Update of river health assessment indicator system, weight, and assignment criteria in China

Comprehensive assessment of river health is challenging due to the diversity of rivers, the complexity of their ecosystem and functional service. This paper updates the river health assessment indicator system, weight, and assignment criteria in China by reviewing and examining the peer-reviewed literature. We propose an indicator system, weight, and criteria, validated by nine case studies and able to assess the country-scale river health. Our analysis shows that the rule layer of indicator system includes hydrology, water quality, aquatic organism, physical habitats, and functional service; its corresponding weights are set to 0.15, 0.21, 0.18, 0.22, and 0.24, respectively. The ten most representative indicators are selected that incorporates the indicator layer with their corresponding weights. The evaluation based on case studies shows that in eight out of nine cases, our results are consistent with those obtained in previous studies. Therefore, the suggested index system, weight, and three-assessment criteria arewell suited for the complex cases in China. This papercan serveas a reference for a river health assessment and present a comprehensive listing of assessment criteria. into three types, the corresponding assignment criteria are proposed, respectively. With this method, the health status of rivers of types more and compared horizontally.


INTRODUCTION
Ecosystem and river health has gradually become an important research field, especially river health assessment for sustainable river management (Wang ). Hence, river management and conservation, originally assessed with water quality index and organic pollutants, is now using the framework of river health assessment more often (Singh & Saxena ). A series of studies on river health assessment started in 1972. For example, one can refer to the Rapid Bioassessment Protocols (Buss & Vitorino ), the River Habitat Survey (Szoszkiewicz ), the Australian River Assessment System Parsons et al. 2002), the EU Water Framework Directive (Voulvoulis ), the Blind Number Theory (Wang ), and the Definition and Connotation (Wen ). These studies provide numerous indicators and assignment criteria.
However, problems in river health assessments remain.
Large spatiotemporal variations in rivers make the multiscale evaluation more complex, as the river geospatial components increasingly differ from each other (southern and northern, inland and outer, urban channel and natural rivers, mainstream and tributary). Other challenges include the evaluation and assignment criteria, and data availability.
Some researchers think that the social functional service of rivers is more important (Meyer ), while others consider their natural properties essential (Frey ). These issues make the task even more difficult to carry out river health evaluations properly and compare the health status of rivers horizontally and vertically. Adverse consequences can affect the decision-making, the management, and the ecological river restoration. Currently, two methods are able to solve these problems related to the river health assessment. One method is to classify different rivers according to their different attributes or characteristics. For example, rivers are divided into large categories (ecological (You ), mountain (Xiao ), urban (Deng et al. )), as well as specific rivers (the Yangtze River (Zheng & Wang ), the Yellow River, and the Pearl River (Lin et al. )). Through the principle of neighboring (also called similarity), the evaluation indicators of one river are transplanted to a neighbor or similar river. However, because this approach is limited, it does not allow a comparison of different types of rivers. An alternative approach is to adopt an universal evaluation index for the health assessment of rivers and lakes at the country level (Hao & Guo ). However, there are very limited studies on weight and assignment criteria in the country-scale approach. This paper attempts to use statistical methods to handle the data from papers related to Chinese river health assessments, published between 2000 and 2018. These include articles, Masters and PhD theses from China National Knowledge Infrastructure, as well as SCI journals about China river health assessment. These statistical methods can provide more comprehensive and reliable results. The work will undergo these following steps: (1) propose an objective and fair indicator system of river health assessment and the corresponding weight to be used at country scale; (2) propose a set of indicator assignment criteria for three different types of rivers (urban, ecological reserve, and casual rivers); (3) use nine case studies to validate the performance of the suggested indicator system, weight, and criteria, and examine the pros and cons of our system with respect to previous studies. With our proposed method, the river health status can be evaluated by a comprehensive score, and the horizontal comparison of the health status of different types of rivers can be implemented.

Comprehensive evaluation of river health
According to the analytic hierarchy process (AHP), indicator, weight, and assignment criteria are three decisive factors for comprehensive assessment of river health. If these three factors are given, the health index based on them can be calculated as: where R is the comprehensive degree of river health in fivepoint scale, R i is the integrity score of the indicator, W i is the weight of indicator i, and E i is the score of the indicator i, which is determined by the assignment criteria according to the value.
For further evaluation, the score is transformed into a centesimal system according to Equation (2): where F is the comprehensive score in centesimal system.
The health level of rivers is divided into five grades: very healthy, healthy, sub-healthy, unhealthy, and morbid, according to the comprehensive score. See Table 1 for details.

Determination of indicator, weight, and assignment criteria
The three decisive factors of river health assessment vary with the river type, which is inevitably subjective. Therefore, in this paper, we assume that their objective existence is endowed by the subjective.

Method to obtain the indicator system
We analyzed statistically how many times an indicator is used in river health assessment in order to build a new indicator system for China. This new indicator system should be quantifiable, easy to use, and be representative of the physical and chemical characteristics of the river health.
Representativeness, universality, quantification, and data availability are therefore essential for the indicator to be rel-

Weights calculation
Weights indicate the importance of various indicators of river health assessment, while it also reflects the importance of people's attention to each indicator of river health under certain social and economic conditions. Therefore, it is fixed during a certain period.
This paper builds statistics on the indicator weight of the representative multi-index river health assessment in China, and calculates its mathematical expectation as the weight of each indicator as follows: where E(x) is the mathematical expectation, and f(x) is the probability density function of weight x.

Method to summarize assignment criteria
River health reflects the public expectation about rivers. water quality can only score 3 or less due to the higher public expectation to water quality. While comparing two types of rivers mentioned above, the difference lies in the assignment criteria: with the same water quality, one scores 4 while the other scores 3. The similarity is that people always care about water quality. In other words, the assignment criteria of indicators have changed; however, the weights of indicators have persisted.
In this paper, a riverwhen determining indicator assignment criteriais first classified, and then the literature is classified according to the river type. Next, statistical methods determine the assignment criteria for each indicator. More specifically, we count, summarize, and combine similar assignment criteria and indicators from different literature sources.

Indicator system
The statistical result based on the literature review are shown in Table 2. This table shows that the assessment

Hydrology
The hydrology indicator contains one IL indicator, I 1 -Flow process variation degree (FVD), which refers to the percentage of the annual discharge to average annual discharge (Li ) and is calculated using Equation (4): where q m is the measured monthly runoff of the assessment year, Q m is the natural monthly runoff of the assessment, and Q m is the mean value of natural monthly runoff in the assessment year.

Water quality
The water quality indicator contains one IL indicator, I 2 -Achievement ratio of water quality in water functional area (ARWA), which refers to the proportion of water function area in which the water quality reaches the target value of the water function area (Li ). It shows whether the water quality is up to the standard which meets the requirements of water resources development, utilization and ecological environment protection at the same time.
ARWA is calculated as follows: where n is the number of water function zones with water quality up to standard, and N is the total number of water function zones.
The second indicator is related to benthos live. Since where n b is the B-IBI value of the assessment river, and N b is the optimal expectation of B-IBI in the water ecological zone near the river.

Physical habitats
Three indicators are used to represent the state of physical habitats. One is the I 5 -Riverbank vegetation coverage ratio (VCR). Riverbank vegetation benefits rivers in various ways by reducing soil erosion, preventing river erosion, purifying water, improving river microclimate, and providing habitat for aquatic organisms (Zhang ; Wei ). VCR is calculated as: where S p is the vegetation area of the riverbank, and S is the total area of vegetation that grows in the riverbank.
I 6 -Longitudinal connectivity (LC) is the second IL indicator, which refers to the connectivity of river branches, lakes, wetlands, and other water systems, and reflects the continuity of water flow and the vertical continuity of the river (Wei ). It is calculated as: where L is the number of gates or dams per 100 km of river. Wei ). WCR is obtained by: where a is the current wetland area, and A is the wetland area in the reference period.

Functional service
Functional service contains two IL indicators. Flood control and disaster reduction depends on the safe flood discharge capacity of the river. The completion of a flood control project is one of the important signs of the safe flood discharge of the river (Yang ; Li ). I 8 -Flood control project completion ratio (FPR) can be selected to reflect the flood control and disaster reduction capacity of the river. The calculation formula is: where h is the number of the completed flood control projects, and H is the flood control projects to be completed.
I 9 -Utilization ratio of water resources (URW) is an indication of the degree of water resources development and utilization and represents a river's use. Experience shows that if a river basin is rich in water, the rate of human development increases appropriately; however, its ratio should be controlled within a certain range. It is calculated as: where U is the water consumption of the area and C is the total water resources.

Weights of indicator
We selected 47 typical evaluation cases from the literature, according to the number of downloads, citations, journal quality, and weight. We also analyzed and sorted the indicator weights.
Weights of RL  Figure 2.

Weights of indicator layer
The weights of IL indicators are obtained through two steps: (  rivers (Deng et al. ). ERR refer to rivers or their reaches that originate from or flow through the protected areas.
Based on the studied references, we proposed the assignment criteria of UR, ERR, and CR, which is a five-point scale and is summarized in Table 3.

Urban rivers
Case 1: Jing-Hang Grand Canal The Beijing Hangzhou Grand Canal is the oldest and longest man-made canal in the world. Zhang () established  Table 4.
The value of I 1 is qualitatively evaluated as 'satisfied', which is in the second class according to the criteria of Case 1, and thus the score of I 1 is 4 in this paper. I 6 is described by the fluidity index with the value of 75%, which is in the third level according to the criteria of case 1, and thus the score is 3. The data of I 4 is missing, thus its weight is assigned to I 3 . The data of I 7 is missing, and thus its weight is allocated to I 5 and I 6 in proportion. This is the same for I 9 , with the weight being assigned to I 8 and I 10 in proportion.
Converting the total score into a percentage system, the comprehensive score is 87. Thus, the Jing-Hang Grand Canal in Huai'an section can be classified as 'very healthy'.
The health status of Case 2 is also 'very healthy', which is consistent with the evaluation result of this paper. The comparison of the indicator scores between this paper and case 1 is shown in Figure 4.

Case 2: The Yangtze River in Wuhan sector
As the largest river in China, the Yangtze River Basin has a prominent social, economic, and ecological status. Zhang () proposed different health evaluation index systems according to river section, and evaluated the health of three typical river sections. The Yangtze River in Wuhan section is selected as Case 2 to test the proposed river health assessment system. Wuhan is located in the middle reaches of the Yangtze River, with a permanent population of more than 11 million. The evaluation process and results are shown in Table 5 for this case.
The selected indicators and their weights are handled in the same way as Case 1: Jing-Hang Grand Canal. Converting the total score into a percentage system results in the comprehensive score of 67.4. Thus, the health degree of the Wuhan section of the Yangtze River is 'sub-healthy'. The evaluation result of Case 2 is also 'sub-healthy', which is consistent with the evaluation result of this paper.   Table 6.
With the comprehensive score of 64.8, the health degree of the Simao River is 'sub-healthy'. The evaluation result of Case 3 is also 'sub-healthy', which is consistent with the evaluation result of this paper. See Figure 5 for the comparison of the indicator score between this paper and Case 3. Note: '*' means that the equation used for the IL indicator in the selected reference for the case study, or the RL indicator in the selected reference was different to our study. We used the IL indicator's grade in the reference to assign the IL indicator score.

Case 4: The Songhua River
The Songhua River is the largest tributary on the right bank of the Heilongjiang River and the largest river in Heilong-  Table 7. Note: '*' means that the equation used for the IL indicator in the selected reference for the case study, or the RL indicator in the selected reference was different to our study. We used the IL indicator's grade in the reference to assign the IL indicator score.  The comprehensive score of river health is 46.4, thus it is classified as 'morbid'. However, the evaluation result in Case 4 is 'healthy', which is different from the result we obtained. The reason for the inconsistency is that the standard used in Case 4 cannot be applied for an ecological reserve river and should be stricter. This leads to an optimistic evaluation result, which is incorrect for river management. Figure 6 shows the comparison of the indicator score between this paper and Case 4. In Case 4, with a value of 30%, the score of I 5 is 78, and with the value of 47%, the score of I 9 is 68, which is not correct.

Case 5: The Buyuan River
The Buyuan River is an important tributary of the lower Lancang River. Yang () selected seven key indexes from 60 references related to the river health evaluation index system and verified them using the Buyuan River. In this paper, the Buyuan River in the Nanban river section was selected as Case 5 for validation. In this section, a fish nature reserve will mitigate the adverse impact of the cascade power station construction on aquatic organisms and their habitats, while maintaining the connectivity of fish habitats. The evaluation process and results are reported in Table 8.
The comprehensive score of the Buyuan River in the Nanban sector is 86.8, thus it is classified as' very healthy', which is consistent with the 'very healthy' status of the river in Case 5.

Case 6: The Lhasa river
The Lhasa River is a tributary of the Yarlung Zangbo River. Chen () constructed a river health assessment index system including 11 indexes based on the characteristic of the Lhasa River, and applied it to the main stream of the Lhasa River. The upper reaches of the Lhasa River were selected for Case 6 for validation.
The river source is the Medica wetland, which is a national natural reserve. The evaluation process and results are given in Table 9.
The comprehensive score of the Lhasa River in the upper reaches is 81.6, and thus it is classified as 'healthy', which is consistent with the results obtained in Case 6 according to Chen ().
The comprehensive score of the Daxi River is 61.2, thus the river status is 'sub-healthy', which is consistent with the evaluation result in Case 7 obtained by Zhang (). Figure 7 shows the indicator score of this paper compared to Case 7 by Zhang ().

Case 8: The Qi River
The Qi River is a first-class tributary on the right bank of the upper reaches of the Yangtze River. The Qi River basin is rich in iron, and forest and tourism resources.
The health of the Qi River directly affects the socioeconomic development and ecological stability of the basin.
Li () established a river health assessment index system based on 16 indicators and a multi-level gray clustering evaluation system which were applied to the Qi River. Case 8 is selected for validation in this  paper. The evaluation process and results are shown in Table 11.
The comprehensive score of the Qi River is 76.6, and thus it is classified as 'healthy', which is consistent with the evaluation result obtained by Li () in Case 8.

Case 9: The Yellow River in inner Mongolia
The Yellow River is the second largest river in China and the birthplace of Chinese civilization. This river's health directly affects the economic development of Inner Mongolia. Tian () established a river health assessment index system in line with the characteristics of the Yellow River in Inner Mongolia, and evaluated its   health with fuzzy evaluation method. We use this study as Case 9. The evaluation process and results are shown in Table 12.
The comprehensive score is 42, meaning that the Yellow River is classified as 'unhealthy'. This is consistent with the evaluation result in Case 9 obtained by Tian ().

CONCLUSION
This paper reviews and examines the previous literature on river health assessment. It also proposes an indicator system and assignment criteria for three types of river.
We use nine case studies to evaluate our suggested method for river health assessment. The main outcomes are: (1) the proposal of a novel indicator system of river health assessment based on statistics and mathematical analysis. This system incorporates five RL indicators and ten IL indicators including their corresponding weights.
With fixed indicators and weights, the assessment system can provide an efficient, clear, and concise evaluation method.
(2) The establishment of three types of assignment criteria for three types of rivers (urban rivers, ecological reserve rivers, and casual rivers) which will improve more accuracy and comparison with different rivers; and (3) the nine validation cases show that the novel method is reliable, rational, and widely usable.