Ecohydrological responses on water diversion in the lower reaches of the Tarim River, China

Transcription

1 WATER RESOURCES RESEARCH, VOL. 44, W08422, doi: /2007wr006186, 2008 Ecohydrological responses on water diversion in the lower reaches of the Tarim River, China Hui Tao, 1,2 Marco Gemmer, 3 Yudong Song, 4 and Tong Jiang 1,3 Received 19 May 2007; revised 21 January 2008; accepted 10 March 2008; published 14 August [1] During the past 30 years, water ceased to flow in the lower reaches of the Tarim River in northwest China. A project was initiated that aims for ecosystem recovery and rehabilitation by means of transporting water through an open canal to the lower reaches of the Tarim River. In this study, the ecohydrological responses of this rare type project are assessed. Water loss-runoff relationships and an index model for water loss rates and runoff are analyzed. The detected ecohydrological responses of the canal project include that water diversion dominates the dynamics of (1) the groundwater depth and (2) the tempo-spatial variation of riparian vegetation close to the water channel. The relationships between groundwater depth, vegetation coverage, species richness and soil water content are clearly the main factors contributing to the riparian vegetation. Variations of water mineralization are significant, both temporally and spatially, at each sampling station within the lower reaches of the Tarim River. The study provides basic information on water diversion and stream corridor restoration in the lower reaches of the Tarim River. The results show associated mechanisms between riparian vegetation and hydrological variation in arid zone. This lays the theoretical and practical foundation for improving the evaluation system for supplementary water delivery and comprehensive improvement in the Tarim Basin. It also provides information on strengths and weakness in current practices. These are needed for the planning of ecological recovery and rehabilitation of damaged ecosystems in this and other arid areas in western and northern China. Citation: Tao, H., M. Gemmer, Y. Song, and T. Jiang (2008), Ecohydrological responses on water diversion in the lower reaches of the Tarim River, China, Water Resour. Res., 44, W08422, doi: /2007wr Introduction [2] Water is scarce in many regions of the world. Demand exceeds supply in many areas even in countries with an overall abundance of water [William and Jorgensen, 2003; Matete and Hassan, 2005]. Water is a determinant of biodiversity, ecological patterns and ecological processes [Wang and Cheng, 1998; Thoms and Sheldon, 2002; Palmer and Allan, 2006; Palmer and Bernhardt, 2006]. As water supplies are frequently fully allocated, they often create a conflict of interests between the needs of riparian ecosystems (in-stream, riparian, and floodplain areas) and the domestic, agricultural, and industrial water demand [Thoms and Sheldon, 2000]. In recent years, the study on the ecohydrological mechanism has become the forefront and focus of ecoenvironmental research. The systematic and intentional transfer of water from one point to another by open canal has become common practice. The application of ecology and engineering has become a method for ecosystem rehabilitation in the western region of China and other areas 1 State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography and Limnology, CAS, Nanjing, China. 2 Graduate School of the Chinese, Academy of Sciences, Beijing, China. 3 Center of Climate Change, China Meteorological Administration, Beijing, China. 4 Xinjiang Institute of Ecology and Geography, CAS, Urumqi, China. Copyright 2008 by the American Geophysical Union /08/2007WR W08422 of the world in order to solve a class of environmental problems [Shields et al., 2003; Mitsch and Jørgensen, 2003; Zhang et al., 2005a, 2005b; Bernhardt et al., 2005, 2007; Hou et al., 2006; Storomberg et al., 2007]. However, most of these projects are carried out at microhabitat scale and generally focus on site-specific habitat-improvement rather than on restoration. Information on restoration are difficult to obtain since government agencies involved in the river restoration may differ based on the restoration clients, techniques applied, and permits required [Moerke and Lambert, 2004]. [3] Supported by the Central Chinese Government, the Ministry of Water Resources decided to invest 10.7 billion RMB (about 100 million Euro) to revive the ecological system of Tarim River. This is supposed to be achieved by transporting fresh water from the upper and middle reaches of Tarim River through the cannel to meet the ecological water demands of the river s lower reaches. [4] As the water diversion project proceeds, Chinese governments at all levels and the international community are paying high attention to the ecological responses. Therefore it is a pressing task to assess the ecohydrological responses after water is transported to the drying-up (no-flow) river sections in the lower reaches of the Tarim River. Key questions are whether (1) the water diversion produces net beneficial or adverse ecological effects and (2) how to implement the water diversions more effectively. [5] This study aims at understanding hydro-ecological linkages between the water and riparian systems through 1of10

2 W08422 TAO ET AL.: ECOHYDROLOGICAL RESPONSES ON WATER TRAN W08422 Figure 1. Sketch Map of the Tarim River Basin. analyzing field data and spatial modeling of the water discharges of the lower reaches of Tarim River. Finally, recommendations for the optimal diversion scheme are made. Indications such as whether or not this form of large-scale water diversion would be beneficial or detrimental to riparian ecosystems in other arid regions in China are provided. 2. Methodology 2.1. Study Area [6] The Tarim River Basin is located in the Northwest of China, known internationally for its abundant natural resources and fragile ecosystems [Tang and Qu, 1992]. The basin is a relatively flat desert region with a mean annual precipitation below 50 mm and m 3 runoff per year. Water resources are principally provided for by high mountain precipitation and seasonal snow and glacier melting which feed the Tarim River through its tributaries. Situated in the southern part of Xinjiang Autonomous Prefecture, the main catchment of the Tarim River is 1321 km long and covers an area of km 2 (35 43 N, E) (Figure 1). It is one of the world s largest closed hydrological drainage systems without outflow. The land and solar resources are perfect for agriculture if enough water is available. [7] The study focuses on the lower reaches of the Tarim River between the Taklamakan and Kuruke deserts. This area is known as the green corridor which protects the station road along the river. [8] The problems of drying-up and ecosystem deterioration coexist in most arid river basins that were subject to short-term benefit development in the world. They have long been a concern for both the public and governments at various levels. In hydrological terms, the Tarim River basin represents a closed hydrological drainage systems linked to the Hotan, Yarkand, and Akesu Rivers. However, since the 1960s, large-scale anthropogenic activities such as flooding irrigation and random reclamation in the upper and middle reaches of the Tarim River have triggered the disintegration of the natural hydrology. At the same time, a warmer climate leading to higher evapotranspiration has occurred. A strong drying-up of the lower region can be observed. After the Daxihaizi reservoir was built in 1972, the streamflow of the 321 km long lower river section completely dried-up [Song et al., 1999]. This drying-up seriously deteriorated the basin s downstream ecosystems. Desertification and salinization expanded Ecological Water Diversion [9] Starting in May 2000, the Management Bureau of the Tarim River Basin started implementing the seventh ecological water diversion (Table 1). The water was intermittently transferred from Bosten Lake to Daxihaizi reservoir, and finally to the dried-up riverbed of downstream Tarim River. [10] The first five water diversions played an important role for ecology recovery in that area. The total volume of water transported from Bosten Lake to Daxihaizi reservoir was m 3, and a volume of m 3 was transported from Daxihaizi reservoir to the lower reaches of Tarim River (Table 1). The first water diversion consisting of m 3 reached the vicinity of the Kardayi section; the second one of m 3 flew 146 km downstream to the Alagan section. The third, fourth and fifth water diversions led water to the terminus of the Tarim River which is the Taitema Lake. This abandoned the desiccation history in the lower reaches of the river over the last 30 years. 2of10

3 W08422 TAO ET AL.: ECOHYDROLOGICAL RESPONSES ON WATER TRAN W08422 Table 1. Transported Water Volume to the Lower Reaches of the Tarim River Water Diversion Period Conveyed Water Volume, 10 8 m 3 Distance to Taitema, km Water Surface Area of Taitema, km 2 First diversion (14 May 2000 to 13 Jul 2000) Second diversion (3 Nov 2000 to 14 Feb 2001) Third (I) diversion (1 Apr 2001 to 6 Jul 2001) Third (II) diversion (12 Sep 2001 to 17 Nov 2001) Fourth diversion (20 Jul 2002 to 10 Nov 2002) Fifth (I) diversion (1 May 2003 to 6 Jul 2003) (double channel) 30 Fifth (II) diversion (4 Aug 2003 to 3 Nov 2003) (double channel) 200 Sixth diversion (3 Apr 2004 to 11 Jul 2004) (double channel) Seventh diversion (20 May 2005 to 30 Sep 2005) (double channel) Total [11] After the third water diversion in November 2001, the water surface area in the Lake Taitema basin was 4.5 km 2 which was increased to 16.7 m 2 following the fourth water diversion in November By the end of 2003, after the second phase of the fifth diversion was completed, the total water surface area in the Taitema Lake expanded to 200 km 2. The main purpose of the two latest diversion programs was to maintain the achievement of the former five water diversions Monitoring of Water Diversion [12] Nine groundwater-monitoring sections (including about 40 monitoring wells) were established along the river courses conveying water to the lower reaches of Tarim River in order to monitor the subsurface water depths and sample subsurface water. They are located in the sections of Akdun (A), Yahepu (B), Yinsu(C), Abudayi (D), Kardayi (E), Tugmailai (F), Alagan (G), Yiganbujima (H), and Kaogan (I) downstream of the Daxihaizi Reservoir. The distances between them are about 20 km for the first six sections and 45 km for the last three sections (Figure 2). The monitored data at each experimental section include discharge, groundwater depth and groundwater mineralization degree, soil water content, salt content and the growth increment of Populus euphratica at different distances from the river course before and after each water diversion. The samples of Populus euphratica from different sections were collected by saw after the six water diversions. The samples were returned to the laboratory, where they were mounted and finely sanded to allow cross dating and measurement of the widths of the rings that grew in recent years. Additionally, the loss rate per unit of river section for the previous water discharge as well as the vegetation plots between different sections were monitored. 3. Observations 3.1. Water Diversion Loss in Main Channel [13] Most water projects suffer from losses in diversion. The losses mainly consist of operation and seepage into the soil from the canal bed and slopes [Chakravorty et al., 1995; Kahlown an Kemper, 2004]. The efficient use of water resources is of vital importance in increasing deteriorated ecology especially in arid region. For the restoration purposes, the most efficient system is clearly achieving highest flows throughout the entire river length without surplus water accumulating and evaporating in Taitema Lake. However, the water from the Daxihaizi reservoir was mostly consumed by the aquifer recharge downstream, except for a small portion that was consumed by surface evaporation of water in the river course and lake. In this study, an inflow and outflow method was used which shows the loss occurring during the first five water diversions in the open canal (Figure 3). [14] The water that was lost between the Daxihaizi reservoir and the Alagan section was m 3. This accounts for 76.2% of the total volume. The river course between the Alagan section and Taitema Lake consumed m 3, accounting for 19.6% of the total volume while the water injected to the Taitema Lake accounted for just 4.2% of the total volume. The spatial distribution of the transported water is therefore heterogeneous. [15] From the implementation of the first five diversions, with the increase of flow velocity (Q), the water loss per unit of channel length (d) all descended rapidly and stabilized in the main section (Figure 4). [16] According to this field data, an index model with the water loss rate and flow velocity of the intermittent diversions to the dried-out river channels in the lower reaches of Tarim River was established [Deng, 2005]. On the basis of this model and the conditions of vegetation and water system, the results of possible diversion schemes can were predicted and analyzed. The model shows that it is reasonable to adopt the double line style water diversion method. Within this method, water is transported from Daxihaizi reservoir to the lower reaches of the Tarim River through both the new river canal and the old Tarim River. This enlarges the restored area Dynamic Variation of Groundwater Depth and Response of the Riparian Vegetation After Water Diversions [17] Water diversions have implications for groundwater depth and vice versa. The monthly profile of the water depth in the lower reaches of the Tarim River is relatively constant before the water diversion [Hou et al., 2006]. The transverse change in groundwater depth is an important criterion for measuring the ecological benefits of the water diversions. The transverse response of the groundwater depth increased in fluctuation with the increase of diversion times. The responses along the river course decreased from the upper stream to the lower stream [Chen et al., 2006]. These responses indicate that the diversions in the lower reaches of the Tarim River play a very important role in raising the groundwater depth close to the river course. [18] Because of the arid climate in the Tarim River basin, the water demand of the natural vegetation is mainly fed from groundwater. Therefore native plants can only survive 3of10

4 W08422 TAO ET AL.: ECOHYDROLOGICAL RESPONSES ON WATER TRAN W08422 Figure 2. The sketch of monitor sections in the lower reaches of the Tarim River. 4of10

5 W08422 TAO ET AL.: ECOHYDROLOGICAL RESPONSES ON WATER TRAN W08422 Figure 3. Water consumption of the river course between the main sections in the lower reaches of the Tarim River. if the groundwater depth is maintained at an appropriate depth. Figure 5 shows the transversal variation of groundwater depth in the different distances (150 m (C3), 300 m (C4), 500 m (C5), 750 m (C6), and 1050 m (C7)) from the water canal at the Yinsu section (section C) after the fifth water diversion. The groundwater depth of the monitoring well near the water channel responds more spontaneous than that of the well further away from the water channel. In general, the groundwater depth increases with the distance from the river course in most arid regions. However, there is a distinct rising of groundwater depth in all monitoring wells after the fourth diversion. [19] The variation of groundwater depth is distinct in our study area due to (1) the differences thickness of clay layers and sand layers in the soil profiles along the transverse sections, (2) different intervals (or duration) of water diversion, and (3) different volumes of water transported. For example, the durations of the second, fourth and the second phase of the fifth diversions were more 100 d. The intervals of the five diversions were 4, 3, 2, 8, and 1 month respectively. Thus the groundwater depth could have been affected almost simultaneously by the different water diversions. Therefore the relationship between the rising groundwater depth and the distance from the river is more variable rather than linear. [20] The analysis of the results from the nine sections along the river course shows that, longitudinally, the level of groundwater depth is reduced with increasing distance from the Akdun section (section A). Although the groundwater depths are all higher following the water diversion scheme, the elevation of groundwater depth reduces gradually the further downstream of the river. This could be as the aquifer layer is saturated at certain depth and further water discharge cannot be handled before the groundwater depth falls again. However, has not necessarily been fully restored. Therefore it is very important to determine the optimal ecological groundwater depth and promote the efficiency of water diversions. [21] Riparian vegetation is an integral component of river systems and is vital for maintaining a number of key ecosystem services. The floodplains of the channel become Figure 4. Relationship between runoff and its average loss rate per unit of river sections for all previous water conveying. 5of10

6 W08422 TAO ET AL.: ECOHYDROLOGICAL RESPONSES ON WATER TRAN W08422 Figure 5. Transversal variation of groundwater depth at different distances from the water channel. drier and more saline because of the hydrological connectivity of the lower reaches of Tarim River altered by the Daxihaizi reservoir over the past half century. The structure and composition of the riparian communities in the lower reaches of the Tarim River were simple and included a constructive species (Populus euphratica), some woody shrubs (Tamarix hispida, T. ramosissima, Halimodendron halodendron) and herbs (Phragmites communis, Glyzyrrhiza inflata, Apocynum venetum, Alhagi sparsifolia and Karelinia caspica, along with Sophora alopecuroides, Scorzonera divaricata, Cynanchum sibiricum, Aeluropus litoralis, Oxtropis glabra and Inula ammophila) [Zhang et al., 2004]. The Populus euphratica forest and its plant communities from Akdun in the middle region to the lower reaches of the Tarim River were severely degraded due to the previous loss of river flow to recharge the groundwater as well as the ecosystems around Taitema Lake [Zhang et al., 2005a, 2005b]. [22] Among the diversity indices, Margalef is regarded as a good index for reflecting the change of species richness [Ma, 1994]: M = (s 1)/lnN, where N is the total number of individual of all species, and S is the total number of species. In this study, we calculated the Margalef index of vegetation plots in the middle section of the Tarim River (from Yinsu to Yiganbujima), which is a typical waterreceiving area with bare land before the water diversion [Zhao et al., 2006]. [ 23] The exponential distribution model (y = e x ) where y is the index of Margalef and x is the distance from the water channel present a relationship between the species richness and the distance from the water channel (Figure 6). 6of10 [24] The natural vegetation dominated by Populus euphratica and Tamarix chinensis in the lower reaches of the Tarim River relies mainly on groundwater and soil water for its existence. Precipitation is low and evaporation is high. Thus shallow groundwater depth directly influences the growth of the vegetation. The change in the landscapes since the start of the water diversions to the lower reaches of the Tarim River is compared and analyzed by using Landsat ETM data in August 1999 and August Figure 6. Variation of species richness in the middle section of the lower reaches of the Tarim River.

7 W08422 TAO ET AL.: ECOHYDROLOGICAL RESPONSES ON WATER TRAN W08422 Figure 7. Branches growth increment in the different distance from the river for Yinsu section (A) and Keardayi section (B). [25] The results shows that a reverse of desertification has occurred in the lower reaches of the Tarim River after the diversions [Sun et al., 2004; Chen et al., 2004]. Since the dominant species in the communities of the lower reaches are Populus euphratica and Tamarix chinensis, in this study the variation in tree rings (the growth increment) of Populus euphratica at the Yinsu and Kaerdayi sections are sampled. With this analyze, the quantitative response of the natural vegetation to the water diversions can be determined. The growth increment is an important index to measure the ecological rehabilitation after the water diversions. [26] From Figure 7 we can see that all the inflexion of growth increments of Populus euphratica start in the year 2000 transversely across the river. The increment of growth is greater when being closer to the river course. At the Yinsu section, the average growth increments are 0.18 cm in 2000, cm in 2001, cm in 2002, and 0.34 cm in These growth increments are all larger than the growth increments at Kaerdayi section (described by Deng [2005]) (Figure 7(B)) for the same years and same distance from the river. Therefore longitudinally, the growth increment of Populus euphratica in the upper reaches is bigger than the one in the lower reaches measured at distance from the river channel. In addition, the growth increment of Populus euphratica in 50 m distance at Yinsu section in 2001 and in 220 m distance at Karerdayi section in 2002 are obviously higher than the others. This is mainly due to the duration of water diversions and the volume of the runoff. [27] Compared to the dynamic variation in groundwater depth, the response of natural vegetation may be small. This is because of (1) the slowness of vegetation to response to the dynamic variation of groundwater depth and (2) the different drought resistance and groundwater depth necessary for the growth of different species as determined by the different physiological and ecological characters of those different species. For example, in the areas with low groundwater depth, herbs were sparse, with almost no other herbs than Athagi pseudathagi that has long and penetrating roots. There is a third reason for small response in natural vegetation. Although the crown radius of Populus euphratica responds to the water diversion, there is very little seed germination due to the shortage of surface soil moisture. Populus euphratica reproduces mostly through rhizomes and suckers, and there are only a few young plants established from seeds on both sides of the river [Zhao et al., 2006]. Thus there was almost no variation in spatial distribution pattern and quantitative characteristic of Populus euphratica. [28] To summarize the relationship between riparian vegetation and ecohydrological factors, Pearson s correlation matrices [Swan and Sandilands, 1995] were used to detect the relationships between riparian vegetation and ecohydrological factors (groundwater depth, soil water content, mineralization degree). The data were collected from the monitoring well at each section during the six water diversion in Table 2 show strong correlations Table 2. Correlation Coefficient of Riparian Vegetation and Ecohydrological Factors Vegetation Coverage, % Groundwater Depth, m Species Richness Transverse Distance, m Soil Water Content, % Mineralization Degree, g/l Vegetation coverage, % 1.00 Groundwater depth, m 0.79 a 1.00 Species richness 0.75 a 0.56 a 1.00 Transverse distance, m Soil water content, % 0.75 a 0.96 a 0.55 a Mineralization degree, g/l a Correlation is significant at the 0.01level (2-tailed). 7of10

8 W08422 TAO ET AL.: ECOHYDROLOGICAL RESPONSES ON WATER TRAN W08422 Table 3. Change in Surface Water Mineralization Before and After the Water Diversions Location Mineralization Degree of Surface Water, g/l Monthly Average ( ) Sample Water (2003) ph Value of the Sample Water in 2003 Kala section Daxihaizi reservoir River course of Yinsu River course of Alagan River course of Kaogan between groundwater depth, vegetation coverage, species richness and soil water content. These relationships clearly identify the main factors contributing to the riparian vegetation, in other words, it may be possible to revive the vegetation through the water diversion in areas that are not too badly degraded Response of Water Quality and Land Desertification [29] Water quality is an important indicator of environmental change [Feng et al., 2005] and an important factor determining the distribution of vegetation in arid zones [Glenn and Nagler, 2005]. In this paper, the change of total dissolved solid (TDS) and the major ions (SO 2 4,Cl,Na, Ca 2+, Mg 2, and HCO 3 ) between the upper and lower reaches of the Tarim River were analyzed. The results show that TDS and the concentration of the major ions initially and quickly increased and then decreased, but finally increased again [Chen et al., 2005]. Because of the drying-up since the 1970s, the degree of water mineralization at Alagan (G) and Kaogan (I) is high. At Kaogan it reached g/l. According to the monitoring data in 2003, the degree of water mineralization apparently decreased after the water diversions. Table 3 summarizes the change of water mineralization traits before and after the water diversions. The mineralization of the water in the Daxihaizi reservoir is much lower than at other locations downstream. However, the degree of water mineralization at Kaogan section decreased about 22.8 times due to the water diversion. At the Kala section, the mineralization only decreased 2.68 times. The amplitude of the decrease in water mineralization increased with the distance from Daxihaizi reservoir. However, the same conclusion could not be drawn from the variations between stations in ph value of the sampled water as only the 2003 values were available. [30] Hydrological changes and the degradation of vegetation increased desertification in the lower reaches of the Tarim River before the implementation of the water diversion scheme. At the end of 1999, desert land occupied more than 90% of the total area of the lower reaches of the Tarim River. The area of serious desertification had increased by an equivalent of 52.7% (Table 4). In the region of the Yinsu and Abudayi sections, 45.7% and 50% respectively of their total cultivated land had been abandoned. [31] With the rising groundwater depth and the decreased degree of groundwater mineralization some dying vegetation and main community-building species such as Populus euphratica and Tamarix chineses have started to regenerate by the end of the fifth water diversion. This reversed the deterioration of the land in the lower reaches of the Tarim River. Because of the severely degraded vegetation and the long-term desiccation in this seriously desertified region, the reversion of desertification in a short period of time seems very difficult and even impossible. 4. Discussion and Conclusions [32] In arid zones, the groundwater depth is the most important and sensitive indicator of ecosystem response to riparian community restoration. The first five water diversions in the lower reaches of the Tarim River produced a significant response in groundwater depth, water quality and vegetation coverage. This effectively stopped the deterioration of the ecoenvironment in the lower reaches of the Tarim River. This study shows an effective way to determine the ecological benefit of water discharges, using groundwatermonitoring wells and vegetation plots as main indicators. This can improve riparian restoration ecology efforts in many parts of the arid zone in China s northwest. According to our analysis, we can conclude. [33] 1. The water that was discharged from the Daxihaizi reservoir recharged the groundwater mostly between Daxihaizi reservoir and the Alagan section. We can calculate reasonably well the volume of water delivery by the model in order to enhance the efficiency of water diversions in the future. [34] 2. The discharges of water into the lower reaches of the Tarim River play an important role in raising the groundwater depth and decreasing the mineralization of groundwater near therivercourse.however,theelevationofgroundwaterdepth reduced gradually from upstream to downstream areas in the lower reaches of the Tarim River, and with the increase of diversion times, the response of groundwater depth also decreased gradually further down the river course. [35] 3. The species richness of vegetation gradually decreased along the groundwater depth gradient because of the dynamic variations of groundwater in the lower reaches of the Tarim River. However, the scope for response by natural vegetation is smaller than the rate of groundwater recharge. The buffer belt in the lower reaches of the Tarim River is highly circumscribed. The degraded Populus euphratica (the dominant species in the middle section of the lower reaches of the Tarim River) is sensitive to the water diversions, but there was almost no variation in spatial distribution pattern and quantitative characteristic of Populus euphratica. Because of different time of water diversions and volumes of runoff, the growth increments are Table 4. Area (10 4 ha) of Land Desertification in the Regions of the Lower Reaches of the Tarim River a Sections Qwenkuer River Old Tarim River Daxihaizi-Akedun (A) Akedun (A)-Yahebumaha (B) Yahebumaha (B)-Yinsu (C) Yinsu (C)-Abudale (D) Abudale (D)-Keardayi (E) Keardayi (E)-Tug ai (F) Tug ai (F)-Alagan (G) Alagan (G)-Yiganbujima (H) Yiganbujima (H)-Kaogan (I) Kaogan (I)-Taitema lake (J) a The monitored data are from the Ecology conservation planning and design bulletin in the lower reaches of the Tarim River, of10

9 W08422 TAO ET AL.: ECOHYDROLOGICAL RESPONSES ON WATER TRAN W08422 different at the different sections and distances from the river course. Therefore the government should deliver the water at a reasonable time and in a suitable volume, to coincide with the period when the Populus euphratica needs the water most during vegetation period. [36] 4. It is feasible to rehabilitate the green corridor through continuous water diversion although the existing water diversions program has not restored the entire riparian system yet. The first three intermittent water diversions were all performed in a line style method through the natural river course from 2000 to They were timely and necessary for the recovery of the natural vegetation along the river course and for stopping environmental degradation. However, the scope is limited since most of water recharges before it arrives at the lowest reaches of the river. If we want to enlarge the water-receiving area of the water diversions, it may be necessary to implement a river dredging project below the Daxihaizi Reservoir. Benefits would be gained from combining the line style method with the space style drainage. The benefit of this combination was indicated during the fourth and fifth diversion when the double-channel style diversion was performed. Furthermore, according to the physiological characteristics of Populus euphratica to seeds germination, it is worth to mention that the volume of water diversions is more crucial to the population of Populus euphratica although the volume and time of water diversion should be in accordance with the demand for germination and maturity time of Populus euphratica seeds. [37] As the restoration of the Tarim River Basin gathers momentum, the regeneration after water diversions will become a hot topic among domestic and foreign scientists. Successful riparian restoration depends not only on understanding the physical and biological processes that influence ecosystems at the watershed scale, but also in the proximity of the restoration effort to the sources of disturbance [Mitsch and Jorgensen, 2003]. At present, the ecological recovery of the lower reaches of the Tarim River mainly relies on Kaidu River, which is the main headstream of the ecological water diversions. Although water diversions alleviated the degradation of the Green Corridor, the problem has not been solved. It is feasible in the short-term to utilize the wet season in order to transport water down the Tarim River. In turn, a crisis will emerge if the Kaidu river catchment experiences a dry period during a long period of implementation. The droughts has resulted in a careful review of contingent transfers because of the water requirements conflict between the Kaidu river basin and the green corridor in the lower region of the Tarim river basin. Therefore stakeholders must consider climate variability for the planning of restoration activities. They should adopt an integrated approach for water allocation at the river basin depth, rather than depending on a sole tributary contribution. [38] Acknowledgments. This work was supported by the National Natural Science Foundation of China (NSFC , , ). Special Fund for Climate Change Research of CMA (LCS ). The authors are grateful to the Management Bureau of the Tarim River Basin for providing river discharge and water mineralization data. References Bernhardt, E. S., et al. (2005), Synthesizing U.S. river restoration efforts, Science, 308, Bernhardt, E. S., et al. (2007), Restoring rivers one reach at a time: Results from a survey of U.S. river restoration practitioners, Restoring Ecol., 15(3), Chakravorty, U., E. Hochman, and D. Zilberman (1995), A spatial Model of optimal water conveyance, J. Environ. Econ. Manage., 29, Chen, Y. N., X. L. Zhang, X. M. Zhu, W. H. Li, Y. M. 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