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Chapter 11. Impact of water transfer on the natural environment
Xu Yuexian and Hong Jialian
Institute of Geography, Academia Sinica, Beijing, China
A CHARACTERISTIC of the natural environment is its high degree of spatial variability. Changes in certain key environmental factors under the influence of human activities usually lead to changes in other factors or even in the entire environmental system itself. Since water projects are aimed at regulating and controlling water, the most active factor in the environment, various kinds of environmental variations also emerge at the same time as benefits are gained. Particularly when the present large-scale and extra large-scale water projects are built, the impact on the natural environment will be more complicated and widespread.
The south-to-north water transfer project in eastern China is a gigantic and enormously complex engineering system consisting of two schemes, the East Route and the Middle Route. It is therefore of great significance to study the impact of this engineering system on the natural environment of the regions involved in the proposed water transfer, predict the trend of environmental changes following the transfer of water and explore methods for providing against possible troubles. An attempt is made here to introduce certain important questions and the results of preliminary research conducted thus far.
IMPACTS ON WATER RESOURCES IN THE CHANG JIANG BASIN
The Chang Jiang is the largest river in China. Its drainage basin contains a population of 342 million (1978) and covers 25 x 106 ha of cultivated land, embracing over 40 large and medium-sized cities and industrial or mining bases. Its grain output is about 40 per cent of the national total and it produces about 35 per cent of the country's cotton. The total value of its industrial output amounts to approximately 40 per cent of the national total. The middle and lower reaches, directly affected by the proposed water transfer, are the chief grain and cotton producing areas within the basin and the most developed industrially and commercially.
The Chang Jiang basin holds such an important position in the nation's economy that some people worry about diverting water from there to other river basins. They note that although the maximum annual runoff recorded at Datong Station in the lower reaches of the Chang Jiang was 1,360 km³ in 1954, the minimum was only 675 km³ in 1978. The runoff during the flood season, April to October, accounts for about 80 per cent of the whole year while that of
the dry season, November to March, is only 20 per cent. Over the last thirty years, more than 40,000 reservoirs of all sizes have been built in the Chang Jiang basin with a total capacity exceeding 100 km³. The irrigated area has expanded to 6.7 x 106 ha and industrial and municipal water use is increasing continuously. Water in the Chang Jiang has already tended to decrease. So, transferring water from the Chang Jiang northward is nothing more than "supporting deficiency with deficiency", if we consider future increases in water use and the fact that the Chang Jiang basin is not all that rich in water resources.
According to actual survey data, the average discharge for a number of years at the Datong Hydrological Station on the lower reaches has been 29,200 m³/sec and the average annual runoff has been 920.9 km³, over 35 per cent of the national total. This volume is 19 times that of the Huang He, 17 times that of the Huai He and 41 times that of the Hai He. Although there is a distinctive change in the amount of water in the Chang Jiang between the flood season and the dry season, the difference is small compared with that of the rivers in northern China. Therefore, we feel that since the flow in the Chang Jiang is relatively abundant and stable there will be no marked effect on water resources in the lower reaches if 1,000 m³/sec is diverted through the East Route. This is only 3.4 per cent of the annual average discharge at Datong Station.
On the Middle Route, it is possible to transfer 10.9 km³ of water at the existing scale of the Danjiangkou Reservoir. After further construction the water transferred will average 23.7 km³ and minimum release from the reservoir into the downstream will be reduced by over 100 m³/sec compared with the average dry season discharge prior to the building of the reservoir. This amounts to approximately 1 per cent of the average flow during the dry season at the Hankou Station and 0.8 per cent of that at the Datong Station. The regulating role of the large lakes will ensure that on the whole there will be no effect on the amount of water in the middle and lower reaches of the Chang Jiang. Any impact on shipping and irrigation in the lower reaches of the Han Jiang can be taken care of by rational reservoir scheduling combined with long-range planning of shipping and by the adoption of necessary engineering measures.
To deal with the problems of water supply and requirements in the Chang Jiang basin, an overall plan should be drawn up linking the long-term goals of economic development with the South-to-North Water Transfer Project. In order to ensure water use in the Chang Jiang per se, water diversion may be suitably reduced or suspended during dry years or seasons.
IMPACT ON THE NAVIGATION CHANNEL OF THE CHANG JIANG
The Chang Jiang is the most important inland navigation channel in China. The total volume of goods transported on the main stem and its tributaries constitutes roughly 65 per cent of total national waterborne freight. Along the main stem the 2,800 km between Yibin and the estuary are ice-free and navigable all year round. The conditions of the navigation channel are more complex for the 1,032 km from Yibin to Yichang than for the 1,800 km in the middle and lower reaches from Yichang to Shanghai, with the exception of the stretch in Hubei Province below Yichang and above Hankou where the river meanders and its bed is unstable. It is anticipated that no additional difficulties will be presented to upstream navigation by the proposed water transfer since neither the Middle Route nor the East Route involves the water of the upper reaches. Although the flow in the middle and lower reaches will decline, the amount of reduction will not have any major impact on the navigation channel.
IMPACT OF THE EAST ROUTE ON SEAWATER INTRUSION IN THE CHANG JIANG ESTUARY
Shanghai Municipality and Jiangsu Province are extremely concerned about seawater intrusion. They have organized scientific research units to conduct specialized research on it.
The Chang Jiang estuary is 90 km across at its widest point. It is divided into two branches by Chongming Island. Salt water dominates the north branch in all seasons, dry and flood, because of the drop in freshwater runoff caused by constant siltation of the shallows in the upper outlet. In the south branch, which serves as the main passage to drain the Chang Jiang runoff, saltwater intrusion is not as serious as in the north, but sea water can still back up beyond Wusong during the dry season.
The Huangpu Jiang is the main source of industrial, agricultural and domestic water for Shanghai Municipality. Since it joins the Chang Jiang in the estuary, seawater intrusion during the dry season often leads to an increase in the chloride content and hardness of the water supplied by the water works at Wusong, Zhabei and Yangshupu. In 1978, as a result of a smaller flow than usual into the middle and lower reaches of the Chang Jiang, salt water even entered the Wangyu He in the vicinity of Changshu County, Jiangsu Province, which is 120 km from the estuary. Nearly all the water works along the Huangpu Jiang were also affected.
According to studies carried out by East China Normal University (see Chapter 25) and the Nanjing Institute of Hydraulic Science, there will be no marked increase in salt water in estuarine districts if water is transferred at 1,000 m³/sec through the East Route when the discharge at Datong Station exceeds 16,000 m³/sec. The impact of saltwater intrusion may be aggravated to different degrees if water is diverted in dry periods when the flow at Datong is less than 16,000 m³/sec. The south-to-north water transfer should thoroughly take this problem into account in its project plans and diversion times so as to avoid any deterioration in the quality of water supplied to Shanghai.
IMPACTS OF EAST ROUTE ON AQUATIC LIFE
Water transfer through the East Route will cause a reduction in the discharge into the estuary of the Chang Jiang, narrowing the area flushed by fresh water, allowing the ocean tides to go upstream and increasing the salinity of the estuary. Some of the fish which feed on halophytes may migrate inwards along the estuary and some of the open-sea fish will tend to come closer to the shore. Migratory paths for certain fish such as yellow croakers and hairtail in the offshore fishing grounds will be affected to a certain extent.
The East Route water transfer project will pass through several freshwater lakes, namely the Hongze, Luoma, Dongping and Nansi. The total lake surface is about 5.6 x 106 mu (380,000 ha) with a water depth of 2 to 4 m. The lake waters contain rich nutrient salts and a large amount of plankton. These lakes serve as important freshwater fishery bases and rich growing areas for aquatic plants such as reeds, lotus roots and water chestnuts.
After water is transferred, a high water level will be maintained in the lakes for a longer time. Static or slow moving water bodies will change into those with swift flows. Hence the sediment content will increase and the transparency of the lake water will decrease. These changes in the lake environments will have some impact on aquatic life. According to the research of the Wuhan Institute of Aquatic Biology of the Chinese Academy of Sciences, (see Chapter 27) following the transfer of water along the East Route the amount of algae, aquatic weeds, plankton and the like will decline in lakes along the route and the yield of benthos such as snail and clam will fall. Corresponding decreases will occur in plant-eating fish such as grass carp and bream as well as fish like carp and crucian carp which spawn in aquatic weeds.
The Shandong Institute of Aquatic Products expects that aquatic resources will increase rather than decrease. This is because the water surface areas of lakes and depressions will be enlarged after water diversion.
QUALITY OF TRANSPORTED WATER
The natural quality of water in the south-to-north water transfer regions is relatively good. The mineralization of the Chang Jiang water is about 0.2 g/1, increasing gradually northward from the Chang Jiang until it reaches about 500 g/1 in the Hai He basin. The contents of the principal pollutants in water bodies along most sections of the water transport routes have not yet exceeded the surface water standards set by the government.
On the other hand, water has been polluted to differing degrees in some areas due to the discharge of industrial waste water and solids. Furthermore, pollution will become more serious with the further development of industry and agriculture. For instance, the phenol, cyanogen and mercury content near the intake of the East Route and in Hongze and Nansi Lakes have already exceeded the surface water standards. Hence the proposed South-to-North Water Transfer Project should take into account the protection of water sources. Industrial and mining enterprises along the route should control the discharge of waste materials so as to prevent the south-to-north transfer of polluted water.
NORTHWARD MIGRATION OF SCHISTOSOMIASIS
Before the founding of the People's Republic of China, schistosomiasis was epidemic in ten provinces and municipalities in the middle and lower reaches of the Chang Jiang. Over 7 million people were afflicted. Thirty years have elapsed and the number of those afflicted has dropped by 70 per cent thanks to the preventive control measures taken by the government at various levels to exterminate snails. At present the northernmost point of snail distribution is in Baoying County, Jiangsu Province (33° 15' N). Waterborne organisms can be carried to remote areas by flowing water and certain pathogenic bacteria can also spread to other places along with the currents of water. A question that has universally aroused interest is whether schistosomiasis would migrate northward with the transferred water.
Since certain hydrometeorological conditions are essential to the existence of the snails which are the intermediate host of schistosomiasis, it is hard for them to survive in the winter cold of the north. The Jiangsu Institute for Research on the Prevention and Cure of Schistosomiasis has carried out preliminary research on the conditions for snail survival north of Baoying by combining on-the-spot investigations with laboratory experiments. Over 90 per cent of the snails die within thirty days at a constant temperature of -2° C. No snails were ever found moving north through the Grand Canal that linked
the south with the north in the past, providing additional proof that the climate in the north is unfavourable for the survival of the snails. According to this analysis, the northward migration of schistosomiasis after a south-to-north water transfer is not too likely.
IMPACTS ON CLIMATE
This topic has so far not been adequately studied. Thus, only preliminary conjectures can be made based on certain experimental data and analytical results.
Generally speaking, soils in irrigated fields are moist with high thermal capacity. Evaporation and air humidity increase and the changes between day and night in soil temperature and near-surface air temperature are relatively moderate. According to experimental data obtained in the western surburbs of Beijing, the maximum surface temperature of irrigated winter wheat fields is 8° to 9° C lower than that of unirrigated fields during April and May and the minimum temperature is 1.0° to 2.5° C higher. The daily range of surface temperature in the irrigated fields is 14° C and in the unirrigated fields it is 23.7° C. After winter wheat land is irrigated, the relative humidity of the air at 13.00 hours is 20 per cent, 13 per cent, 12 per cent and 2 per cent higher at 20 cm, 50 cm, 100 cm and 150 cm respectively above the ground surface than that of the unirrigated land. The air temperature of the irrigated fields at 20 cm above the surface is about 4° C lower than that of the unirrigated land with a daily range of 16.7° C compared with 21.5° C for the latter. The daily temperature range at 150 cm above the ground surface tends to be identical for irrigated and unirrigated fields, however: 16.1° C and 16.0° C respectively. The South-to-North Water Transfer Project would add or improve a total of 9.4 x 106 ha in irrigated land. It is estimated that the air humidity and temperature will be modulated in the irrigated areas, with a marked impact on the meso-and microclimates.
Both the water surface and the area under irrigation will expand greatly after a transfer of water northward. This will certainly cause an increase in total evaporation over the water-using regions. Preliminary estimates are that the average annual evaporation will increase by 20 to 30 per cent (or 16 x 109 m³). The increase in evaporation will in turn accelerate the process of water circulation which will induce changes in other climatic factors. Some scholars feel that precipitation in the dry season (such as the month of May) will increase by 2 to 4 per cent (see Chapter 24).
SALINIZATION OF IRRIGATED AREAS
This subject has aroused heated arguments in discussions of the water transfer projects. Some scholars maintain that salinization in irrigated areas will beyond doubt be aggravated as a result of water diversion. Their view is based on the fact that the irrigated land in some other countries has suffered from extensive swampiness and salinization and on the many complications and setbacks which China has encountered during the past thirty years of improving saline soil. They point out that the control of drought, flooding and salinization should be considered in a comprehensive manner in the project programme.
Others note that transferring water for irrigation is by no means in contradiction with saline soil improvement given the numerous successful experiences in this area both in China and abroad. They suggest that since transferring water from south to north is urgently needed for the development of agriculture in north China, we cannot "try to save a little only to lose a lot" or "give up eating for fear of choking".
More people maintain that soil salinization is a key issue which will determine the success or failure of the water transfer. We must therefore conduct intensive research to seek out ways of solving this problem.
The land to be irrigated by the northward water transfer may be divided from west to east according to soil geochemical processes (Chen Jinsheng, 1962): (1) a piedmont alluvial plain zone in which the depth of groundwater exceeds 5 m and mineralization ranges from 0.5 g/1 to 1.0 g/1. The principal soil geochemical process is leaching and there is no problem of salinization; (2) a flovial-alluvial plain zone formed alluvially by rivers such as the Huang He and the Hai He. Here the terrain is level but the meso- and microtopography is crisscrossed with ridges and depressions. The groundwater is 2 to 4 m deep and in some areas it is closer than 2 m. Its mineralization increases from 1 to 2 g/ 1 up to 5 g/1. Accumulation is the main soil geochemical process with extensive distribution of inland saline soils; and (3) a coastal plain zone located in narrow coastal belts 30 to 40 km wide with an elevation less than 10 m above sea level. The groundwater is no more than 1.5 m deep and has a mineralization over 10 g/ 1. Since its soil geochemical processes are affected by sea water, this zone consists in the main of coastal saline soil.
Over the past thirty years the area of the saline soil in the proposed water transfer region has fluctuated constantly. In the three provinces of Hebei, Henan and Shandong there was a total of 1.9 x 106 ha of saline soil in the mid-1950s. This area expanded to 2.3 x 106 ha in the early 1960s due to the diversion of the Huang He for irrigation and to water storage on the plain. The saline area dropped to 1.4 x 106 ha by the mid-1970s, however, because of improvements in the standard of drainage with the harnessing of the Hai He and other river basins and the lowering of the water table due to the development of pump well irrigation; but the tendency to expand seems to have returned in the late 1970s. At the end of the 1970s the area was about 1.9 x 106 ha. This corresponds to the rise in the water table along both sides of certain rivers associated with the construction of water-impounding dams. At present there is a total of 2.7 x 106 ha of saline soil, amounting to about 15 per cent of the total arable land from the Shaying He system of the Huai He basin in the south to Beijing and Tianjin in the north. Moreover there is an additional 4.7 x 106 ha of potential saline soil which is most vulnerable to secondary salinization if affected by detrimental factors.
Although the Huang-Huai-Hai Plain has good heat conditions and abundant land resources, its grain yields are low and unstable because of salinization and other natural disasters. An investigation of 287 counties revealed that the average annual grain yield over the three-year period from 1976 to 1978 was below 2.25 t/ha in 173 of them, 60 percent of the total. In 71 counties (25 per cent) the average was between 2.25 and 3.0 t/ha. In only 43 counties (15 per cent) was the average over 3.0 t/ha.
The area to be irrigated along the East Route and most of that along the Middle Route is situated in the fluvial-alluvial plain zone. The original water table is relatively high with rather serious salinization. With the present level of management and technical conditions, the import of a large amount of water from outside will inevitably cause the water table to rise and the area of saline soil will expand correspondingly. Furthermore, places along both sides of the main conveyance routes and the different levels of distributaries, areas where subsurface runoff is retarded, the surroundings of water storage projects and areas where well irrigation has been converted to canal irrigation will be the first to be affected.
Two kinds of measures should be adopted as steps to preventing secondary salinization. The first one is to raise the level of existing irrigation management, strictly control the volume and times of irrigation, improve' irrigation systems and methods and as much as possible prevent the water table of the farmland from rising greatly. The second is to propagate energetically the effective experiences of experimental areas and plots and adopt suitable technical measures such as building drainage systems; constructing projects which combine irrigation and drainage; combining canal irrigation and well irrigation; afforestation; increasing soil fertility; and reducing the effect of lateral seepage as much as possible by constructing necessary seepage prevention and interception works along both sides of canals and around impoundments. By doing all this, the adverse effects of secondary salinization can be mitigated. Therefore, in transferring water northward we should consider whether or not the level of management and technical conditions are well developed in the areas to be irrigated. We must move cautiously in carrying out this project.
Reference
Chen Jingsheng, 1962, "The Landscape of the North China Plain and its Geochemical Characteristics", Acta geographica sinica, No. 3.