1. Introduction
The water cycle supports the formation of water resources and is the main driving force for the evolution of water environments and ecosystems. With the development of the social economy and acceleration of urbanization and other human activities, the regional water cycle has gradually changed from a single “natural” water cycle model to a “natural–social” dual water cycle model. The basic process of a natural water cycle is precipitation–runoff–evaporation. The basic process of a social water cycle is water intake–water use–water supply–water drainage–water reuse. Natural and social water cycles influence each other and system structure coupling. A set of “natural-social” dual water cycle models based on “water supply–water use–water consumption–water drainage–water reuse” is formed.
Zhengzhou City is in the north-central part of Henan Province (
Figure 1). The water cycle is greatly influenced by the urbanization rate and urbanization processes with problems including severe water pollution, a sharp decline in groundwater depth, and water shortage. In 2020, Zhengzhou’s total water resources were 859.12 million cubic metres. The per capita water resources were only 68 cubic metres, one-tenth of the national per capita share, far below the internationally recognized 500 cubic metres, signifying an extreme water shortage warning line. In general, Zhengzhou is in a state of severe water shortage. In recent years, the rapid economic development in Zhengzhou has led to increased water supply demand. In 2020, the total amount of sewage discharge reached 105.125 million tons, which restricted the long-term social development in Zhengzhou. Maintaining a healthy water cycle has become a significant proposition for the sustainable development of human society.
At the beginning of the 21st century, China’s urban water system developed rapidly. Many experts and scholars began to study the healthy circulation of urban water systems from the overall direction of the water environment and ecology. Zhang et al. [
1] proposed a healthy water cycle according to the water crisis caused by human behaviours and emphasized the importance of recycling water models for water cycle health. Some scholars have adopted different methods to study this concept from different dimensions. For example, Jia et al. [
2] evaluated the water cycle health in Ningxia and five other cities using principal component analysis. Zhang et al. [
3] analysed the water cycle health in Beijing and comprehensively considered the influence of the South–North Water Diversion Project on the water cycle in Beijing, thus analysing the urban water cycle’s influencing factors more scientifically. Luan et al. [
4] proposed a farmland healthy water cycle and evaluated the water cycle health of the Jun Liu irrigation area. Research on water cycle in foreign countries has obtained many research results. Chu [
5] established an index system based on “S-E”, thought to evaluate the health status of the urban water cycle. Maria et al. [
6] used life cycle methods to evaluate urban water cycles in the Mediterranean region. Deng et al. [
7] proposed an improved entropy-based fuzzy matter element model to evaluate the Taihu Lake Basin in China. Healthy water cycles are a fuzzy concept.
The TOPSIS model ignores the linear relationship between indicators, leading to the failure of Euclidean geometry. In addition, Euclidean distance can only reflect the evaluation object and reason from position distance. The closeness of the solution cannot reflect similar geometry. Mahalanobis distance and grey relational analysis improve the TOPSIS method, which can evaluate healthy water circulation and provide references for water cycle health and ecological management in Zhengzhou.
In this context, Zhengzhou was chosen for case studies, and an evaluation system covering four criteria layers was established, namely water abundance, water ecology, water use, and water quality, including 19 evaluation indexes such as mean annual precipitation, annual water storage change of reservoir, and eco-environmental water demand. The combined weights of subjective and objective indexes were obtained by combining the three-scale AHP and EFAST algorithms. On this basis, we established an improved TOPSIS model to evaluate the water cycle in Zhengzhou. We used the TOPSIS model and FCE for comparison and verification, aiming to provide scientific reference for the realization of benign water cycles and sustainable development in Zhengzhou City.
3. Results and Discussion
3.1. Weight Calculation Results
The subjective weight of the evaluation index was calculated using a three-scale AHP. After calculation,
; therefore, the consistency test was passed. The objective weights of evaluation indexes in different years were determined by the EFAST algorithm. The minimum information entropy principle combines the subjective and objective weight values. Finally, the combined weight value of each evaluation index was calculated. The weight values of four criteria layers and 19 indicators of the evaluation system are shown in
Figure 5.
3.2. Evaluation of Index Layer
The health state for the index layer is shown in
Figure 6 according to the threshold of each indicator. According to evaluation results of the index layer, most indicators were developing towards a healthy trend annually.
Among them, the indicators were healthy in the past decade, showing that the sewage treatment rate in Zhengzhou was high, the industry’s water consumption per unit of value (CNY 10,000) was reasonable, and the safety of drinking water sources was high. were opposing indicators and have been in sub-disease and morbid states for a long time in the past decade. The health level of was poor, and the health score was only 1.0. Apart from the general health level in 2016, was sub-morbid for nine years, and the score was only 2.0. The health level of improved in the past three years, but it was still sub-morbid. These three indicators indicated that Zhengzhou had to improve the drainage pipeline density, increase the water consumption of the ecological environment, and draw attention to reservoirs’ annual water storage change. status continued to improve and tended to be healthy. experienced three consecutive years of health from 2011 to 2013, but the next seven years were sub-health, showing that the protection of water resources in Zhengzhou City was increased and resulted in improved water ecological functions such as river and lake storage capacity and water function area compliance rates. was morbid in 2021 due to the once-in-a-millennium heavy rainfall in Zhengzhou on 20 July 2021, showing that the precipitation in Zhengzhou was unstable. Therefore, the precipitation trend in Zhengzhou should be better monitored. If the precipitation was excessive, there should be a timely warning to avoid the adverse effects of jade. The health status of was volatile, showing the dependence of Zhengzhou on groundwater and the sensitivity of indicators.
3.3. Factors Analysis of Water Circulation Health Disorders
According to Formulas (17)–(19), the obstacle degrees of 19 evaluation indexes in Zhengzhou City from 2011 to 2021 were calculated. Six indexes with large obstacle degrees were selected for statistical analysis to accurately understand the distribution of obstacle degrees. The calculation results are shown in
Table 4. The greatest obstacle factor was the ecological environment’s water consumption (
), and the average obstacle degree was 19.65. The main obstacle factor of the minimum obstacle degree was the green coverage rate in a built-up area (
); the average obstacle degree was 7.3.
Table 4 shows eco-environmental water demand (
), the proportion of river length above class III water quality (
), the industry’s water consumption per unit of value (CNY 10,000) (
), water resources amount (
), effective utilization coefficient of irrigation water (
), and the green coverage rate of the built district (
) when the obstacle degree was high. The six factors with a higher obstacle degree were the main influencing factors.
and
belonged to the water utility criterion layer,
belonged to the water abundance criterion layer,
and
belonged to the water ecology criterion layer, and
belonged to the water quality criterion layer.
In order to visually display the obstacle degree change of the six main obstacle factors, the trend was plotted as a broken line, as shown in
Figure 7.
From 2011 to 2021, the eco-environmental water demand (), the green coverage rate of built districts (), the water resources amount (), and the industry’s water consumption per unit of value (CNY 10,000) () showed an increasing trend of obstacle degree annually. Therefore, Zhengzhou should increase ecological environment water consumption and improve the green coverage of built-up areas. In residents’ daily life, there should be more water-saving instruments to improve the water resources shortage in Zhengzhou City. In terms of industrial structure, the production mode should be improved to reduce the industry’s water consumption per unit of value (CNY 10,000).
3.4. Evaluation of Target Layer
Through the value of each index and the corresponding weight, we established the improved TOPSIS model using Formulas (1)–(16), and the relative progress of Zhengzhou City from 2011 to 2021. The specific results are shown in
Table 5.
From 2011 to 2021, the overall health status of the water cycle in Zhengzhou City developed a healthy trend. Due to the low eco-environmental water consumption from 2011 to 2012, the compliance rate of the water function zones was low, and the health level of the water cycle in Zhengzhou City from 2011 to 2012 was grade IV. From 2020 to 2021, the water cycle’s health degree in Zhengzhou City decreased mainly due to the rare heavy rainfall weather on 20 July 2021; the annual average precipitation rose to more than 1000 mm, resulting in severe urban waterlogging, slightly lowering the water cycle’s health degree in 2021 compared with 2020. In recent years, with the implementation of the most stringent water resources management system by the government and the strengthening of natural environmental protection, the water cycle health of Zhengzhou City has developed healthily and steadily.
Based on our analysis of the evaluation results, the countermeasures to improve Zhengzhou City’s water cycle health are as follows:
- (1)
Continue to implement the most stringent water resources management system. In terms of natural environmental protection, we must increase the ecological protection of rivers and lakes and reduce groundwater exploitation.
- (2)
The precipitation in Zhengzhou City is higher in summer; therefore, reservoir water storage and other water storage works should be conducted for subsequent use. If the precipitation is excessive, the reservoir should be discharged to avoid danger.
- (3)
Zhengzhou’s advantages as a transportation hub should be considered, increasing idea exchange, and continuously exploring new ways to develop green ecology.
- (4)
Government departments should improve the temporary emergency response capacity for natural disasters, further improving the flow capacity of urban drainage channels, and accelerating the establishment of safety monitoring facilities for small and medium-sized reservoirs.
3.5. Method Validity Test
We selected two conventional assessment methods, the FCE method and the traditional TOPSIS model, to compare our evaluation results to the improved TOPSIS model and verify the effectiveness of the improved TOPSIS model. The results are shown in
Table 6.
The evaluation results calculated by the three methods were consistent. There were some differences between the traditional TOPSIS model and the improved TOPSIS model in the evaluation results from 2013 to 2015 and 2021 because, in the improved TOPSIS model, the traditional Euclidean distance was replaced by the weighted Mahalanobis distance and the grey correlation analysis degree. As a result, the correlation degree between some indexes was comprehensively considered, and the position and form indicated the health grades of each year. The FCE method comprehensively considers the subjective opinions of experts and has strong subjectivity [
20], whereas the improved TOPSIS method does not need subjective judgment and reflects the relative progress of each evaluation scheme from the original data. The improved TOPSIS model is applicable and scientific in water cycle health evaluation.