Water ecological carrying capacity (WECC) refers to the ability of a water ecosystem to support and withstand economic and social development. WECC assessments can provide helpful information for resolving water issues. Since WECC involves a wide range of influence factors, indicator-based methods are useful tools for this type of evaluation. However, there are still some imperfections in the existing indicator-based methods for WECC evaluation, primarily in the aspects of index frameworks, indicator selection and evaluation criteria. Based on the pressure-support force-state index framework, this paper presents an indicator-based WECC evaluation method (PSSM). Using PSSM, overall WECC evaluation can be obtained by assessing the pressure of socio-economic development on the water ecosystem, the support from the water ecosystem for human development, and the health status of the water ecosystem. PSSM is directly focused on the pressure-support effect, and fully considers the determinant influence on WECC of the health status of the water ecosystem. The application of PSSM in Tieling City shows that further reduction in the pressure that comes from socio-economic development is still required, and the health status of the water ecosystem should be further improved.

INTRODUCTION

Generally, the primary cause of water issues is that communities excessively pursue economic benefits and rapid development while ignoring the performance of the natural environment. This short-sighted development model has led to an overwhelming of water ecosystems in some areas due to unreasonable levels of human activity (Qin et al. 2011). Water ecological carrying capacity (WECC) expresses the ability of a water ecosystem to both support and withstand socio-economic development (Zhang et al. 2014). WECC links water ecological problems and socio-economic development together through carrying capacity, seeking to establish a sustainable development model (Arrow et al. 1995). This concept no longer simply focuses on human needs, but measures the overall coordination between human development and water ecosystems.

Objective assessments of WECC are the key link in the understanding and application of this concept in practice (Yang et al. 2014). Since WECC involves a large number of influence factors, indicator-based methods are very useful in the assessment process (Liu & Cai 2012). Normally, the main procedures of these methods are: first, to build up a WECC evaluation indicator system in accordance with a logical framework; secondly, to convert these indicators into a composite index using a mathematical approach (generally, the indices are dimensionless values from 0 to 1 or 0 to 100) (Du et al. 2011); and finally, to use this composite index to comprehensively assess the WECC (Gong & Jin 2009). The index frameworks for WECC evaluation have been used in existing research as follows: (a) the driving forces-pressures-state-impacts-responses (DPSIR) framework (Maxim et al. 2009); and (b) the subsystem framework, such as water resources, water environment, water ecology, and socio-economic subsystem (Zhang et al. 2014). The most popular indicators that are frequently adopted include the following (Liu & Cai 2012): population, natural population growth rate, gross domestic product (GDP) per capita, the proportion of three industries, water consumption per unit of GDP, Engel coefficient, forest coverage rate, arable land per capita, intensity of pollutant emission, waste water discharge, and municipal sewage treatment rate (Li et al. 2011b).

WECC evaluation is essentially a systematic procedure that involves the comprehensive measurement of many factors. Therefore, the WECC assessment index system is not simply a display of indicators, but should be built up based on a logical framework and in accordance with scientific principles (Liu & Cai 2012). However, most currently applied subsystem frameworks are not able to reflect the causal connections of carrying capacity. Moreover, there are overlaps among the water resource subsystem, water environment subsystem, and water ecological subsystem (Yang et al. 2014). The DPSIR framework using indicator categories too much may in turn weaken the impact of the key indicators. There are also some problems in indicator selection: (a) similar indicators and highly relevant indicators are repeated redundantly due to unsatisfactory framework designs; (b) indicators that weakly or unclearly affect WECC are adopted (such as arable land per capita, and the Engel coefficient); and (c) emphasis is put on socio-economic driving force and pressure indicators, while ignoring water ecosystem health status (Liu & Cai 2012). In addition, the existing indicator-based methods generally adopt one single composite index to determine whether WECC is overloaded (Zhang et al. 2014). The conclusions obtained by such methods may be unreliable because the water ecosystem health status cannot be fully known using such indicators.

In this paper, the guidelines for WECC evaluation are examined. The authors take human–water harmony as the purpose and rule of WECC. WECC evaluation results should be obtained through assessing the pressure of socio-economic development on the water ecosystem, the level of support force from the water ecosystem for socio-economic development, and the overall health status of the water ecosystem. According to the above analysis, based on the pressure-support force-state index framework (PSS), this paper proposes an improved WECC evaluation method (PSSM). PSSM directly focuses on the pressure-support effect analysis which is the core content of WECC evaluation. PSSM considers the determinant influence on WECC from the health status of a water ecosystem, a factor that is often inadequately considered in other methods (Yang et al. 2014). In addition, PSSM with diversified criteria and measures is more reliable than the methods using one single composite index.

METHODOLOGY

Guidelines for WECC evaluation

The general guidelines for WECC evaluation are as follows. (a) Human–water harmony is the goal and principle of WECC evaluation. It is necessary to achieve a coordinated and harmonious human–water relationship, which can ensure that even a limited water ecosystem will provide long-term support for socio-economically sustainable development. (b) The analysis of the pressure-support effect is the core content of WECC evaluation. WECC essentially reflects the pressure-support interactions between the water ecosystem and the socio-economic system. Therefore, a stress analysis of the interactions between socio-economic pressure and water ecosystem support force is the key point in evaluating WECC. (c) Th water ecosystem health state is an important standard for discerning whether WECC is overloaded. Only a healthy water ecosystem can provide long-term support for socio-economic development.

The PSS framework for an index system

This paper puts forward a WECC assessment index system based on three categories: pressure index (PI), support force index, and water ecosystem health state index (HSI). The PSS framework has a clear cause–effect relationship, which reflects the interaction between the elements of WECC and emphasizes the direct effective factors in the relationship. Socio-economic pressure is the sum of water consumption pressure, water pollution pressure, and water ecological damage pressure. The water ecosystem support force reflects a water ecosystem's ability to constantly provide support for socio-economic development, and bear the stresses caused by human society to produce a relatively stable situation. The water ecosystem support force is a natural attribute of a water ecosystem. If the health of a water ecosystem cannot be guaranteed, then the water ecosystem support force will not be sustainable. Therefore, the health state of a water ecosystem is also an important factor for evaluating WECC.

Indicator selection

This study chose the indicators for the WECC evaluation index system through theoretical analysis and literature research (Liu & Cai 2012; Zhang et al. 2014). WECC evaluation is a complicated systematic project involving society, economy, water resource, water environment, water ecology, and other factors. All the selected indicators should be scientifically based, systematic, representative and comparable. As far as possible it is also necessary to select indicators with high impact factors, good disaggregation, clear response effects, and strong numerical sensitivities, and then exclude or merge some to make highly relevant indicators. The selected indicators that express socio-economic pressure, water ecosystem support force and water ecosystem health status are proposed based on the PSS framework (indicator explanations are given in Table 1).

  1. Socio-economic pressure indicators: the human activities of production and living are the main cause of changes in the state of health of a water ecosystem. There are six indicators for evaluating socio-economic pressure: population density, economic strength per unit of land area, water consumption per unit of GDP, per capita water consumption, pollutant emissions per unit of GDP, and sewage per capita emissions. Among these indicators, population density, per capita water consumption and sewage per capita emissions mainly reflect pressures from population; and economic strength per unit of land area, water consumption per unit of GDP, and pollutant emissions per unit of GDP express the pressures deriving from the development of the economy.

  2. Water ecosystem support force indicators: the support of a water ecosystem for socio-economic development is the foundation of WECC. The level of this support is determined by the supply capacity of local water resources and the assimilative capacity of the water environment (Yang et al. 2014). Four indicators of support were selected in accordance with this understanding: water resources per capita, forest coverage rate, river runoff per unit of land area, and the precipitation coefficient of variation (Gong & Jin 2009). Water resource per capita is an important factor that affects water supply capacity. The remaining three indicators are significant factors affecting water supply capacity and the assimilative capacity of the water environment.

  3. Indicators of water ecosystem health status: a healthy water ecosystem should demonstrate physical, chemical and biological integrity, and be self-recycling (Müller et al. 2000). Water ecosystem health state evaluation includes the evaluation of aquatic organism health and habitats (Holguin-Gonzalez et al. 2013). In this paper, four evaluation indicators of water ecosystem health were selected as follows: ecological water requirement satisfaction rate, river habitat quality index, the water quality compliance rate of functional areas, and the aquatic biological integrity index (Barbour et al. 1999; An et al. 2002; Zhang et al. 2009, 2013).

Table 1

Recommended indicators for the WECC evaluation index system (Zhang et al. 2014)

CategoryNo.Name of indicatorsUnitsIndicator explanation
Pressure Population density Persons/km2 Population/Land area 
Economic strength per unit of land area 104 Yuan/km2 GDP/Land area 
Water consumption per unit of GDP m3/104 Yuan Annual water consumption/GDP 
Per capita water consumption L/person/day Average daily water consumption/Population 
Pollutant emissions per unit of GDP kg/104 Yuan Annual emissions of pollutants/GDP 
Pollutant emissions of sewage per capita kg/person/day Pollutant emission of sewage/Population 
Water resources per capita m3/person Total water resources/Population 
Supportive force Forest coverage rate Woodland area/Land area 
River runoff per unit of land area m3/km2 River runoff/Land area 
10 Precipitation coefficient of variation – Reflect changes in precipitation 
11 River habitat quality index (HQI) – Represent the physical habitat quality conditions of a river (Barbour et al. 1999
12 Water quality compliance rate of functional areas The proportion of water function zones compliant 
Health state 13 Environmental flow guarantee rate Environmental flow requirements for guaranteeing the physical condition of aquatic habitats (Richter et al. 2012
14 Index of biotic integrity (IBI) – Integrated index of multiple biological parameters for expressing water ecosystem health status (Barbour et al. 1999; An et al. 2002
CategoryNo.Name of indicatorsUnitsIndicator explanation
Pressure Population density Persons/km2 Population/Land area 
Economic strength per unit of land area 104 Yuan/km2 GDP/Land area 
Water consumption per unit of GDP m3/104 Yuan Annual water consumption/GDP 
Per capita water consumption L/person/day Average daily water consumption/Population 
Pollutant emissions per unit of GDP kg/104 Yuan Annual emissions of pollutants/GDP 
Pollutant emissions of sewage per capita kg/person/day Pollutant emission of sewage/Population 
Water resources per capita m3/person Total water resources/Population 
Supportive force Forest coverage rate Woodland area/Land area 
River runoff per unit of land area m3/km2 River runoff/Land area 
10 Precipitation coefficient of variation – Reflect changes in precipitation 
11 River habitat quality index (HQI) – Represent the physical habitat quality conditions of a river (Barbour et al. 1999
12 Water quality compliance rate of functional areas The proportion of water function zones compliant 
Health state 13 Environmental flow guarantee rate Environmental flow requirements for guaranteeing the physical condition of aquatic habitats (Richter et al. 2012
14 Index of biotic integrity (IBI) – Integrated index of multiple biological parameters for expressing water ecosystem health status (Barbour et al. 1999; An et al. 2002

Comprehensive evaluation index calculation

The calculation of the WECC evaluation indices can be divided into the following steps.

Indicator data standardization

In the multi-index comprehensive evaluation, indicators of different dimensions cannot be directly compared and calculated. Thus, mathematical transformations should be employed to standardize the indicators and to eliminate dimensional effects. In this paper, the standardization formulas are as below (Tian & Gang 2012): 
formula
1
 
formula
2
 
formula
3
where are the actual values of the indicators; are the corresponding dimensionless values of after standardization; and is the optimized value of the indicator. Positive indicators are standardized according to Equation (1); negative indicators are standardized according to Equation (2); and the indicators, i.e., ‘the closer a certain value is the better’, are standardized according to Equation (3).

Indicator weight determination

Indicator weight directly affects the value of the comprehensive evaluation index, and thus the accuracy of the evaluation result. The methods available for indicator weight determination are: the Delphi method, equal weighting, the analytic hierarchy process, principal component analysis, the entropy weighting method, the literature research method, and other situationally appropriate methods (Li et al. 2011a). In this study, the weights of the indicators were determined by the equal weighting method.

Aggregated index calculation

A weighted linear combination method was adopted to calculate the aggregated indices of socio-economic pressure, water ecosystem support, and water ecosystem health state. At this point, a final evaluation index that synthetically reflects the level of WECC could be developed. Socio-economic pressure is affected by many factors. Assume that socio-economic pressure P is determined by x1, x2, x3xn, and then P can be presented as 
formula
4
If the dimensionless values of x1, x2, x3xn are the standardized data of P1, P2, P3Pn, and the weight of each factor is Wi, then socio-economic PI can be calculated by Equation (5). The larger the PI value is, the more likely it is that socio-economic pressure is being put on the water ecosystem. 
formula
5
In the same way, the water ecosystem support force index (SI) and water ecosystem HSI can be calculated by Equations (6) and (7): 
formula
6
 
formula
7
where is the value of a support force factor after standardization, and is its weighting. The larger the SI value is, the greater the support capacity of the water ecosystem in socio-economic development is; is the evaluation value of a water ecosystem health state indicator, is its weighting. The larger the HSI value is, the healthier the water ecosystem is.
The pressure-support degree PS is determined by the ratio of PI and SI, and the formula for calculating PS is 
formula
8
Finally, the comprehensive WECC evaluation index is presented through the weighted linear combination method. The formula for calculating this index is 
formula
9
where WECCI is the comprehensive evaluation index of WECC, and ,, and are the weights of PI, SI, and HSI.

Evaluation standards

(a) Evaluation standard of HSI: according to the general methods of water ecosystem health assessment (Zhang et al. 2009), HSI is divided into five classes (Table 2). In general, 0.6 is the demarcation point for determining whether the water ecosystem is in a healthy status or not. (b) Evaluation standard of PS: to present the pressure-bearing capacity, the value of PS reflects the correlation of PI and SI. If PS < 1, WECC is in a carrying situation; if PS = 1, WECC is on the critical point; if PS > 1, WECC is in an overload situation. (c) Evaluation standard of WECCI: WECCI is divided into four levels as shown in Table 2 (Zhang et al. 2014).

Table 2

Evaluation criteria for HSI, PS, and WECCI (Zhang et al. 2009, 2014)

 GradesStandardsLevels
HSI [0.8, 1] Healthy 
II [0.6, 0.8] Sub-healthy 
III [0.4, 0.6] Poor 
IV [0.2, 0.4] Very poor 
[0, 0.2] Extremely poor 
PS [0, 1] Carrying 
II Critical situation 
III [1, +∞] Overload 
WECCI [0.75, 1] Good carrying 
II [0.5, 0.75] Weak carrying 
III [0.25, 0.5] Weak overload 
IV [0, 0.25] Severe overload 
 GradesStandardsLevels
HSI [0.8, 1] Healthy 
II [0.6, 0.8] Sub-healthy 
III [0.4, 0.6] Poor 
IV [0.2, 0.4] Very poor 
[0, 0.2] Extremely poor 
PS [0, 1] Carrying 
II Critical situation 
III [1, +∞] Overload 
WECCI [0.75, 1] Good carrying 
II [0.5, 0.75] Weak carrying 
III [0.25, 0.5] Weak overload 
IV [0, 0.25] Severe overload 

The final decision method for WECC evaluation indices

HSI focuses on the health state of the water ecosystem; PS focuses on presenting the interactions of pressure and support force; and WECCI focuses on reflecting the comprehensive situation of WECC. Based on the guidelines for WECC evaluation, a final decision method for WECC evaluation is proposed. The steps are as follows (Figure 1). (a) The HSI decision process. HSI is the primary basis for WECC evaluation, and should be prioritized as the reject condition. If HSI meets the criteria, then the decision process moves to the WECCI determination step; otherwise, WECC should be identified as being in an overload condition. (b) The WECCI decision process. If WECCI meets the requirements, then the decision process moves to the PS determination step; otherwise, it should be concluded that WECC is in an overload condition. (c) The PS decision process. If PS meets the standard, WECC is capable of carrying the projected load, otherwise, the decision process moves to the PI changing trend judgment step. (d) The PI decision process. If PI has been stable or downward in recent years, then WECC is capable of carrying the projected load, otherwise, it is in an overload condition. The above comprehensive decision method is in accordance with the principles of WECC evaluation, and is in accordance with good logic and ease of operation.

Figure 1

Decision process of PSSM.

Figure 1

Decision process of PSSM.

CASE STUDY

Study area

Tieling City was chosen as a target city for a case study. Covering an area of 12,985 km2, this city is located in Liaoning Province, China. The total population was 3.04 million in 2011, and GDP was 87.38 billion RMB Yuan. Water resources per capita for Tieling city are 839 m3, which is only one-ninth of the world per capita amount (data sources: Tieling Statistical Yearbook and Water Resources Bulletin of Tieling). Population expansion and further development of the social economy will constrain the sustainable development of this region, if water issues cannot be better addressed. WECC evaluation may provide technical support for the city at this stage in its development.

Results

PI, SI, and HSI

The calculated results for Tieling City's PI, SI, and HSI from 2005 to 2011 are shown in Figure 2. The variation range of PI is between 56.22 (in 2011) and 64.17 (in 2005). It is obviously on a downward path, which indicates that pressure from the social economy on the water ecosystem was declining in general even though there was a 330% growth in GDP during the same period. The possible reasons for this pattern could be: (a) for the period 2005–2011, there was almost no population growth in Tieling City, thus, the pressure from residential sources may not have increased; and (b) through the implementation of water conservation and waste reduction measures, per capita water consumption and pollutant emission intensity may have significantly decreased.

Figure 2

Calculation results of PI, SI, HSI, PS, and WECCI.

Figure 2

Calculation results of PI, SI, HSI, PS, and WECCI.

The variation range of SI is from 44.85 (in 2006) to 65.09 (in 2010). This measure fluctuates greatly with no obvious regularity. Only in 2005 and 2010 are the SI values above 50, which indicates that the water ecosystem provides weak support for socio-economic development. A significant correlation between precipitation and SI was found through analyzing their numbers.

The variation range in HSI is from 57.04 (in 2007) to 72.13 (in 2010). From 2005 to 2009, the values are stable, while in 2010 and 2011, the values are obviously higher than in the previous 5 years. Combined with the variation trends of PI and SI, conclusions can be drawn by analyzing the variety of HSI values, as follows. (a) The water ecosystem in Tieling City experienced a poor state of health from 2005 to 2009, and was in a sub-healthy state between 2010 and 2011. (b) 2010 was a wet year, and the precipitation was 86.3% more than the long-time average annual value. This is the main reason for the significant improvement of HSI in 2010, because abundant rainfall could meet the environmental flow requirements of the river ecosystem, which in turn helped to dilute pollutants and improve the health of the water ecosystem. (c) Precipitation in 2011 was the lowest between 2005 and 2011, but HSI still remained at a high level. This may indicate that water conservation and waste reduction measures have played an important role in reducing the pressure of socio-economic development on the water ecosystem.

PS and WECCI

The calculated values for Tieling City's PS and WECCI from 2005 to 2011 are shown in Figure 2. The variation range of PS is 0.91 (in 2010) to 1.38 (in 2007). The PS figures from 2005 to 2011 are less than 1 except in 2010. The variation range of WECCI is 46.67 (in 2007) to 59.27 (in 2010). The WECCI figures from 2005 to 2011 are less than 50 except in 2010 and 2011. According to the evaluation standards in Table 2, it can be concluded that: (a) from 2005 to 2009 the WECC of Tieling City was in a state of overload; (b) the WECC of Tieling City was within carrying capacity in 2010; and (c) in 2011, PS was 1.12 (weak overload), but WECCI was 53.76 (weak carrying). There is a contradiction between the PS result and the WECCI result.

The final results of the WECC evaluation

The final results of the WECC evaluation were obtained according to the decision process outlined in Figure 1. From 2005 to 2009, the WECC of Tieling City was in a situation of overload (HSI < 0.6); the WECC of 2010 was in a situation of weak carrying (HSI, WECCI, and PS are all in line with the requirements); and the WECC of 2011 was also in a weak carrying state (HSI and WECCI meet the requirements). In this period, although PS was above 1, the trend of PI was downward. From 2005 to 2011, the mean value of PI was 61.3, while the mean value of SI was only 50.8. This implies that a long-term pressure-support contradiction has damaged the health of the water ecosystem in Tieling City. The WECC evaluation results show that even though a number of improvement measures have been carried out in Tieling City in recent years, the current situation of the water ecosystem protection is still grim. Further reduction in the pressure that comes from socio-economic development is still required, and the water ecosystem health state should be further improved.

DISCUSSION

Compared with the existing indicator-based methods for WECC assessment (Peng 2013; Zhang et al. 2014), PSSM shows the following improvements. (a) PSSM directly focuses on the pressure-support effect which is the core content of WECC evaluation, and fully considers the dominant influence on WECC from the health state of a water ecosystem, which is often overlooked in other WECC evaluation methods (Zhang et al. 2014). (b) The PSSM index framework is simpler and clearer than that of DPSIR, and to some extent can avoid the recurrent selection of similar indicators. (c) The decision method of PSSM considers the overall impact of factors on the carrying capacity while preserving the independence of these important indices instead of lumping them into a single complex index (Peng 2013; Yang et al. 2014). Therefore, PSSM is more reasonable and may improve the reliability of WECC evaluation.

Owing to the complexity of the human–water relationship, there is no doubt that some components within PSSM are oversimplified and imperfect. The authors wish to highlight some of the limitations and uncertainties, which are mainly as follows. (a) Indicator data, such as for IBI and HQI, which reflect the water ecosystem's state of health, are usually hard to obtain. If there are no such data from existing studies or relevant literature, a great deal of on-site investigation needs to be carried out. (b) There remains some uncertainty and subjectivity when applying the PSSM procedure to the evaluation of WECC, in particular weighting determination, indicator data standardization, and evaluation standards.

CONCLUSIONS

WECC-relevant studies have been gradually emerging in recent years. The concept of WECC, as a link between the water ecosystem and the socio-economic system, focuses on their interactions and provides a path for promoting the sustainable development of communities in relation to available water resources. Based on the PSS indicator framework, a WECC evaluation method is put forward in this paper (PSSM). In PSSM, there are some improvements to the existing indicator-based methods in respect of the index framework, indicator selection, evaluation criteria, and the decision process. PSSM directly focuses on a pressure-support effect analysis of the water ecosystem and the socio-economic system, and fully considers water ecosystem states of health. The index framework of PSSM is helpful in selecting the indicators which are closely related to WECC. In addition, by adopting ecological indicators, the philosophy of human–water harmony and ecological preferences can be implemented. Furthermore, PSSM adopts a comprehensive decision framework, which may improve the reliability of WECC evaluations. A case study in Tieling City shows that WECC from 2005 to 2009 was in a situation of overload, and from 2010 to 2011 was in a situation of weak carrying state. The long-term pressure-support contradiction has damaged the health of the water ecosystem in Tieling City. The assessment results based on PSSM are expected to provide helpful information for the sustainable development of Tieling.

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