|Long Distance Water Transfer: A Chinese Case Study and International Experiences (UNU, 1983)|
Institute of Hydrology
University of Bayreuth, Bayreuth, Germany
THIS STUDY was undertaken without any field work. Therefore, it should be considered as preliminary and may only provide some indications of what environmental impacts could occur when a substantial water transfer from the Chang Jiang to the North China Plain is undertaken.
Without thorough knowledge of the hydraulic engineering measures and the irrigation scheme, it is impossible to make detailed statements on the environmental impacts of the proposed interbasin water transfer projects. These statements are based primarily on general hydrological investigations especially by Henning (1968) and Greer (1979).
Important general research on interregional water transfer was published by Golubev and Biswas (1979). The papers of Biswas (1979), Fisher (1979), and Golubev (1979) provide valuable advice about adverse impacts of long-distance water transfers on the environment.
The river basins of the Huang He and the Chang Jiang are both under the climatic influence of the East-Asian monsoon. Therefore, there exists a distinct annual runoff pattern with maximum discharge in summer and minimum discharge in winter, with a definite dry season in the greatest part of the two river basins. The climatic aridity grows according to the major atmospheric flow pattern from south to north.
The river basin of the Chang Jiang is more than double the size of the Huang He. Whereas the Huang He flows through arid and semi-arid lands with a mean annual rainfall of no more than N = 500 mm a-1, the river basin of the Chang Jiang receives a mean annual rainfall of about N = 1,000 mm a-1. These facts result in a different mean annual runoff in the two river basins with Q = 35 x 106 m³ s -1 in the Chang Jiang and Q = 1,360 m³ s -1 in the Huang He, which means an annual runoff of A= 517 mm in the Chang Jiang basin and A = 55mm in the Huang He basin. The evapotranspiration in the Chang Jiang basin amounts to about V = 483 mm, and in the Huang He basin it amounts to V= 445 mm, which is an indicator of the aridity of that river basin.
IMPACT OF WITHDRAWAL OF WATER ON THE AQUATIC ENVIRONMENT
Considering the question of possible deleterious effects to the Chang Jiang basin if part of its water is transferred northward, one may argue there may be none because of the high mean runoff of MQ = 35 x 103 m³ s-1from which even a high total consumptive use of Q= 600 m³ s-1 could be diverted. But even if a diversion of this amount of water is technically possible, there are some serious questions to be answered concerning the deterioration of the aquatic ecosystems.
At a ten-year minimum water flow of NNQ10 = 3,500 m³ s -1 and a NNQ = 1,900 m³ s-1 at Ichang one should compute the overall effect of the project on:
(1) the tidal waters and the intrusion of brackish and salt water further up
the river; and
(2) the decrease of the waste assimilative capacity at low water if part of the water is diverted. This may adversely affect the biological life in the downstream reaches.
The computation may be done by the following two-dimensional flow model and a water quality model. The basis of the hydrodynamic computation is formed by the following equations:
(i) Equation of motion, x-direction:
(ii) Equation of motion, y-direction:
(iii) Equation of continuity:
(iv) Equation of transport and chemical reaction:
The equations may be solved numerically by a finite difference method. In these equations the basic unknown quantities are:
|qx||: flow in the x-coordinate direction|
|qy||: flow in the y-coordinate direction|
|h||: tidal amplitude|
|d||: water depth|
|g||: acceleration of gravity|
|?||: Coriolis parameter|
|Sex||: energy slope in x-direction|
|Sey||: energy slope in y-direction|
|(the influence of the river runoff may be accounted|
|for by the energy slope)|
|Xw||: wind stress in x-direction|
|Yw||: wind stress in y-direction|
|e||: evaporation rate|
|c||: concentration of a constituent|
|a||: velocity in x-direction|
|v||: velocity in y-direction|
|Ex, Ey||: instantaneous dispersion coefficients|
|k||: first order chemical decay coefficient|
Even if there is only a maximum silt concentration of 0.56 per cent, and a mean silt concentration of 0.1 per cent at Zhijiang (Henning, 1968) which is less than 50 per cent of the silt concentration of the Huang He, a substantial diversion of Chang Jiang water may perhaps result in some silting of the river course and the intake structures of the canal.
There will certainly be some environmental impacts on the lower Chang Jiang and the estuary by the planned transfer of water to the North China Plain. Furthermore, it will be very important to study the impacts on the shelf waters. There may be an influence by reducing the amount of fresh water. Besides, a higher concentration of pollutants may adversely affect the shelf waters' ecosystems.
As the mean discharge of the Chang Jiang is about 35 x 103 m³ s-1 and the diversion would be in the order of 600 m³ s -1, the transfer is only 1.7 per cent. Taking into account this comparison, the withdrawal is rather small and no considerable impact should be expected. Yet, the transfer deserves a more precise investigation at low water when there may be a diversion of up to 30 per cent of the Chang Jiang river flow.
IMPACT OF WATER TRANSFER ON NEIGHBOURING TERRESTRIAL AND FRESHWATER ECOSYSTEMS
Both the Eastern and Middle Routes will have distinct environmental impacts. As the Eastern Route would follow the Grand Canal through or along various lakes near the eastern coast, the groundwater level would rise as would the level of the lakes by percolation of the diverted water from the channel and possibly by percolation of irrigation water from irrigation schemes which may be developed along both sides of the channel.
Rising of the groundwater table may be accompanied in the northern dry part of the course of the channel by salinization of the soils. This phenomenon will be discussed later.
The increase in lake levels may be followed by a change in the nearshore terrestrial ecosystems. Furthermore, agricultural lands may be inundated by lake water. During the construction phase of the channel, environmental impacts will result from soil erosion and disturbance of natural drainage. In some areas there will be interference with the water table by which the groundwater will be open to the air and groundwater resources downstream may be contaminated by pollutants. Moreover, during the construction phase, surface waters may be polluted and heavily silted. There will be destruction of wildlife habitats, parks, recreation areas and historic sites.
During the operation of the channel some of the environmental impacts of the construction phase will disappear, but others (e.g. groundwater contamination) will continue. Because of the high silt content and the low slope of the Eastern Route, the channel may be badly silted. The overall effect of the silt on the channel is not known, but is likely to be significant and adverse. Consequently dredging and spoil disposal may be necessary, which could have severe environmental impacts. Indirect impacts are also likely, such as contiguous land use.
In contrast, the influx of river water with a high content of nutrients and silt into the lakes may have beneficial effects like turbidity reduction, oxidation of organic material and coliform reduction, but there may also be detrimental effects like algae blooms, siltation, build-up of inorganic substances and lower reaeration. The most important impact on the biogeochemistry of the lakes will be a decrease of dissolved oxygen in the hypolimnion because of the differentiation of the reaeration and oxygen production in the epilimnion and the consumption of oxygen in the hypolimnion by the oxidation of organic materials originating in the epilimnion. Out of this, anaerobic conditions in the hypolimnion will follow. Because of the change of the redox milieu there will be a change of the oxidation number of Fe and Mn resulting in dissolution of precipitated Fe and Mn from the sediment into the water. By this co-precipitation, poisonous heavy metals will be freed and will have adverse effects on the aquatic habitats. Similarly there may be a reflux of PO4 from the sediments which will speed up the eutrophication of the lakes.
The Middle Route would link at least one greater reservoir to the Chang Jiang. Further, this section would require great earth- and stoneworks. The reservoir would be subject to silting up and an adequate design of the spillway and the silt outlet, as well as the channelling of silty water, could prolong the service of the reservoir endangered by sedimentation.
Because of the huge earth- and stoneworks necessary along the Middle Route throughout the construction phase, there will be major environmental impacts like displacement of people, noise, soil erosion and disturbance of natural drainage, water pollution, and so on.
The construction of the channel provides clear benefits to the areas neighbouring it, but there are important impacts of the canal on hydrological, biotic and human health features. The increase in both water-borne and water-based diseases due to major water developments should be considered carefully (Biswas, 1979). Incidences of such diseases should be kept to a minimum by appropriate countermeasures. Furthermore, interbasin transfer routes and reservoir sitings must take into consideration earthquake-prone areas (Biswas, 19?9).
ENVIRONMENTAL IMPACTS OF IRRIGATION DEVELOPMENT
The additional water which would be brought to the North China Plain demands the planning of large irrigation works. The potential benefit of such diversion is great in the unpredictable environment of the North China Plain. The increase of evapotranspiration may have some effects on the microclimate of the irrigated area, but no deleterious effects to the climate of the North China Plain are to be expected. Indiscriminate irrigation without drainage, combined with poor soil conditions, will cause elevation of the underground water table, especially in the poorly drained coastal areas. This will result in soil salinization.
In a modelling process for assessing water quality problems in irrigated agriculture, Skogerboe, Walker and Evans (1979) present a planning framework for arriving at cost-effective solutions to water pollution problems resulting from subsurface irrigation return flows. Similar solutions may be found for solving saline water pollution problems resulting from surface return flows.
The environmental impacts of large irrigation works are manifold. The most important are: (i) The combination of seepage and deep percolation losses may cause a rise of the groundwater table. This means severe waterlogging; (ii) Because of the aridity of the North China Plain a soil water potential gradient will develop in such a way that water will move upwards resulting in salinization of the root-zone soil horizons and the upper soils. This process of salinization will cause severe deterioration of agriculture, and expensive relief measures must be undertaken. The balance of sufficient irrigation and leaching of salts out of the soil zone must be reached by a drainage system; (iii) The return water of this drainage system of the irrigation works may contribute to the increase in salinity of the rivers and-even worse-of the groundwater. Besides, the return water may contain dissolved nutrients depending on the absorption characteristics of the soils, the intensity of fertilization and the type of irrigation. This will result in severe eutrophication of the receiving waters. Furthermore, pesticides and fecal pollution may adversely affect the receiving water which then may not be usable as drinking water. The return water has different quality, depending on whether underground or surface irrigation is applied. Since phosphates, for example, are strongly adsorbed on humic material and clay minerals and thus are not leached, only surface irrigation is likely to result in a higher phosphate concentration in the tail water.
As the loose-packed loess of the North China Plain is very easily eroded, erosion control, therefore, seems to be unavoidable. As the erosion is also very much dependent on the soil stability of the saline soils, the colloidal dispersion of clay minerals and the clay humic material complexes may be stabilized by a supply of CaSO4(~10 meq 1-1). By this measure also the salinity can be reduced. Furthermore, check dams in the gullies, field terraces, and improvement of cultivation methods have to be implemented, together with the planting of trees and provision of shrub and grass cover in non-agricultural areas to stop the silting of canals and river courses by erosion.
OPTIMIZATION TECHNIQUES IN INTERREGIONAL WATER TRANSFER
So far as the environmental impacts of large interregional water transfer is considered, no intensive modelling with validation by actual data has been done. A helpful tool for the authorities when analysing regional environmental impact (impact caused by interregional water transfer) may be some sort of economic optimization technique. The components of the computational framework may include models of the cause-effect relationships between different hydraulic engineering measures and their environmental impacts, and models of the relationship between environmental goals and the minimum cost of accomplishing them as optimization models (Gass and Sisson, 1975; Biswas, 1976, 1981; Bishop and Grenney, 1976; Simonsen, 1979).
Bishop, A. and Grenney, W., 1976, "Coupled Optimization-Simulation Water Quality Model," Journal of Environmental Engineering Division, American Society of Civil Engineers, Vol. 102, No. EE5, pp. 1071-1086.
Biswas, A. K., 1981, "Models for Water Quality Management," McGraw-Hill, New York.
Biswas, A. K., 1979, "North American Water Transfers: an Overview", In: Interregional Water Transfers, Golubev, G. and Biswas, A. K., (Editors), Pergamon Press, Oxford, pp. 79-90.
Biswas, A. K., 1976, "Systems Approach to Water Management", McGraw-Hill, New York.
Fisher, A., 1979, "Some theoretical and measurement issues in economic assessment of interbasin water transfers," In: "Interregional Water Transfers, " Golubev, G. and Biswas, A. K. (Editors), Pergamon Press, Oxford, pp. 137-146.
Gass, S. I. and Sisson, R. L., 1975, "A Guide to Models in Governmental Planning and Operations," Sanger, Potomac.
Golubev, G. 1979, "Environmental issues of large interregional water projects," In: "Interiegional Water Transfers", Golubev, G. and Biswas, A. K., (Editors), Pergamon Press, Oxford, pp. 177-186.
Golubev, G. and Biswas, A. K., 1979, "Interregional Water Transfers," Pergamon Press, Oxford.
Greer, C., 1979, "Water management in the Yellow River Basin of China," University of Texas Press, Austin.
Henning, I., 1968, "Hwang Ho und Yangtze Kiang. Ein Beitrag zur Potamologie," In: 1. Bericht der IGU-Commission on the International Hydrological Decade: Flussregime und Wasserhousholt. Geogr. Institut, Selvstverlag, Freiburg, pp.87180.
Simonsen, J., 1979, "Regional Water Quality Management," In: State of the Art in Ecological Modelling. Jorgensen, S. E. (Editor), Pergamon Press, Oxford, pp. 443 -450.
Skogerboe, G. V., Walker, W. R. and Evans, R. G., 1979, "Modelling process for assessing water quality problems and developing appropriate solutions in irrigated agriculture," In State of the Art in Ecological Modelling, Jorgensen, S. E. (Editor), Pergamon Press, Oxford, pp. 269-298.