Assessment of impact of water diversion projects on ecological water uses in arid region

Song-hao SHANG*, Hui-jie WANG,

1. State Key Laboratory of Hydroscience and Engineering, Tsinghua University, Beijing 100084, P. R. China

2. Yellow River Engineering Consulting Co. Ltd., Zhengzhou 450003, P. R. China

Assessment of impact of water diversion projects on ecological water uses in arid region

Song-hao SHANG*1, Hui-jie WANG1,2

1. State Key Laboratory of Hydroscience and Engineering, Tsinghua University, Beijing 100084, P. R. China

2. Yellow River Engineering Consulting Co. Ltd., Zhengzhou 450003, P. R. China

In arid regions, large-scale water diversion from rivers leads to significant changes in river flow regimes, which may have large impacts on ecological water uses of river-dependent ecosystems, such as river, lake, wetland, and riparian ecosystems. To assess the integrated impact of water diversion on ecological water uses, we proposed a hierarchy evaluation model composed of four layers representing the evaluation goal, sub-areas of the influenced region, evaluation criteria, and water diversion schemes, respectively. The evaluation criteria for different types of ecological water uses were proposed, and the analytical hierarchy process was used for the integrated assessment. For a river ecosystem, the percentage of mean annual flow was used to define the grade of environmental flow. For a lake ecosystem, water recharge to the lake to compensate the lake water losses was used to assess the ecological water use of a lake. The flooding level of the wetland and the groundwater level in the riparian plain were used to assess the wetland and riparian ecological water uses, respectively. The proposed model was applied to a basin in northern Xinjiang in northwest China, where both water diversion and inter-basin water transfer projects were planned to be carried out. Based on assessment results for the whole study area and two sub-areas, an appropriate scheme was recommended from four planning schemes. With the recommended scheme, ecological water uses of the influenced ecosystems can be maintained at an acceptable level. Meanwhile, economical water requirements can be met to a great extent.

arid region; water diversion project; environmental impact assessment; ecological water use; environmental flow; riparian forest; analytic hierarchy process

1 Introduction

In arid regions, increasing water diversion from rivers for domestic, industrial, and agricultural uses over the past several decades has caused significant changes in river flow regimes (Ye et al. 2006). The decrease of river flow may have large impacts on ecological water uses of river-dependent ecosystems (Shang et al. 2006), such as river, lake, wetland, and riparian ecosystems that are sensitive to large changes in river flow regimes. The impact of river flow change on ecological water uses has become the focus of environmental impactassessment for water diversion projects in arid regions.

The impacts of river flow change and water diversion/transfer projects on different river-dependent ecosystems are usually assessed separately. For river ecosystems, over 200 flow assessment methods have been proposed to define the minimum or suitable ecological (environmental) flow (Tharme 2003; Jain 2012). These methods are usually classified as hydrological (Tennant 1976), hydraulic rating (Gippel and Stewardson 1998; Shang 2008), habitat simulation (Stalnaker et al. 1995), and holistic (King et al. 2000) methodologies. Of these methods, the Tennant (1976) method is the simplest and most widely used (Tharme 2003). For lake ecosystems, ecological water levels can be assessed with the historical water level method, lake morphology analysis, habitat analysis, or species-environment models (Xu et al. 2004; Beca 2008). The fluctuation of water level is also important for lake ecosystems (Wantzen et al. 2008). For wetland ecosystems, the influence of water level has also been studied (Kingsford 2000; Li et al. 2009; Tian et al. 2010). For riparian ecosystems, regeneration relies on the flooding level (Hughes and Rood 2003; Shang and Mao 2010), while the growth relies mainly on the groundwater level (Ye et al. 2010). These studies illustrate the impact of river flow change on different types of river-dependent ecosystems. However, few studies have considered an integrated assessment where all river-dependent ecosystems are taken as a whole.

The main objective of this study was to develop a hierarchy model for integrated ecological impact assessment of water diversion projects in arid regions. The model was applied to a river basin in northern Xinjiang in northwest China, where large-scale water diversion and inter-basin water transfer projects were planned to be carried out.

2 Ecological impact assessment model and assessment method

For the integrated assessment, many methods have been proposed (Chen et al. 2004), such as the statistical analysis method, fuzzy assessment method, artificial neural networks, multiple attribute decision method, and analytic hierarchy process (AHP). Of these methods, the AHP method (Saaty 1990; Satty 2008), as a structured technique for organizing and analyzing complex decisions, has been widely applied to decision and assessment problems. The AHP method was used in this study for the impact assessment of water diversion projects on ecological water uses.

2.1 Hierarchy model for assessment of impact of water diversion projects on ecological water uses

In arid regions, large-scale water diversion and transfer projects exert different impacts on related river, lake, wetland, and riparian ecosystems in the water source and water-receiving areas. Thus, it is necessary to assess the integrated impact of water diversion and transfer projects on ecological water uses. In this study, a hierarchy model was used to assess the impacts of water diversion and transfer projects on ecological water uses in aridregions (Fig. 1). This hierarchy model consists of four layers, including the goal, sub-area, criterion, and scheme layers. The whole area under consideration was further divided into several sub-areas, such as the water source sub-area and the water-receiving sub-area. Four types of ecosystems, the river, lake, wetland, and riparian ecosystems, which are closely associated with river flow regime, were selected as the assessment criteria. The lowest layer was the schemes of water diversion projects to be assessed.

Fig. 1Hierarchy model for integrated impact assessment of water diversion projects on ecological water uses

Based on the hierarchy model in Fig. 1, different schemes can be assessed with the AHP method (Saaty 1990). First, the schemes were assessed with respect to each criterion (ecological water use) to obtain the priorities of different schemes, which will be explained below. Then, ecological water uses were assessed with respect to each sub-area to obtain the priorities of ecological water uses and consequently the priorities of different schemes. Finally, the sub-areas were assessed to obtain the overall priorities of the schemes. Generally, a scheme with higher priority was preferable to a scheme with lower priority.

2.2 Criterion and method for river ecological flow assessment

Different grades of ecological flow of rivers provide different levels of protection for the river ecosystem. The Tennant (1976) method is one of the most widely used methods for ecological flow assessment. With this method, the percentage of mean annual flow was used to classify the ecological flow into five grades.

wherep1ik(i=1,2,…,m;k=1,2,…,n) is the priority of theith scheme with respect to the ecological flow in thekth sub-area.x=(x11k,x12k,…,x1mk)Tis the principal eigenvector ofA.

2.3 Criterion and method for lake ecological water use assessment

A lake’s water level and its fluctuations have large impacts on ecological processes and patterns of lake ecosystems. To compensate for water loss due to evapotranspiration and leakage, a certain amount of water is required to recharge the lake, which is referred to as ecological water consumption of a lake at a specified ecological lake level. Considering a one-year period for lake level fluctuations, water flowing or diverted into lakes was used to assess the ecological impact of water diversion projects. If the percentages of annual water recharge to ecological water consumption were in the ranges of above 80%, 60% to 80%, 40% to 60%, 20% to 40%, and below 20%, the lake ecosystem was considered to be in grades one to five, respectively.

The assessment of the schemes regarding to lake ecological water use was similar to that of ecological flow, and the priority of theith scheme with respect to lake ecological water use in thekth sub-area,p2ik(i=1,2,…,m;k=1,2,…,n), was derived.

2.4 Criterion and method for wetland ecological water use assessment

Knowing that the wetland ecosystem is mainly influenced by the flooding level, the flooding level was chosen as the assessment index for wetland ecological water use. However, the relationship between the wetland ecosystem and the flooding level has not been well recognized, and characterizing the grading of the flooding level is difficult. For the wetland ecosystem in arid regions, it is reasonable to assume that a higher flooding level is preferable to a lower flooding level.

Under this assumption, the impact of the flooding level on the wetland ecosystem could be assessed using the indirect method, where the 0-to-2 scale was used for paired comparison. If the flooding level of schemei(i=1,2,…,m) was significantly (say 10 cm or more) lower or higher than that of schemej(j=i,i+1,…,m), the comparison coefficients between schemesiandjwereaij=0 or 2, andaji=2 or 0, respectively. Otherwise, schemesiandjwere assumed to be similar with respect to the wetland ecosystem, andaij=aji=1. Then, the comparison matrix based on the 0-to-2 scale could be obtained, and was transformed to the 1-to-9 scale comparison matrix (Xu 1994) to obtain the priority of theith scheme with respect to the wetland ecosystem in thekth sub-area,p3ik(i=1,2,…,m;k=1,2,…,n).

2.5 Criterion and method for riparian ecological water use assessment

The growth of riparian forests in arid regions relies heavily on the groundwater level,which is mainly influenced by the river water level. The groundwater level in the riparian area at different river water levels can be estimated by the groundwater dynamics models and used to assess the impact of water diversion on the riparian ecosystem. The assessment procedure used in this study was similar to that for the wetland ecosystem. The priority of the ith scheme with respect to riparian ecological water use in the kth sub-area, p4ik(i=1,2,…,m; k=1,2,…,n), was obtained.

2.6 Integrated impact assessment of water diversion projects on ecological water uses in each sub-area and whole area

Through impact assessment of each type of ecological water use (as shown in the criterion layer in Fig. 1), the impact of water diversion projects on ecological water uses in each sub-area and the whole area can be assessed using the AHP method. For sub-area k, the priority of the ith scheme, qik(i=1,2,…,m; k=1,2,…,n), can be calculated as

where wjkis the weight of the jth (j=1, 2, 3, 4) ecological water use in sub-areak( k=1,2,…,n), and it can be determined from the relative importance of different ecological water uses by using the AHP method. For the whole study area, the priority of the ith scheme, qi( i=1,2,…,m), can be calculated as

where wkis the weight of sub-areak( k=1,2,…,n), and it can be determined from the importance of sub-areas in regard to river-dependent ecosystems using the AHP method.

3 Study area

The study area is located in northern Xinjiang in the northwest of China. It includes the main stream area of the Irtysh River from section S1 to section S6 and two branch (the Burjin River and the Kelan River) areas of the main stream (Fig. 2). As shown in Fig. 2, section S7 is located downstream the head of the water transfer canal where water is diverted from the Burjin River to the main stream. The study area is an arid region, with a mean annual precipitation of about 150 mm, and a mean annual evaporation of about 1 700 mm from a 20-cm evaporation pan.

The drainage area of the main stream in the study area is 2 076 km2, and the mean annual discharge at section S1 is about 105.5 m3/s. For the Burjin River, the drainage area is 1 554 km2, and the mean annual discharge is about 138.9 m3/s. Runoff from the Kelan River is less than that from the main stream and the Burjin River, but a large area of the wetland has developed along the downstream reach of the Kelan River as a result of the interaction between the main stream and the Kelan River. Therefore, the ecological water use of the wetland is considered part of the ecological water uses in the main stream area. The wholestudy area was divided into two sub-areas, the main stream sub-area and the branch (the Burjin River) sub-area. Ecological water uses in the main stream sub-area include ecological flow, riparian forest water use, wetland water use, and water use for Ulungur Lake, while those in the branch sub-area include ecological flow and riparian forest water use of the Burjin River.

Fig. 2Sketch of study area

The main water diversion projects in the study area include water diversion from the main stream and branches for local irrigation water use, water diversion from the main stream to Ulungur Lake, and inter-basin water transfer from the main stream to neighboring basins and from the Burjin River to the main stream (Fig. 2). Flow regimes of the main stream and branches will change with the increase of water diversion from rivers, affecting ecological water uses in the study area. Therefore, it is urgent that we assess the impact of water diversion projects on ecological water uses. Five schemes were assessed, including a current state scheme (scheme 0) that does not consider inter-basin water transfer, and four planning schemes (schemes 1, 2, 3, and 4), which consider different scales of water diversion and inter-basin water transfer (Table 1).

Table 1Brief description of current state (scheme 0) and four assessed schemes (1 to 4)

4 Results and discussion

4.1 Assessment results of river ecological flow

Considering natural flow of the main stream and the Burjin River, the flow ranges ofdifferent grades for ecological flow were calculated (Table 2). Six representative sections, sections S1 to S6 (Fig. 2), were used to assess the ecological flow regime of the main stream. Section S7 was used to assess the ecological flow regime of the Burjin River. River flows in different sections of the main stream and the Burjin River were obtained from the results of water resources planning according to the water balance method. The corresponding percentages of mean annual flow and flow grades are given in Table 3. With the enhancement of water diversion from the main stream to neighboring basins for local uses (schemes 0 to 1) , river flow in the main stream decreases significantly, especially in the lower river reaches (sections S4 to S6). With the enhancement of water diversion from the Burjin River to the main stream (schemes 2 and 3), the flow regime in the main stream improves significantly, while the flow in the Burjin River decreases moderately. With more water diverted from the main stream and the Burjin River (scheme 4), the flow in the Burjin River and the upper reaches of the main stream decreases further.

Table 2Grades of ecological flow and corresponding flow ranges for study river reaches

Table 3Percentages of mean annual flow and flow grades of main stream and Burjin River

Using the 1-to-9 scale of AHP, the priorities of different schemes were obtained (Table 4). The results show that the current state scheme is superior to the four planning schemes. However, instream flow will decrease unavoidably in the future with the increasing demand of industrial and agricultural water uses and offstream environmental water uses. Besides the current state scheme, schemes 3 and 4 are also acceptable for the main stream, while schemes 1, 2, and 3 are acceptable for the Burjin River.

Table 4Priorities of different schemes with respect to ecological flow

4.2 Assessment results of lake ecological water use

According to water resources planning results, the ecological water requirement of Ulungur Lake can be met with all schemes. In other words, the water diversion projects have no impact on lake ecological water use. Therefore, the five schemes were considered to have the same priority (0.2) with respect to water recharge to the lake.

4.3 Assessment results of wetland ecological water use

The flooding level of the wetland is mainly influenced by flooding of the Kelan River and the main stream, and the flooding depth was estimated to be 1.4 m, 1.3 m, 1.55 m, 1.66 m, and 1.51 m for schemes 0 to 4, respectively. The increase of flooding level in schemes 2, 3, and 4 was mainly caused by water diversion from the Burjin River to the Kelan River. Using the 0-to-2 scale for paired comparison, the priorities of schemes 0 to 4 were determined to be 0.077, 0.077, 0.209, 0.427, and 0.209, respectively. Therefore, scheme 3 is the best, and schemes 2 and 4 are also acceptable as far as the wetland ecosystem is concerned. These results show a positive effect of water diversion to the Kelan River on wetland ecological water use.

4.4 Assessment results of riparian ecological water use

The water requirement of the riparian forest was estimated by the Penman-Monteith method (Allen et al. 1998). The requirement was 50 million m3and 51 million m3for the riparian forest in the main stream area (Shang et al. 2006) and the Burjin River area, respectively. Considering that the distribution area of the riparian forest from sections S1 to S3 is relatively small and the hydrological regimes of sections S1 to S3 are quite similar, section S1 was taken as a representative of a riparian forest from sections S1 to S3. From the flow rates at sections S1, S4, S5, and S6 across the main stream and section S7 across the Burjin River, the river water level was estimated from the water level-flow rate relationships(Table 5).

The change of the groundwater level near the river is close to that of the river water level, while the groundwater level far away from the river is less influenced by the change of the river water level. Therefore, the average changes in the groundwater level in the riparian area were assumed to be half the changes in the river water level. As a result, the impact of water diversion projects on the riparian forest was similar to that on the river flow. The priorities of different schemes were obtained based on the 0-to-2 scale for paired comparison (Table 6). Besides the current state scheme, schemes 3 and 4 are acceptable for the main stream, while schemes 1, 2, and 3 are acceptable for the Burjin River.

Table 5River water levels at different sections for different schemes

Table 6Priorities of different schemes with respect to riparian ecosystem

4.5 Assessment results of two sub-areas and whole study area

For the main stream sub-area, all four types of ecological water uses were considered. Since river flow is the base for river-dependent ecosystems, the ecological flow was considered more important than others. Water uses of wetlands and riparian forests were considered to be of the same importance, but they were both more important than the water uses of Ulungur Lake. Using the 1-to-9 scale for paired comparison, the weights of ecological water uses for river, lake, wetland, and riparian ecosystems were calculated to be 0.424, 0.122, 0.227, and 0.227, respectively. The priorities of different schemes with respect to the four related ecosystems were calculated to be 0.355, 0.099, 0.125, 0.247, and 0.174 for schemes 0 to 4, respectively (Fig. 3). As seen from these results, schemes 3 and 4 are acceptable for themain stream sub-area.

For the Burjin River sub-area, only two types of ecological water uses were considered, the ecological water uses of river and riparian ecosystems. Similar to the main stream sub-area, the weights of ecological water uses for river and riparian ecosystems were calculated to be 0.667 and 0.333, respectively. The priorities of different schemes with respect to related ecosystems were calculated to be 0.317, 0.317, 0.219, 0.102, and 0.045 for schemes 0 to 4, respectively (Fig. 3). Besides the current state scheme, schemes 1, 2, and 3 are also acceptable for the Burjin River sub-area.

Fig. 3Priorities of water diversion schemes for ecological water uses in whole study area and two sub-areas

The ecological water uses in the main stream sub-area are more important than those in the Burjin River sub-area, and the weights for these two sub-areas were calculated to be 0.667 and 0.333, respectively, using the 1-to-9 scale for paired comparison in AHP. As a result, the priorities of water diversion schemes for ecological water uses in the whole study area were calculated to be 0.342, 0.172, 0.156, 0.199, and 0.131 for schemes 0 to 4, respectively (Fig. 3). The results show that scheme 3 is the second best option after the current state scheme, while scheme 4 is the most inferior one for the whole study area due to over-diversion of water from rivers. Scheme 3 is the best among the four planning schemes for four types of ecological water uses in the main stream sub-area, and the second or third best option for two types of ecological water uses in the Burjin River sub-area. Considering the coordination between ecological and economical water uses and the harmonization between the main stream and the Burjin River sub-areas, scheme 3 is recommended to be put into practice, and scheme 4 is not acceptable.

With the recommended scheme 3, the ecological water uses of the influenced ecosystems can be maintained at an acceptable level. For ecological flow, the main stream can be maintained at about grade 2 (grades 1 to 4 for different sections), and the branch at grade 2 as well. This scheme provides the same level of lake ecological water use and the best regime of wetland ecological water use of the four planning schemes. It also provides better water conditions for the riparian forest than scheme 4.

The AHP method can be used to assess the overall impact of water diversion schemes on various types of ecological water uses in different sub-areas and is appropriate in the selection of preferable schemes from the view of ecological impact assessment. However, the assessment results are subjective to a certain extent, because the relationships between river-dependent ecosystems and river flow regimes have not been fully understood and not fully considered in the assessment. Therefore, the mechanism of river-dependent ecosystems in response to hydrological change needs further investigation, so as to improve the objectivity of the AHP assessment.

5 Conclusions

A hierarchy model was proposed to assess the integrated impact of water diversion projects on ecological water uses. The model is composed of four layers describing the goal, sub-area, criterion, and scheme. The assessment criteria and methods for ecological water uses of river, lake, wetland, and riparian ecosystems were proposed, and the AHP method was used for the assessment.

The proposed model was applied to a basin in northern Xinjiang in northwest China, where both water diversion and inter-basin water transfer projects were planned to be carried out. Based on the assessment results of the whole study area and two sub-areas, an appropriate scheme with a moderate amount of water transfer from both the main stream and the branch (scheme 3) was recommended from the four planning schemes. With this scheme, the ecological water uses of the influenced ecosystems can be maintained at an acceptable level, and economical water requirements can be met to a great extent.

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(Edited by Pei WANG)

This work was supported by the National Natural Science Foundation of China (Grant No. 50879041).

*Corresponding author (e-mail: shangsh@tsinghua.edu.cn)

Received Feb. 24, 2012; accepted Jan. 8, 2013