Excessive socioeconomic activities in the Weihe River region have caused severe ecosystem degradation, and the call for the recovery and maintenance of the river health has drawn great attention. Based on the connotation of river health, previous research findings, and status quo of the Weihe River ecosystem, in this study, we developed a novel health evaluation index system to quantitatively determine the health of the Weihe River in Shaanxi Province. The river in the study area was divided into five reaches based on the five hydrological gauging stations, and appropriate evaluation indices for each river section were selected according to the ecological environmental functions of that section. A hybrid approach integrating analytic hierarchy process (AHP) and a fuzzy synthetic evaluation method was applied to measure the river health. The results show that Linjiancun-Weijiabao reach and Weijiabao-Xianyang reach are in the “moderate” level of health and Lintong-Huaxian reach and downstream of Huaxian reach are in the “poor” health rating, whereas Xianyang-Lintong reach is in the “sick” rating. Moreover, the most sensitive factors were determined, respectively, for each reach from upper stream to lower stream in the study area.
From an economic point of view, water resources are composite assets that provide a variety of goods and services for consumptive and productive activities of human being [
However, the problems of water scarcity and deteriorating water quality, due to rapid socioeconomic development [
Catchment and riparian degradation has caused declining ecosystem health of streams worldwide [
China possesses total water resources of 2812.4 billion m3 ranking the 6th in the world, while its per capita water resource only accounts for a quarter of world average in terms of per capita water resources [
This study developed an evaluation index system for the health of the Weihe River in Shaanxi Province, where the Weihe River (Shaanxi section) was divided into five sections based on water function zoning method and then the river health was evaluated section by section. This study improves the indicator evaluation system for river health; the methods and results concerned will provide theoretic and practical support for river health assessment, river development, utilization, and management, and river ecological restoration of the Weihe River.
The Weihe River, originating from north of Niaoshu Mountain with an altitude of 3485 m above sea level, is the largest tributary of the Yellow River. It runs across 818 km through the provinces of Gansu and Shaanxi and joins the Yellow River from the right bank in the city of Tongguan, from where the Yellow River turns to the east (Figure
Study area showing hydrological gauging stations.
The Weihe River flows across about 502.4 km with a drainage area of 67,100 km2 in Shaanxi Province where the well-known Guanzhong Plain in Northwest China is located. The river basin plays a great role in social, ecological, and economical development of Shaanxi Province (Figure
The most important topographic feature of the Weihe River Basin is the Loess Plateau in the north, which is the main source of sediments in the river [
The climate of the Weihe River Basin in Shaanxi Province is continental, warm, and semihumid controlled by the East Asian monsoon climate [
A large number of groundwater pumping wells were constructed near the river, which potentially reduces the infiltration of river water into the adjacent aquifers of the Weihe River. Agricultural irrigation is the largest water consumer, representing 60% of the total amount of water consumption, which is withdrawn from the river and aquifers. Even in the low flow period, 80% of water in the stream is impounded by dams for agriculture irrigation in the Baoji district [
Cumulative volume of deposited sediment in the channel of the lower Weihe River.
In addition, the Weihe River is the major sewage discharge channel in the Guanzhong Region. Large amount of untreated industrial wastewater and domestic sewage is directly discharged into the Weihe River. Also, the nonpoint source of contaminated stormwater washing off parking lots, roads and highways, and lawns (containing fertilizers and pesticides) is drained into the river. According to the survey data from Shaanxi Provincial Environmental Protection Bureau, a total number of 245 sewage discharge ports are distributed on both sides of the Weihe River, and more than 700 million tons of sewage is discharged into the river annually, which has resulted in serious water pollution [
There are five hydrological gauging stations (Linjiacun, Weijiabao, Xianyang, Lintong, and Huaxian) along the Weihe River in Shaanxi Province (Figure
Fuzzy synthetic evaluation method applies fuzzy mathematical principles to evaluate things and phenomenon affected by variety of factors [
Firstly, evaluation factors are defined. In order to obtain accurate assessment results for river health conservation and protection, the factors reflecting the river health are determined based on the following rules. (1) Naturality: The factors reflect the essential attribute of the ecosystem functions of each section for the river [
Based on these rules, the main ecosystem function of the Weihe River and the water environmental function zoning of Shaanxi Province was enacted by provincial government in 2004. Factor set of evaluation object is determined, which is expressed by
Secondly, remarkable grades of evaluation factors are determined: the rationality of the evaluation criteria directly affects the accuracy of evaluation results. Considering field investigation, expert advice, and literature review, the five remarkable grades including excellent, good, moderate, poor, and sick for the significance of the evaluation factors are defined, which is expressed by set,
Thirdly, the factor weights are determined. The accuracy determination of the factor weight is of great importance to the evaluation of estimation results. Many methods have been developed to calculate factor weight, which can be roughly divided into subjective method and objective method according to the different sources of the original data. Subjective method, such as Delphi method and analytic hierarchy process (AHP), depends on the expert’s experience and judgment, and so on. In contrast, objective method, such as principal component analysis method, standard variance method, and maximizing deviation method, is based on measured data and thus has less subjective errors caused by human judgment. Objective ways cannot reflect the subjective requirements of decision-makers though they avoid human interference. This study used AHP, one commonly used method, to evaluate the health of the Weihe River in Shaanxi Province. A formal questionnaire for evaluation factor weights is proposed, the values of which are suggested by researches and governors. Then, the weight for each evaluation factor is determined, which is expressed by
Fourthly, the fuzzy membership function is estimated. There are some commonly used forms of functions to calculate membership degree, such as normal type, partial large-scale, partial small, triangular fuzzy numbers, lower semitrapezoidal, trapezoidal, and ridge [
For the first grade (
For the second, third, and fourth grade (
For the fifth grade (
Formulas (
Finally, the hierarchical fuzzy evaluation is calculated. The fuzzy relation matrix
The first-grade fuzzy comprehensive evaluation set
Based on formula (
Then, the second-grade fuzzy synthetic evaluation set
According to the principle of maximum membership degree, the greatest value in the set
Sensitivity analysis is a useful method to determine which factors are the key drivers to river health [
The reasonability of determining the evaluation indicator will directly affect the evaluation results of river health. Up to now, there has been no cohesive standard to assess the river health in China [
Evaluation indicators, weight, and current values for five reaches of the Weihe River in Shaanxi Province.
Reach | Criterion layer (weight) | Indicator layer | Evaluation criterion | Current value | ||||
---|---|---|---|---|---|---|---|---|
Excellent | Good | Moderate | Poor | Sick | ||||
LW | Water quality (0.25) | COD (mg/L) | 15.00 | 20.00 | 30.00 | 40.00 | 50.00 | 19.16 |
NH3-N (mg/L) | 0.15 | 0.50 | 1.00 | 1.50 | 2.00 | 0.69 | ||
Ecological function (0.59) | Ratio of wetland area (%) | 5.00 | 4.00 | 3.00 | 2.00 | 1.00 | 1.24 | |
Riparian vegetation coverage ratio (%) | 90.00 | 70.00 | 50.00 | 30.00 | 10.00 | 59.89 | ||
Guaranteed rate of discharge in dry flow season (%) | 80.00 | 60.00 | 40.00 | 20.00 | 0.00 | 44.00 | ||
Recreation function (0.16) | Recreation value index | 90.00 | 70.00 | 50.00 | 30.00 | 10.00 | 40.00 | |
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WX | Water quality (0.32) | COD (mg/L) | 15.00 | 20.00 | 30.00 | 40.00 | 50.00 | 53.77 |
NH3-N (mg/L) | 0.15 | 0.50 | 1.00 | 1.50 | 2.00 | 8.57 | ||
Ecological function (0.34) | Riparian vegetation coverage ratio (%) | 90.00 | 70.00 | 50.00 | 30.00 | 10.00 | 60.97 | |
Proportion of fish species (%) | 0.80 | 0.60 | 0.40 | 0.20 | 0.00 | 0.35 | ||
Guaranteed rate of discharge in dry flow season (%) | 80.00 | 60.00 | 40.00 | 20.00 | 0.00 | 40.00 | ||
Recreation function (0.34) | Recreation value index | 90.00 | 70.00 | 50.00 | 30.00 | 10.00 | 36.00 | |
|
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XL | Water quality (0.34) | COD (mg/L) | 15.00 | 20.00 | 30.00 | 40.00 | 50.00 | 49.60 |
NH3-N (mg/L) | 0.15 | 0.50 | 1.00 | 1.50 | 2.00 | 7.25 | ||
Ecological function (0.14) | Ratio of wetland area (%) | 5.00 | 4.00 | 3.00 | 2.00 | 1.00 | 8.90 | |
Riparian vegetation coverage ratio (%) | 90.00 | 70.00 | 50.00 | 30.00 | 10.00 | 60.43 | ||
Guaranteed rate of discharge in dry flow season (%) | 90.00 | 70.00 | 50.00 | 30.00 | 10.00 | 51.85 | ||
Flood control (0.52) | Variation rate of streambed gradient (%) | 100.00 | 90.00 | 80.00 | 60.00 | 40.00 | 87.00 | |
Capacity of flood discharge (%) | 95.00 | 85.00 | 75.00 | 60.00 | 40.00 | 54.60 | ||
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LH | Water quality (0.20) | COD (mg/L) | 15.00 | 20.00 | 30.00 | 40.00 | 50.00 | 41.46 |
NH3-N (mg/L) | 0.15 | 0.50 | 1.00 | 1.50 | 2.00 | 6.01 | ||
Ecological function (0.37) | Riparian vegetation coverage ratio (%) | 90.00 | 70.00 | 50.00 | 30.00 | 10.00 | 65.08 | |
Guaranteed rate of discharge in dry flow season (%) | 90.00 | 70.00 | 50.00 | 30.00 | 10.00 | 61.00 | ||
Flood control (0.43) | Variation rate of streambed gradient (%) | 100.00 | 90.00 | 80.00 | 60.00 | 40.00 | 44.00 | |
Requirement rate of bankfull discharge (%) | 100.00 | 80.00 | 60.00 | 40.00 | 20.00 | 39.00 | ||
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DH | Water quality (0.16) | COD (mg/L) | 15.00 | 20.00 | 30.00 | 40.00 | 50.00 | 37.26 |
NH3-N (mg/L) | 0.15 | 0.50 | 1.00 | 1.50 | 2.00 | 5.46 | ||
Ecological function (0.30) | Ratio of wetland area (%) | 5.00 | 4.00 | 3.00 | 2.00 | 1.00 | 1.64 | |
Guaranteed rate of discharge in dry flow season (%) | 80.00 | 60.00 | 40.00 | 20.00 | 0.00 | 37.50 | ||
Flood control (0.54) | Variation rate of streambed gradient (%) | 100.00 | 90.00 | 80.00 | 70.00 | 60.00 | 71.00 | |
Requirement rate of bankfull discharge (%) | 100.00 | 80.00 | 60.00 | 40.00 | 20.00 | 62.50 |
According to the principle of maximum membership degree and the estimated value from the criterion layer (Table
The evaluation results for the criterion layer of Weihe River in Shaanxi Province.
Section | Criterion layer | Excellent | Good | Moderate | Poor | Sick |
---|---|---|---|---|---|---|
LW | Water quality | 0.04 | 0.81 | 0.16 | 0.00 | 0.00 |
Ecological function | 0.00 | 0.20 | 0.47 | 0.05 | 0.28 | |
Recreation function | 0.00 | 0.00 | 0.50 | 0.50 | 0.00 | |
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WX | Water quality | 0.00 | 0.00 | 0.00 | 0.00 | 1.00 |
Ecological function | 0.00 | 0.19 | 0.76 | 0.05 | 0.00 | |
Recreation function | 0.00 | 0.00 | 0.21 | 0.79 | 0.00 | |
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XL | Water quality | 0.00 | 0.00 | 0.00 | 0.00 | 1.00 |
Ecological function | 0.33 | 0.18 | 0.49 | 0.00 | 0.00 | |
Flood control | 0.00 | 0.40 | 0.11 | 0.42 | 0.09 | |
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LH | Water quality | 0.00 | 0.00 | 0.00 | 0.48 | 0.53 |
Ecological function | 0.00 | 0.72 | 0.28 | 0.00 | 0.00 | |
Flood control | 0.00 | 0.00 | 0.00 | 0.55 | 0.46 | |
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DH | Water quality | 0.00 | 0.00 | 0.09 | 0.42 | 0.50 |
Ecological function | 0.00 | 0.00 | 0.48 | 0.38 | 0.15 | |
Flood control | 0.00 | 0.02 | 0.49 | 0.49 | 0.00 |
The estimated values of membership degree for evaluation indicator of the Weihe River in Shaanxi Province.
Reach | Criterion layer | Indicator layer | Value of membership degree | ||||
---|---|---|---|---|---|---|---|
Excellent | Good | Moderate | Poor | Sick | |||
LW | Water quality | COD (mg/L) | 0.07 | 0.93 | 0.00 | 0.00 | 0.00 |
NH3-N (mg/L) | 0.00 | 0.68 | 0.32 | 0.00 | 0.00 | ||
Ecological function | Ratio of wetland area (%) | 0.00 | 0.00 | 0.00 | 0.14 | 0.86 | |
Riparian vegetation coverage ratio (%) | 0.00 | 0.49 | 0.51 | 0.00 | 0.00 | ||
Guaranteed rate of discharge in dry flow season (%) | 0.00 | 0.10 | 0.90 | 0.00 | 0.00 | ||
Recreation function | Recreation value index | 0.00 | 0.00 | 0.50 | 0.50 | 0.00 | |
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WX | Water quality | COD (mg/L) | 0.00 | 0.00 | 0.00 | 0.00 | 1.00 |
NH3-N (mg/L) | 0.00 | 0.00 | 0.00 | 0.00 | 1.00 | ||
Ecological function | Riparian vegetation coverage ratio (%) | 0.00 | 0.58 | 0.42 | 0.00 | 0.00 | |
Proportion of fish species (%) | 0.00 | 0.00 | 0.85 | 0.15 | 0.00 | ||
Guaranteed rate of discharge in dry flow season (%) | 0.00 | 0.00 | 1.00 | 0.00 | 0.00 | ||
Recreation function | Recreation value index | 0.00 | 0.00 | 0.21 | 0.79 | 0.00 | |
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XL | Water quality | COD (mg/L) | 0.00 | 0.00 | 0.00 | 0.00 | 1.00 |
NH3-N (mg/L) | 0.00 | 0.00 | 0.00 | 0.00 | 1.00 | ||
Ecological function | Ratio of wetland area (%) | 1.00 | 0.00 | 0.00 | 0.00 | 0.00 | |
Riparian vegetation coverage ratio (%) | 0.00 | 0.53 | 0.47 | 0.00 | 0.00 | ||
Guaranteed rate of discharge in dry flow season (%) | 0.00 | 0.02 | 0.98 | 0.00 | 0.00 | ||
Flood control | Variation rate of streambed gradient (%) | 0.00 | 0.79 | 0.21 | 0.00 | 0.00 | |
Capacity of flood discharge (%) | 0.00 | 0.00 | 0.00 | 0.83 | 0.17 | ||
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LH | Water quality | COD (mg/L) | 0.00 | 0.00 | 0.00 | 0.95 | 0.05 |
NH3-N (mg/L) | 0.00 | 0.00 | 0.00 | 0.00 | 1.00 | ||
Ecological function | Riparian vegetation coverage ratio (%) | 0.00 | 0.86 | 0.14 | 0.00 | 0.00 | |
Guaranteed rate of discharge in dry flow season (%) | 0.00 | 0.58 | 0.42 | 0.00 | 0.00 | ||
Flood control | Variation rate of streambed gradient (%) | 0.00 | 0.00 | 0.00 | 0.10 | 0.90 | |
Requirement rate of bankfull discharge (%) | 0.00 | 0.00 | 0.00 | 0.99 | 0.01 | ||
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DH | Water quality | COD (mg/L) | 0.00 | 0.00 | 0.17 | 0.83 | 0.00 |
NH3-N (mg/L) | 0.00 | 0.00 | 0.00 | 0.00 | 1.00 | ||
Ecological function | Ratio of wetland area (%) | 0.00 | 0.00 | 0.00 | 0.71 | 0.29 | |
Guaranteed rate of discharge in dry flow season (%) | 0.00 | 0.00 | 0.96 | 0.04 | 0.00 | ||
Flood control | Variation rate of streambed gradient (%) | 0.00 | 0.00 | 0.02 | 0.98 | 0.00 | |
Requirement rate of bankfull discharge (%) | 0.00 | 0.04 | 0.96 | 0.00 | 0.00 |
Comparison of impounding water by dams and the runoff in Baoji area [
Month | Jan. | Feb. | Mar. | Apr. | May | Jun. | Jul. | Aug. | Sep. | Oct. | Nov. | Dec. |
---|---|---|---|---|---|---|---|---|---|---|---|---|
Impounding water (106 m3) | 38.0 | 34.5 | 38.8 | 34.0 | 31.0 | 46.2 | 55.3 | 81.6 | 65.9 | 71.0 | 47.2 | 40.1 |
Runoff (106 m3) | 44.7 | 40.7 | 60.6 | 63.6 | 88.2 | 99.3 | 166.8 | 181.5 | 165.2 | 184.3 | 76.7 | 48.5 |
Ratio of impounding water to runoff | 0.85 | 0.85 | 0.64 | 0.53 | 0.35 | 0.47 | 0.33 | 0.45 | 0.40 | 0.39 | 0.62 | 0.83 |
Membership matrix and the corresponding weights in the evaluation criteria layer of each reach were used to calculate the health situation of the Weihe River in Shaanxi Province (Table
Health evaluation results of the Weihe River in Shaanxi Province.
Reaches | Excellent | Good | Moderate | Poor | Sick | |
---|---|---|---|---|---|---|
River health index (RHI) | LW | 0.01 | 0.32 | 0.40 | 0.11 | 0.17 |
WX | 0.00 | 0.06 | 0.33 | 0.29 | 0.32 | |
XL | 0.05 | 0.23 | 0.13 | 0.22 | 0.39 | |
LH | 0.00 | 0.27 | 0.10 | 0.33 | 0.30 | |
DH | 0.00 | 0.01 | 0.42 | 0.45 | 0.13 |
In this study, the sensitive degree of an indicator to the river health was determined by comparing the difference of measurement values between keeping the whole indicators and taking out of the indicator. Based on the result variation of each indicator before and after being removed, the influence degree, that is, sensitivity degree of the indicator to the health evaluation results of the Weihe River, was determined.
For the reach of LW, the guaranteed rate of discharge in the dry flow season is the most sensitive factor, and the second sensitive factor is the ratio of wetland area (Table
Sensitive analysis results for the health evaluation of the Weihe River in Shaanxi Province.
Reach | Removed indicators | Value of membership degree | ||||
---|---|---|---|---|---|---|
Excellent | Good | Moderate | Poor | Sick | ||
LW | Non | 0.01 | 0.32 | 0.40 | 0.11 | 0.17 |
COD (mg/L) | 0.01 | 0.32 | 0.40 | 0.11 | 0.17 | |
NH3-N (mg/L) | 0.00 | 0.29 | 0.44 | 0.11 | 0.17 | |
Ratio of wetland area (%) | 0.02 | 0.35 | 0.36 | 0.11 | 0.17 | |
Riparian vegetation coverage ratio (%) | 0.01 | 0.38 | 0.54 | 0.08 | 0.00 | |
Guaranteed rate of discharge in dry flow season (%) | 0.01 | 0.23 | 0.39 | 0.12 | 0.25 | |
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WX | Non | 0.00 | 0.06 | 0.33 | 0.29 | 0.32 |
COD (mg/L) | 0.00 | 0.06 | 0.33 | 0.29 | 0.32 | |
NH3-N (mg/L) | 0.00 | 0.06 | 0.33 | 0.29 | 0.32 | |
Riparian vegetation coverage ratio (%) | 0.00 | 0.00 | 0.39 | 0.29 | 0.32 | |
Proportion of fish species (%) | 0.00 | 0.10 | 0.31 | 0.27 | 0.32 | |
Guaranteed rate of discharge in dry flow season (%) | 0.00 | 0.10 | 0.29 | 0.29 | 0.32 | |
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XL | Non | 0.05 | 0.23 | 0.13 | 0.22 | 0.39 |
COD (mg/L) | 0.05 | 0.23 | 0.13 | 0.22 | 0.39 | |
NH3-N (mg/L) | 0.05 | 0.23 | 0.13 | 0.22 | 0.39 | |
Ratio of wetland area (%) | 0.00 | 0.24 | 0.16 | 0.22 | 0.38 | |
Riparian vegetation coverage ratio (%) | 0.07 | 0.21 | 0.12 | 0.22 | 0.38 | |
Guaranteed rate of discharge in dry flow season (%) | 0.07 | 0.24 | 0.09 | 0.22 | 0.38 | |
Variation rate of streambed gradient (%) | 0.05 | 0.03 | 0.07 | 0.43 | 0.42 | |
Capacity of flood discharge (%) | 0.05 | 0.44 | 0.18 | 0.00 | 0.34 | |
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LH | Non | 0.00 | 0.27 | 0.10 | 0.33 | 0.30 |
COD (mg/L) | 0.00 | 0.27 | 0.10 | 0.24 | 0.40 | |
NH3-N (mg/L) | 0.00 | 0.27 | 0.10 | 0.43 | 0.21 | |
Riparian vegetation coverage ratio (%) | 0.00 | 0.21 | 0.16 | 0.33 | 0.30 | |
Guaranteed rate of discharge in dry flow season (%) | 0.00 | 0.32 | 0.05 | 0.33 | 0.30 | |
Variation rate of streambed gradient (%) | 0.00 | 0.27 | 0.10 | 0.52 | 0.11 | |
Requirement rate of bankfull discharge (%) | 0.00 | 0.27 | 0.10 | 0.14 | 0.49 | |
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DH | Non | 0.00 | 0.01 | 0.42 | 0.45 | 0.13 |
COD (mg/L) | 0.00 | 0.01 | 0.41 | 0.38 | 0.21 | |
NH3-N (mg/L) | 0.00 | 0.01 | 0.44 | 0.51 | 0.05 | |
Ratio of wetland area (%) | 0.00 | 0.01 | 0.57 | 0.34 | 0.08 | |
Guaranteed rate of discharge in dry flow season (%) | 0.00 | 0.01 | 0.28 | 0.54 | 0.17 | |
Variation rate of streambed gradient (%) | 0.00 | 0.02 | 0.68 | 0.18 | 0.13 | |
Requirement rate of bankfull discharge (%) | 0.00 | 0.00 | 0.17 | 0.71 | 0.13 |
For the reach of WX, the riparian vegetation coverage ratio, the proportion of fish species, and the guaranteed rate of discharge in the dry flow season had an impact on the results of the river health evaluation in this river section (Table
For the reach of XL, the indicators that had greater impacts on the comprehensive evaluation results included the riparian vegetation coverage ratio, variation rate of streambed gradient, and capacity of flood discharge (Table
For the reach of LX, pollutant of COD and the requirement rate of bankfull discharge are the most influential factors affecting health evaluation results. In general, the requirement rate of bankfull discharge was slightly more sensitive than COD to the river health (Table
For the reach of DH, each indicator directly impacts the results of comprehensive health assessment to some extent (Table
River health is an indicator of the harmony between human and water resources; therefore river health assessment is an important tool for human to develop, utilize, and manage the river in a sustainable way. This study applied a hybrid approach combining AHP and fuzzy comprehensive evaluation to calculate the river health of the Weihe River in China. The results reveal that the reaches of LW and WX were in the “moderate” level of health, and the reaches of LH and DH were in the “poor” health rating, whereas the reach of XL was in the “sick” rating. The key factors which influence the river health are the guaranteed rate of discharge in the dry flow season, water quality, capacity of flood discharge, requirement rate of bankfull discharge, and variation rate of streambed gradient, respectively, for each reach from upper stream to lower stream in the study area.
Many factors in the evaluation process affect the results of river health assessment in varying degrees, such as indicator chosen, index weight determination, basis of reference, and evaluation methods. There are various factors affecting river health, which mainly include natural factors and human disturbance. Natural factors mainly include precipitation decrease, significant reduction in vegetation, and severe soil erosion in the Guanzhong area, which have increased the pressure on flood control in the downstream. As for human disturbance, great increasing of water demand due to rapid development of industry and agriculture and population growth in recent years has greatly aggravated the situation of insufficient river baseflow and even caused river blanking in parts of reaches, which has reduced stream’s self-purification capacity and exacerbated water pollution. In general, river health is more intensely affected by human activities, and thus more attention should be paid in aspects of planning water use, water saving, water protection, and promoting harmony between human and water.
The health of the Weihe River is closely related to the benefits of local people in the river basin, and thus the aspects of a healthy river that supplies their basic living needs should be fully investigated and incorporated in the evaluation system. Their concepts and choice of a healthy river are affected by the socioeconomic development situation and their education and awareness. The expectation of river health from local people according to their living needs and so on requires more comprehensive social and economic studies; therefore, it is not included in this study.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This study is jointly supported by National Natural Science Foundation of China (Grant no. 51379175), Specialized Research Fund for the Doctoral Program of Higher Education (Grant no. 20136101110001), Program for Key Science and Technology Innovation Team in Shaanxi Province (Grant no. 2014KCT-27), and Program for New Century Excellent Talents in University (Grant no. NCET-11-1045).