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4: Water resources and agricultural environment in arid regions of China

Introduction
Water resources
Water utilization and agricultural environment
Improving water management for sustainable agricultural development
Conclusions
References

Wang Tao and Wu Wei

Introduction

In arid regions, water is one of the most challenging current and future natural-resources issues. For a sustainable agriculture and, hence, a healthy economy, water is the key to success. The importance of water in arid regions is self-evident indeed.

The arid regions occupy a vast area in north-western China that mainly includes the Alxa Plateau in the western part of Inner Mongolia, the northern part of Ningxia Hui Autonomous Region, most of Qinhai and Gansu provinces and the Xinjiang Uygur Autonomous Region - about 2.5 million km² or one-quarter of the Chinese territory. In these regions, mean annual rainfall is less than 250 mm, and even less in the western plains (50-150 mm) and the Taklimakan Desert (less than 25 mm). The annual evaporation is more than 1,400 mm in general, and about 2,000-3,000 mm in desert areas. Because of the arid climate, about 70 per cent of the total arid regions are unusable areas such as sandy deserts, gravel deserts, and other wildernesses. Although there are enough wasteland, light, and heat resources, the local economy depends only on irrigated agriculture and animal husbandry because of the limited water supply.

Water is not only the most precious natural resource in arid regions but also the most important environmental factor of the ecosystem. Human impact on the water supply will certainly cause a chain reaction within the ecosystem. Since ancient times, water utilization has always had a decisive impact on local socioeconomic development. But the increased intensity of human activities and overuse or misuse of water resources caused, and quickly spread, agroenvironmental degradation, including salinization, vegetation degeneration, and sandy desertification. Water resources are already under great pressure to support agricultural production in the arid regions and will face a much more difficult situation in the future. Therefore, understanding the relationships between water and environment, and water and development, and recognizing how to practice sound water management, are crucial subjects to study for sustainable agriculture and a stable environment in the arid regions.

Water resources

In the arid regions of China, the water resources are present as rainfall, glaciers, surface water, and groundwater. Rainfall is the basic supply source for all kinds of water resources. Its variation in time and space controls the water conditions and glacial development, as well as directly influencing the formation and distribution of surface water and groundwater. Additionally, there are frequent transformations and interactions between surface water and groundwater.

Rainfall Resources

Rainfall varies from place to place in the arid regions of China (fig. 1). Most parts of the plains receive less than 100 mm annually, but some plains, like the Yinchuan Plain, the eastern part of the Hexi Corridor Region in Gansu, and the northern area of Xinjiang, receive from 100 to 250 mm. The extremely arid centres in China, as shown in table 1, obtain no more than 25 mm yearly. However, the mountain areas share much more rainfall during some periods. For instance, there are 500 mm or even 1,000 mm of rainfall in the western part of the Tianshan Mountains, and about 400 x 108 m³ surface run-off are formed and come down to the plains. In the Qilian Mountains of Gansu, 350400 mm can be expected, which results in 70 x 108 m³ of water being supplied to the Hexi Corridor Region every year.

According to the isohyets, the annual precipitation in China's arid regions (including mountain areas) is estimated at over 5,000 x 108 m³, which converts into an average rainfall of 175 mm and which constitutes the only reliable guarantee for subsistence and development in these regions.

Glacial Resources

Glaciers and permanent snow are special water resources preserved in the mountains of the arid regions, and they play an important role in regulating run-off. The glaciers cover a wide area of about 26,000 km² from the Qilian Mountains in the east to the Tianshan Mountains in the west, and from the Altayshan Mountains in the north to the Kulunshan Mountains in the south, with water reserves of 29,000 x 108 m³ (Qu 1986) supplying 230 x 108 m³ of water to the run-off annually (Gao and Shi 1992). The glacial run-off percentage of the surface run-off is 20.8 per cent in Xinjiang, 12.0 per cent in the Hexi

Figure 1 Precipitation Isogram of the Arid Regions of China

Table 1 The Centres Extreme Aridity in the Arid Regions of China

a. Year recorded, in parentheses.

Table 2 River Run-off Resources in the Arid Regions of China

Source: Gao and Sui (1992).

Corridor Region, and 15.6 per cent in Qinghai Province. So it can be said that the glacial resources serve not only to store water in a certain quantity but also to improve the stability of the water supply and the efficiency of water utilization in the regions.

River Run-off-Resources

The rainfall in the mountains and melt water from the glaciers are the major supply sources to the surface run-off in the regions and can be used as water resources once they have been transformed to surface run-off and have flowed into the plains and basins. In other words, the usable water resources in the arid regions are the surface water and the groundwater in the plains. Based on the average annual runoff volume flowing through the mountain passes to the plains and basins, the amount of surface run-off was estimated at about 1,400 x 108 m³ (Gao and Shi 1992) in the arid regions of China (table 2).

The natural river flow in the regions provides high-quality fresh water that can meet any purpose of water use. The degree of mineralization generally ranges from 0.1-0.3 g/litre at the river heads in the mountains to 0.1-0.5 g/litre at the mountain passes. Table 3 shows the degree of mineralization in some rivers in Xinjiang in 1982 (Tarim River in 1983). The degree of mineralization obviously increases to 1.0-5.0 g/litre if the water is used for irrigation in the plains and permeates the ground, especially in the low reaches of rivers like the Tarim, Heihe, and Ulungurhe rivers. At present, although in some cities waste water is discharged directly into the nearby rivers and lakes and causes water quality pollution in varying degrees, industrial pollution is not yet serious in the regions

Table 3 Annual Changes of Degree of Mineralization in Some Rivers in Xinjiang

Source: CAS (1989).

Groundwater Resources

Groundwater is a very important component of the water resources, and an indispensable form of movement, transformation, and utilization of water in the regions. Because of the arid climate conditions, only a very small part of the groundwater is supplied as rainfall and the largest portion stems from the permeation of surface water. When the rivers come down to the plains from the mountains, a great quantity of water seeps through the ground to become groundwater, and the groundwater spills over as springs in lower-lying areas. Such "seeping-spilling" forms the basic pattern of the water cycle between surface and groundwater in the arid regions. For example, in Xinjiang, about 185 x 108 m³ of river water seeps into the ground, and 60 x 108 m³ overflows to the surface again every year. Sometimes, the cycle seems to repeat itself in some places.

The groundwater resources are widely dispersed in the Piedmont plains, basins' fluvial plains, and desert areas. In the four biggest Piedmont plains, the annual natural supply of water to groundwater is about 316 x 108 m³ (table 4) and 60-90 per cent of that is transformed from surface water (Gao and Shi 1992).

The groundwater in lake basins and fluvial plains is provided mostly by underground flow and permeation of surface water. In the eight largest areas there are about 33 x 108 m³.. Since those areas are located in the lower reaches of the water supply and are seriously affected by human activities, some problems, such as the lowering of the groundwater level, the contraction of lake basins, and the exhaustion of groundwater supplies, have recently become more and more severe.

There are other kinds of water resources, such as soil water and phreatic water. Soil water depends on the water exchanges between rainfall, surface run-off, and groundwater in the soil, which are so complex that it is difficult to make a quantitative evaluation. The phreatic water can be found in deserts. Since most of the interior basins in the arid regions are occupied by deserts more than 694,000 km² and it is impossible for surface and groundwater from the outside to enter, rainfall is the main source for phreatic water here, which is estimated at about 50 x 108 m³ each year.

Table 4 Groundwater Resources in the Piedmont Plains of the Arid Regions of China

Source: Gao and Sui (1992).

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