high in other provincial capitals and nearby cities in Northwest China, except for Qinghai Province where the data is seriously missing. Such spatial agglomeration and a distribution trend are not obvious. W-E-F-E nexus is in a relatively static state in each city or region, that is, the spatial gap of W-E-F-E nexus development has not been significantly narrowed.
From 2012 to 2017, the overall trend of the W-E-F-E nexus in Northwest China was on the rise, with the nexus increasing significantly. The number of cities or regions with W-E-F-E nexus imbalance decreased from 34 in 2012 to 26 in 2012,

FIGURE 3 | Spatial evolution of the W-E-F-E nexus in Northwest China.
and in 32 of the 38 cities or regions covered by the data, the W-E-F-E nexus improved to varying degrees. Among these cities or regions, W-E-F-E nexus coupling coordination values of Hami of Xinjiang Province, Baiyin of Gansu Province, and Weinan of Shaanxi Province increased by 0.33, 0.29, and 0.24, respectively, and all of them got rid of the state of W-E-F-E nexus imbalance. From the perspective of spatial dimension, the W-E-F-E nexus in provincial capitals and nearby cities is still relatively high, showing an obvious trend of spatial agglomeration. Among the provincial capitals, the W-E-F-E nexus of Xi'an in Shaanxi Province reaches 0.77, realizing the moderate coordination of W-E-F-E nexus. These phenomena indicate the enhancement of the W-E-F-E nexus in Northwest China. With the strengthening of social ecological awareness and the implementation of the concept of coordinated development, the concept of development first followed by governance in the past has been gradually abandoned, replaced instead by development and governance. The concrete embodiment of the action is to strengthen the intensive use of water resources, pay attention to the development of new energy sources, develop efficient agriculture, and strengthen ecological restoration efforts. These actions have effectively improved the coordinated development of water, energy, food, and the ecosystem in Northwest China.

Spatial Correlation of the W-E-F-E Nexus
Since the W-E-F-E nexus in Northwest China shows obvious regional differences, does the coupling coordination of complex systems in this region have a spatial correlation effect? In this paper, GeoDa is