Chapter 28. Effect of diverting water from south to north on the ecosystem of the Huang-Huai-Hai plain

Zhu Shouquan, Wang Zunqin and Hseung Yi
Institute of Soil Science, Academia Sinica, Nanjing, China

THE HUANG-HUAl-HAl PLAIN, an important agricultural region, is the largest alluvial plain in China. The interconnected disasters of drought, flooding and salinization which have plagued this region throughout history have rendered its unique ecosystem so complex and unstable as to seriously affect the development of agriculture.

The proposed south-to-north water transfer is a major engineering measure aimed at solving the problems of industrial and agricultural water use in the HuangHuai-Hai Plain. If this vast project is not done properly, it will disrupt the existing ecological balance and cause the ecosystem to deteriorate. It is therefore imperative to study with great care the possible effects of the various water transfer proposals on the ecosystem of the plain in the predesign and design stages of the project.

CHARACTERISTICS OF THE HUANG-HUAI-HAI PLAIN ECOSYSTEM

The Huang-Huai-Hai Plain has been formed by the repeated and interconnected inundation and deposition of the water systems of the Huang, Huai and Hai rivers. The relief is flat but the meso- and microtopography undulates, with rises, slopes and depressions, causing poor drainage of surface and subsurface water. In addition, because the upper reaches of the Huang and Hai river systems flow through loess, their waters have a very high silt content which settles in the gentler plains channels. The consequent elevation of the river beds above the surrounding land surface has intensified lateral seepage and caused the water table to rise.

Climatically, the region is semi-humid, but because of the Pacific monsoon, annual precipitation is unevenly distributed. Summer is blazing hot and rainy, with rainfall concentrated in July and August storms. Spring is windy with little rain, so evaporation is very intense. With these natural conditions, the plain often suffers from spring drought, summer flooding and salinization (Hseung and Xi, 1965; Institute of Soil Sciences, 1978).

Quite often, insufficient moisture is available for spring sowing, and sometimes throughout the entire wheat-growing period. The resulting intense accumulation of salts in the soil presents some difficulties to spring sowing and the normal growth of summer-harvested crops. It is very difficult under these circumstances to gain high yields without irrigation.

Yields of fall crops are low and unstable, with no harvest at all guaranteed in the depressions. The light and heat conditions of this region are suitable for two crops a year or three crops in two years. But present water-related conditions permit many areas to grow only one crop a year (rice alternating with wheat) or three crops in two years (with low and unstable yields).

Water and soil are the predominant elements in the ecological environment of the plain. Water is the more active of the two. Drought, flooding and salinization are all closely related to water and are exhibited through the soil. The regulation and management of water is the key to the control of these natural hazards. If there is too little water in the soil there is drought, while too much leads to waterlogging. Salinity in the soil follows the movement of water; for example, evaporation leaves an accumulation of salt on the surface. Water also acts as a solvent, reactant and transporter of soil components and thereby plays an important part in the processes of soil formation, plant growth and salinization (Kovda, 1977; White, 1978).

It is difficult to change the climate and relief of the Huang-Huai-Hai Plain, but water and soil conditions can be improved. Experience has shown that water is the key element in the ecological environment of the Huang-Huai-Hai Plain. A series of comprehensive measures combining irrigation and drainage and making joint use of wells and canals have had pronounced effects in preventing drought, flooding and salinization. Soil fertility has also been raised in this way and the ecosystem has been improved. All this has built a good base for further improvement.

In the late 1950s, however, water was diverted blindly from the Huang He for irrigation in order to solve the drought problem. Drainage systems were not provided. A large number of reservoirs were built in the plain, blocking the natural drainage channels. The canals carried water for long periods, resulting in considerable seepage which raised the water table. Secondary salinization and swamping of the soil occurred extensively. This lesson has driven home to us that the ecosystem of the plain is unstable. Once the ecological balance is disrupted, it is difficult to return to the original conditions in a short time.

At present, the ecological balance obtained by well irrigation and drainage is extremely fragile. If we are just a little imprudent, all that has been achieved will be spoiled.

Environmental improvement must begin with the adjustment and regulation of the water-salt movement and balance. Improved drainage is the essential first step, making it possible to defend against floods and flooding and to irrigate without causing salinization. It is especially necessary to drain off the excess seepage in canal diversions. Even where irrigation and drainage are both done with wells or where well and canal irrigation are combined, level and open drains are needed to prevent flooding and soil waterlogging. In an engineering system, therefore, the diversion, storage, irrigation, drainage and management of water should be regarded as a single entity, beginning with the design and installation of a drainage system prior to irrigation. Unfortunately, this is rarely done in practice and therefore the soil-plant ecology has not been improved.

EFFECT ON THE ECOSYSTEM OF DIVERTING WATER FROM THE SOUTH TO THE NORTH

A key question of the proposed south-to-north transfer is whether it will aggravate the secondary salinization of the soil which is a threat north of the Huang He and in coastal areas (White, 1978). The project should therefore stress the utilization and management of water in the region to the north of the Huang He.

Either diversion route will certainly increase the water input of the Huang-HuaiHai Plain, fundamentally altering the present water-salt balance. Preliminary plans are for the East Route to deliver approximately 15 km³ per year into the area north of the Huang He (see Yao and Chen, Chapter 9). If the utilization rate of the canal system is 0.4 to 0.5 and the irrigated area is about 2 to 2.6x 10⁶ ha, then an additional 2,250 to 3,750 m³/ha will be used for irrigation districts which rely on wells or the joint use of wells and canals, this can serve to replenish the water sources. Given the present condition of tertiary irrigation and drainage projects and the current level of management, however, gravity-flow irrigation districts with poor natural drainage conditions could be replenish the water sources. Given the present condition of tertiary irrigation and drainage projects and the current level of management, however, gravity flow irrigation districts with poor natural drainage conditions could be confronted with a rising water table and an aggravated threat of soil salinization and marshification.

The proposed East Route would yield relatively great benefits in developing irrigated agriculture and improving the ecological environment of northern Jiangsu and its coastal area, but the main canal would greatly interfere with the existing drainage systems and impede the eastwards flow of natural drainage. The conveyance of water for long periods of time in the main and subsidiary canals would lead to a large amount of seepage into the groundwater, especially in stretches where the level of transported water is high. This would gradually form high water table zones on both sides of the canals. Like an underground dam, these zones would block the drainage of underground runoff and might lead to salinization along the canals.

This is almost a general rule for irrigation canal systems without antiseepage facilities. For example, in 1960 the irrigation district of the People's Victory Canal increased the annual diversion from the Huang He from 177 to 358 days and the amount of water from 405 to 1,096 x106 m, the water table rose greatly and salinization occurred in soils thoughout the irrigated area, causing agricultural production to fall precipitously. In another case, when the Communist Canal carried 200 m³/sec for 60 days, the surrounding water table rose by 0.8 to 1.0 m, salinizing soils within 2 km.

The effect of water storage on the ecology is also important. If Hongze and Nansi lakes south of the Huang He are used as regulation reservoirs, then the elevation of the original storage level of the lakes or the lengthening of the period of high storage will have a definite effect on flood control and the drainage of excess surface water in upstream areas north of the Huai He and in southwest Shandong Province. The area north of the Huang He would rely mainly on plains and river networks for water storage. In general their water levels would be higher than the land surface. This would certainly change the runoff regimes of the surface and underground runoff and would raise the water table in adjacent areas. This could lead to widespread secondary salinization of the soil in some areas, even within the irrigated areas.

Topographical conditions are highly favourable along the proposed Middle Route. Because it is located higher, the trunk canal would not interfere greatly with the existing irrigation and drainage systems and water could be allocated to the irrigation districts entirely by gravity. The area under control would be larger and the stretches that the main canal would pass through are mostly irrigated by wells or jointly by wells and canals. The water table is relatively deep there, the natural runoff flows unimpeded and there is little danger of secondary salinization or swamping of the soil along most of the route. If the watercourse, irrigation and drainage projects are brought about and their management perfected, this project will improve the ecosystem of the Hai He plain, and especially benefit agricultural and industrial development in the piedmont plain. However, after the water is transferred, the cropping system in the irrigated areas will be changed and the area of rice cultivation will be increased. At the same time, it is possible that canal irrigation will partly or entirely supplant well irrigation. This may bring about a deterioration in the regional water-salt regime and disrupt the ecological balance in the area where well irrigation and drainage are now carried out. It may even induce a renewed salinization of improved land. On balance, however, the Middle Route would appear to be more beneficial to the environment and the control of drought, flooding and salinization than the East Route (Hseung, 1979).

Whichever route is adopted, including the diversion of water from the Huang He, the amount of water diverted and the extent of its irrigation coverage will be limited. The imported water can only serve as a supplementary source. Local water resources of the area should still be used to the fullest extent possible. Practice has shown that well irrigation and drainage is effective for the control of drought, flooding and salinization, and the integration of well and canal irrigation still seems to be the proper course for development in the Huang-Huai-Hai Plain even after the diversion of water from the south.

The comprehensive control of drought, flooding and salinization must be considered in a unified manner so as to improve the ecology of the plain and make the project fully effective. Consequently, we propose the following:

(1) That further research be done on the characteristics of the ecosystem of the Huang-Huai-Hai Plain and an ecological regionalization be prepared concentrating on the comprehensive control of the water-related disasters in keeping with the local conditions.
(2) That the diversion of water from the south involves not only the planning and implementation of the main conveyance canal project but also good planning of the areas to be irrigated from the angle of ecological balance, including drainage and the joint use of canals and wells. The solution to the contradictions between diversion, storage, drainage and irrigation must be based on taking care of drainage first and providing complete irrigation and drainage systems.
(3) That a scientific management system be worked out for the conveyance and supply of transferred water. This is necessary for so vast a project and includes the adoption of advanced technology for managing the irrigated area, the establishment of effective management organization and the formulation of rational economic policies.
(4) That more intensive scientific research be carried out on the ecological impact of the proposed water diversion in order to provide a reliable scientific basis for making the project as effective as possible.

References

Hseung Yi, 1979, "The Prevention of Soil Salinization in the Huang-Huai-Hai Plain should be taken into account in the proposed diversion of water from the south to the north", Soils, No. 4, pp. 121-123.

Hseung Yi and Xi Changfan, 1965, "Soils of the North China Plain", Science Press, Beijing.

Institute of Soil Sciences, Chinese Academy of Sciences, Nanjing (Ed.), 1978, "Soils of China", Science Press, Beijing, pp. 79-110.

Kovda, V. A., 1977, "Arid Land Irrigation and Soil Fertility: Problems of Salinity, Alkalinity, Compaction". In E. B. Worthington (Ed.), Arid Land Irrigation in Developing Countries: Environmental Problems and Effects. Pergamon Press, Oxford, pp. 211-236.

White, Gilbert F. (Ed.), 1978, "Environmental Effects of Arid Land Irrigation in Developing Countries", MAB Technical Note No. 8, UNESCO, Paris.