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Water utilization and agricultural environment
In the arid regions of China, water utilization has a long history because of irrigation agriculture. Since the Han Dynasty (206 B.C. 220 A.D.), the regions have been opened up on a large scale. The people have accumulated rich experience and achieved phenomenal success in the development, utilization, and protection of water resources. A very good example is an ancient water conservation measure used in Xinjiang, the karez well, an irrigation system of wells connected by underground channels. This system can draw water automatically into the fields, just like artesian springs. There were more than 1,;'00 channels of the karez well, with an overall length of 5,000 km (3-4 km on average and 30 km the longest) in Xinjiang in the 1950s. According to 1985 statistics (CAS 1989), there were still 1,016 channels used to distribute 4 x 108 m³ of water to irrigate 20,000 ha of farm land (table 5).
Before the 1950s, there were just a few water-conservation facilities in the regions, and the total irrigated land area was only about 1.3 million ha in 1949. Since the 1950s, the construction of water-conservation facilities has achieved quite good results. Excepting the two biggest reservoirs of Liujiaxia and Longyangxia along the Yellow River, there are 1,168 reservoirs of different types with a storage capacity of 77 x 108 m³. Among them are 195 large and middle-sized reservoirs with a storage capacity of 67 x 108 m³.. Many different installations have been built, including 4,300 projects for diverting water automatically, 1,300 engineering facilities for pumping water, 75,700 power-driven wells, and 250,000 km channels on different scales (Gao and Shi 1992). Those installations can effectively irrigate 4.5 million ha of farm land, 127,000 ha of range land and 429,000 ha of orchards and gardens. Table 6 shows the situation of water utilization in the arid regions of China in the 1980s
Table 5 Distribution and Flow Capacity of Karez Wells in Xinjiang in 1985
|County||Channel||Flow capacity (108 m³)|
|Yiwu and Barkol||15||0.19|
Source: CAS (1989).
In the arid regions, the decisive factor in the ecosystem is water, which will directly affect the environment by the changes in its quantity, quality, and regional distribution. The reclamation and utilization of the water resources in the arid regions played a key role in the development of society and the economy. Certainly, the impact of human activities on water management has improved the environment to be favorable for agricultural development on a large scale, especially thanks to the construction of reservoirs and of irrigation and drainage systems. Several dreams have come true, such as expanding the agricultural areas of the old oases, exploiting the wasteland, and increasing the artificial woodland and range land. Those changes have brought about a great advance in agricultural production. But the management of water resources is still the most important task for sustainable development in the arid regions, not only because the promotion of economic prosperity is limited by water scarcity but also because water management is involved in exploiting other natural resources and protecting the environment. In view of the laws governing water movement, transformation, and circulation, and the role of water in the arid ecosystem and in sustainable agriculture, there have been many harmful effects on the agricultural environment from poor water management, which can be summed up as follows.
Shortened Rivers, Shrunken or Dried Lakes and Degenerated Water Quality
Every continental river basin in the arid regions is a unit composed of surface water and groundwater forming an independent water-resources system and an integrated ecosystem. Given the limitation of water resources, if the channels and water storage were increased excessively in the upper reaches this would cause not only a decrease in the water supply, a river shortened in many cases, and the deterioration of water quality in the lower reaches, but also an imbalance in the ecosystem, degradation of the environment, and destruction of other resources.
Table 6 Water Utilization in the Arid Regions of China
|Province||Agricultural irrigation(108 m³)||Range-land irrigation(108 m³)||Industrial water use(108 m³)||City use(108 m³)||Countryside use(108 m³)||Surface water use(108 m³)||Groundwatera use(108 m³)|
|Hexi Corridor Region, Gansu||63.81||3.95||2.01||0.09||0.69||48.5||24.10|
Source: Gao and Sui (1992).
a. Includes water diverted from springs.
b. Up to the Hekou hydrometric station, Lanzhou.
Unfortunately, many rivers, such as the Tarim, Keriya, Hotan, Yarkant, Konqi, Shule, Heihe, and Shiyan, in the arid regions are facing such problems. For example, the Tarim River valleys converge to a river system originating from the Kulun and Tianshan Mountains. There used to be enough run-off so that Lake Taitema could survive for a long time at the end of the river. But, during the last five years, owing to a sharp increase in the water consumed for agriculture in the upper reaches, the water supply to the lower reaches has decreased constantly, as table 7 shows. The artificial Daixihaizi Reservoir has become "the end of the lake." Each decade, the lower reaches received less and less sluice water from the reservoir. From table 7 it can easily be seen that, during the last three decades, the run-off volume has shown only small variations compared with the average volume of 49.2 x 108 m³ at the Aral Hydrometric Station, which represents the volume of water supply contributed to the upper reaches of the Tarim River by its tributaries, but has decreased station by station from the upper to the lower reaches until only about one-quarter of its original volume of 1957-1960 remains at Qara Station in 1986.
Even worse was the fact that more than 300 km of river bed and all of Lake Taitema have been dried up for many years. The groundwater level on both sides of the river bed declined quickly from 3-5 m to 8-10 m or more below the ground surface. For instance, the groundwater levels were 3-5 m in two wells of the Aragan Region in the 1950s and descended to 11-13 m in 1985 (Wang 1986). Table 8 displays another example of shortening of a seasonal section of the Keriya River in Xinjiang.
In the 1950s, there were 52 lakes of over 5 km² in area in Xinjiang, totalling 9,700 km², but that number had decreased to 4,700 km² by the early 1980s. The famous Lake Lup Nur (3,000 km²) dried up in 1964 and others, such as Lake Manas (550 km²) in 1960, Lake Taitema (88 km²) in 1972, and Lake Aydingkol (124 km²) in the 1980s, dried up in succession. Lake Ebinur (1,070 km²) and Lake Ulungur (745 km²) have been reduced to one-half and one-tenth their original size, respectively, since the 1950s. In the Alxa Plateau of Inner Mongolia, the Gaxun Nur Lake (262 km²) dried up in the 1970s and the Sogo Nur Lake in the 1980s.
Since expansion of the irrigation areas in the upper reaches has increased the proportion of backwater (recharged from the irrigated land), the degree of mineralization has increased in the lower reaches, which has caused water-quality deterioration. The degree of mineralization has changed at the Aral Station as follows: initially, 0.33-1.28 g/litre, with an average of less than 1 g/litre year-round except in May (the driest season) before the upper area was irrigated on a large scale; subsequently, more than 1 g/litre year-round except in the flood season, with 2.5-5.5 g/litre in the dry season. The degree of mineralization for groundwater from Aragan to Lake Taitema was raised from less than 1 g/litre in the 1950s to 2-10 g/litre in the 1980s along the Tarim River, and reached over 400 g/litre at Lake Taitema in 1982 (Zhou 1983). In
Table 7 Run-off Volume (108 m³) Passing the Main Hydrometric Stations (Points) along the Tarim River
|Period||Aral||Qiman||Taba Luntai||Confluence one a||Confluence two b||Qara||Sluice from Daxihaizi Reservoir||Yengisu||Argan||Luobuzhuang Lake Taitema|
|1961-1970||51.3||44.7||33.2||(17)||(11)||9.4||3.6||flood water only||2 m³/s (Oct. 1965)|
|1971-1980||44.0||35.2||26.8||(15)||(8)||6.3||0.5||Dried up (1974)|
|1981-1985||46.2||37.7||24.6||(13.5)||(3.8)||3.7||0.6 (1985)||Dried up||Dried up|
|1986||48.0||35.0||20.9||(11.0)||(2 6)||3.4||No sluice||Dried up||Dried up||Dried up|
Source: CAS (1989).
a. Run-off flowed in the Tarim river from the Wushiman river.
b. From the Ogan river.
Table 8 Shortening Situation of a Seasonal Section of the Keriya River
|Period||Type of run-off||Place reached||Distance from Yutian (km)||Extent of shortening (km)|
Source: Tian (1986).
Table 9 Irrigation in Southern Xinjiang in 1985
|Region||Water use(108 m³)||Irrigation area(ha)||Irrigation quota(m³/ha)||Canal utilization coefficient|
Lake Borten (1,019 km²), the degree of mineralization changed from 0.39 g/litre in the 1950s to 1.5 g/litre in the 1970s, and to over 1.8 g/litre in the 1980s; the lake level has descended from an elevation of 1,048.5 m in the 1950s to 1,047.5 m in the 1960s, 1,046.0 m in the 1970s, 1,045.6 m in 1985, and 1,044.8 m in 1986, a total drop of 3.70 m.
Water conservation is an essential prerequisite for constructing new oasis agriculture in the arid regions. A vast area of wasteland has been opened up, dependent solely on the water-supply system. But if the water management is poor and inappropriate, the new productive oasis could become wasteland again. For a long time in the past, much attention was paid to broadening water sources, but less to reducing water wastage. The waste of water, or overuse of water resources, was a very common irrigation practice, resulting from the backwardness of such systems as flood irrigation. Channel permeation wasted water in great quantities, too, since only 0.5-1.0 per cent of the total number of channels had been treated to be waterproof. Under those conditions, a high irrigation quota was impossible to avoid. Table 9 shows the situation of irrigation in the southern part of Xinjiang in 1985. The gross irrigation quota in the area was more than 14,850 m³/ha and even reached 19,000 m³/ha. Very disadvantageous also was the fact that many of the irrigated areas were not fitted with drainage systems. Such a practice not only wasted water resources but also did not meet the water need for crops in good time and sufficient quantity, and caused the rising of groundwater levels and the creation and expansion of land affected by salinization. Up to the late 1980s, about 1.15 million ha of land had been salinized to a serious degree, one-third of the total irrigated farmland in the arid regions of China.
Table 10 Degradation of Populus Diversifolia Woodland in the Lower Reaches of Some Rivers in the Arid Regions of China
The unfavourable changes in the water supply and the degree of mineralization resulted in serious vegetation degeneration, especially of woodlands (mostly composed of Populus diversifolia), in the regions. Of course, felling the trees to open up wasteland and to gather firewood for heating and cooking destroyed the woodland even more quickly. But in the lower reaches of the rivers, a more important factor was the water. Table 10 shows examples of the degradation of P. diversifolia woodland in the lower reaches of some rivers in the region.
Vegetation has also been degraded by overuse of groundwater in oases that are located at the lower reaches of rivers. Take the Minqin Oasis of the Shiyang River as an example. Because the surface run-off to the oasis has been lowered continuously (5.46 X 108 m³ in the 1950s, 4.49 x 108 m³ in the 1960s, 3.23 x 108 m³ in the 1970s, and 2.22 x 108 m³ in the 1980s), groundwater has been pumped extensively since the 1970s (1-3 x 108 m³ annually), and the accumulated total for the following 15 years amounted to 36.28 X 108 m³, which greatly exceeded the quantity of recharged water in the same period. The utilization ratio of groundwater for agriculture has increased from 4-5 per cent in the 1950s to 50 per cent in the 1980s. For these reasons, the groundwater level has declined by a large margin, by about 4-17 m from place to place at the oasis. The natural and artificial vegetation has withered and died. There were 220,000 ha of arboreal and shrub woodland in the late 1950s, of which 72,600 ha in good growth were left in the late 1980s; thus, about 67 per cent of the woodland has degraded. The vegetation cover has decreased from 44.8 per cent to 15 per cent (Zhu and Chen 1994).
Sandy desertification is a major part of environmental degradation in the arid regions of China (Zhu and Chen 1994), and is mainly caused by excessive human activities facilitating wind erosion. Wind erosion damages the structure and composition of soil and leads to a rapid decline of biomass production and potential productivity of the land. The features of the land surface will deteriorate under the impact of wind erosion. Wind erosion occurs after the vegetation has been destroyed by overcultivation, overcollection of fuel wood, overgrazing, and misuse of water resources.
A very good example here can illustrate what constitutes misuse of water resources. Salinization was caused principally by the overuse of water in the upper and middle reaches of the rivers, while the sandy desertification spread because there was no more water available in the lower reaches. Many areas of farm land had to be abandoned along the lower reaches since the water supply had been cut off. Those areas were subject to erosion by wind and became decertified land some years later. Since the 1950s, more than 132,000 ha of farm land have been decertified in the regions along the lower reaches of the Tarim River and Konqi River, 25,400 ha along the Shiyang River and 30,000 ha along the Hotan River. Also, much range land and woodland has been degraded in the same period. In total, 343,000 ha of abandoned land have been decertified in the southern part of Xinjiang (Wang 1996) and much more in the arid regions as a whole. The degradation of the agricultural environment because of misused water resources in the arid regions can be seen in summary in figure 2.
Improving water management for sustainable agricultural development
Water resources are the most important condition for agricultural development and hence economic development and progress of the society in arid regions. Water management and utilization have made great contributions to agriculture, but were accompanied by some environmental and social problems because of the misuse of water resources. At present, developing agricultural production is limited by the degree to which the water supply occurs in the right amount and at the right moment in the arid regions of China. Consequently, the urgent challenge before us is the improvement of water management and utilization, which not only is required to ensure the sustainable development of the economy, but also is needed to protect the agricultural environment. Some suggestions based on typical examples of good water management in the region can be made as follows.
1. Take the Continental River Basin As an Integrated Ecological System to Unify Water-Use Planning with Due Consideration for All Concerned
In the arid regions, the formation, distribution, and transformation of water resources originate from each continental river basin through the link between surface run-off and groundwater, which constitute an integrative valley ecosystem from the upper to the lower reaches of the river. The oasis agriculture in the river basin depends on the water supply. Any unsuitable water use will cause an imbalance of the ecosystem and environmental degradation, and consequently endanger the agricultural production. So it is a vital task to take the river basin as a whole ecosystem to unify the water-use plan. In accordance with the principles of overall consideration of all factors in the upper, middle, and lower reaches of the river, of unified management and utilization of surface and groundwater resources, and of centralized distribution of water supply along the river, the former intensive water use should be regulated and the scope of land use should be maintained at the level of the maximum water capability for irrigation. A good example is the well management of the Manas River basin in the south-western fringe of the Gurbantunggut Desert in Xinjiang in the aspects of water use and water-conservation projects.
Figure 2 Diagram of Degradation of the Agricultural Environment by the Misuse of Water Resources in the Arid Regions of China
2. Increase the Utilization Ratio of Water Use and Establish a Stable and Highly Efficient Artificial Ecosystem in Each River Basin
In the arid regions, agriculture can be practiced only in the oases, and over 90 per cent of farmland relies on irrigation. The average grain yield is 2,100-2,500 kg/ha, but 3,700-4,000 kg/ha in many high-yield fields (Wang and Zhu 1989).
The land's productivity has a great potential to be exploited. Under the present conditions of available water and favourable heat and light resources, along with gigantic efforts to increase the production so as to increase the multiple crop index, to choose crops in the light of water-supply variation in different seasons, to ameliorate the soil, and to control salinization, a stable and highly efficient artificial ecosystem will not be so difficult to establish. Again, the example is the artificial oasis ecosystem in the Manas River basin in Xinjiang. Here, the utilization ratio of water use was increased to as high as 85 per cent in the 1980s. The areas of artificial oasis agriculture expanded from 1,200 km² in the 1950s to 7,200 km² in the 1980s.
3. Improve the Conveyance System and Irrigation Technique
Although many water-conservation facilities have been constructed, most of them still need to be completed by adding conveyance systems, and the trunk and branch canals also have to be treated with seepage-proof materials, so that more benefits of water use can be obtained. For example, in the Shihezhi reclamation area of Xinjiang, the irrigation system with over 40 per cent seepage-proof canals has effectively saved water since the canal utilization coefficient reached 0.63 and the irrigation quota decreased to 5,460 m³/ha.
The irrigation techniques, such as flood and string irrigation, are very backward in the arid regions, too, which results in the large gross quota of irrigation (table 9); capital construction on farmland and better techniques (furrow and border method of irrigation) should therefore be carried out. A series of experiments on the Yarkant River of Xinjiang shows that the gross quota of irrigation could be decreased on average to 2,300 m³/ha when the better technique was practiced, and about 3.87 x 108 m³ of water could be saved annually over a total of 167,000 ha land if spring-sown crops were adapted to the border method of irrigation along the river only. Information regarding the advanced technique of spray and drip irrigation should be spread and applied, although we would not expect that to be on a large scale at present because of the higher cost.
4. Protect the Natural Vegetation and Develop an Artificial Shelter Belt for a Better Agricultural Environment
The oasis is the foundation of agriculture. But only 3-15 per cent of the river basin area is constituted by oases in the arid regions, which are surrounded by deserts and face many natural disasters such as drought damage, frost injury, hail, flooding, sandstorms, dry and hot winds, and wind erosion. Vegetation serves to withstand these disasters; thus, on the one hand, it is a foundation to safeguard the stability of the oasis and on the other hand it is the most stable part of production in the arid ecosystem. It is, consequently, very necessary to ensure a volume of water for use on woodland and range land, which will certainly have the effect of protecting the oasis ecosystem.
Based on the experience of oasis shelter-belt construction in the arid regions, the forestry should keep a certain proportion in the oasis area. In the Shihezhi reclamation area of Xinjiang, the shelter-belt covers 7-15 per cent of the irrigation area on the edge of deserts and 5-10 per cent in the oases. In the Hexi Corridor Region the proportion is 5-10 per cent. Under normal conditions, the shelter forest is planted along the canal or around the crop land, so the seepage water from the canal and land can be used by the forest. That being the case, the forest can fully save and utilize the farmland irrigation water, as well as providing biological drainage to avoid salinization. At present, the total forest land in the arid regions comprises about 5-10 per cent of irrigation land, which still should continue to increase. The water supply for the shelter-belt and woodland should be about 10 15 per cent of total irrigation water in the oases.
Water is one of the most challenging current and future natural resources issues in the arid regions of China. For sustainable agricultural development and, hence, economic growth and society's progress, water is the key to success. Although there are vast wastelands and light and heat resources, the local economy depends only on irrigated agriculture and animal husbandry because of the water limitations. In the arid regions of China, water utilization has a long history and human activities in water management have improved the agricultural environment to be favourable for subsistence and development on a large scale. Along with the construction of reservoirs, irrigation and drainage systems, and other water-conservation facilities, the old oases have been expanded and new oases and artificial woodland and range land have been created. These have brought about great advances in agricultural production. But, owing to increased human requirements and overused and misused water resources, agro-environmental degradation, such as salinization, vegetation degeneration, and sandy desertification, has been caused and spread quickly. The existing water resources are already under great pressure from agriculture, and will face a much more difficult situation in the future. However, some typical examples have proved that agricultural development could be sustained if water management was improved. But how to conduct sound water management is still the most important question for sustainable development when we face agriculture that is limited by water.
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