Liu Zhiming, Wang Guiling, Lin Wenjing

Institute of Hydrogeolgy and Environmental Geology, CAGS, Shijiazhuang, 050061

^* Supported by the Ministry of Science and Technology, China

The first author: LIU Zhiming (1969-), male, doctorial candidate, associate researcher, engaged in study on water resources. E-mail: liuzhimingpeople@peoplemail.com.cn

Abstract: Based on analyzing the current status of groundwater resources utilization in the plain of Shiyang River Basin, this paper has discussed problems during the process of groundwater resources utilization in the study area in combination with historical utilization records. The study results show that because of the utilization degree improvement of surface water and the successive overexploitation of groundwater for many years, the level of groundwater has declined greatly, the flux of springs and the incoming water of Hongyashan Reservoir has decreased year by year, the recharge of groundwater in Minqin Basin has also decreased, the utilization contravention of water between upriver area and downriver area has become very sharp, and at same time the environmental problems have occurred such as the depravation of water quality, the desertification of land, the degradation of vegetation etc. In order to solve these problems, this paper has put forward some suggestions for the reasonable utilization of groundwater.

Key words: the plain of Shiyang River, groundwater resources, utilization, problem

1 Study Area

The study area, consisting of Wuwei Basin, Minqin Basin and Changning Basin, lies in Shiyang River Basin of the eastern end of Hexi Corridor in Northwest China and extends from 101°00'-104°12'of longitude east to 37°02'-39°20'of latitude north, and borders wide Tengger Desert on the eastern side, adjoins to Qilian Mountain on the southern side, closes to Yongchang Apophysis on the western side, and extends to Beishan Mountain on the northern side. The study area is about 19,549 km², of which, the Wuwei Basin and Minqin Basin account for 18,549 km² and the Changning Basin accounts for about 1,000 km² (Li Youlin, 2000).

It belongs to the typical arid and semi-arid inland climate, which is sweltering and transitory in summer and cold and longer in winter. The average annual temperature is 5-10℃. The average annual precipitation is 150-200 mm, and 74% of the annual precipitation occurs in the period from June to September. The annual evaporation is above 2,000 mm.

2 Water Resources Conditions

2.1 Surface water

In the Shiyang River Basin, there are many rivers originating from the Qilian Mountains. The bigger ones that can flow out of the mountains are the Daqing River, the Gulang River, the Huangyang River, the Jinta River, the Zamu River, the Xiying River, the Dongda River and the Xida River. In addition, there are over 60 streams that flow at the shorter distance and the flow directions are vertical with tend towards of the mountain. The total of the average annual flux that run out of the mountain is 1,422 million m³.

As shown in Fig.1, the runoff change trends of the bigger rivers reduced gradually but not very distinct from 1950 to 2000. However, the flux of Shiyang River that is formed by springs in the plain has changed quite distinctly. Because of the reduction of natural recharge, its annual flow has decreased from 512 million m³ in 1950s to 141 million m³ in 1990s (Fig.2).

Fig.1 The runoff curves of bigger rivers

Fig.2 The runoff curves of the Shiyang River and the Jinchuanxia River

When flowing out of the mountains, rivers flow into the South Basin of Hexi Corridor like broom, and then, in the Gobi zone with high permeability, surface water all recharge groundwater, and at the foot of the diluvium-alluvium fan, groundwater discharges to Shiyang River and Jinchuangxia River in the form of springs, and finally, the two rivers flow into Minqin County and Hexipu of Jinchuang and disappeared through the irrigation and evaporation.

In history, flood that comes from the upriver mountain area could flow straight to Shiyang River and then runs into the end lakes, namely, the Qingtu Lake and the Baitu Lake, through Minqin county seat. But now, many reservoirs have been built in the region. In the upriver mountain area, there are the Huangcheng Reservoir, the Xidahe Reservoir, the Xiyinghe Reservoir, the Nanyinghe Reservoir, the Huangyanghe Reservoir, the Shibali Reservoir, the Caojiahu Reservoir, the Dajinghe Reservoir and so on, and in the middle reaches, there are the Jinchuangxia Reservoir and the Hongyashan Reservoir etc. Hence, there has been almost no flood discharged to the downriver area, so that the end lake has dried up.

2.2 Groundwater

In 2000, the total recharge of groundwater in study area was 1,092 million m³, and the natural recharge resource was 546 million m³, among which 422 million m³ was in the Wuwei Basin and there the modulus was 453 thousand m³/(a·km²); 24 million m³ was in the Minqin Basin and there the modulus was 186 thousand m³/(a·km²). The exploitable resource of groundwater was 725 million m³/a, among which 328 million m³/a was in the Wuwei Basin, 351 million m³/a was in the Minqin Basin and 46 million m³/a was in the Chuangning Basin.

3 Current Status of groundwater Resources Exploitation

Before 1970s, surface water was mainly utilized, but the simple use of surface water led to results that the groundwater level was very shallow, the soil was salted and the water quality was deteriorated. Since founding that groundwater level decline could control the soil salinization effectively in 1980s, groundwater has been exploited and utilized both in a great quantity, and the vast of salted land has become cultivated fields. Currently, because the utilization degree is very high, it is very difficult to exploit surface water again. The utilization degrees of groundwater all have exceeded 100% and the total exploitation is far more than recharge, so the groundwater level has declined in the region successively, especially it has declined the most greatly in Wuwei City and Minqin County.

By 2000, there were 14,852 pumping wells and the mean density was 2.16 wells/km². 5,015 wells in the Wuwei Basin and the mean density 1.49 wells/km²; 8,122 wells in the Minqin Basin and the mean density 3.68 wells/km²; 1,715 wells in the Chuangning Basin and the mean density 1.32 wells/km². The total pumping yield was 1,491 million m³/a, of which 733 million m³/a in the Wuwei Basin, 571 million m³/a in the Minqin Basin and 187 million m³/a in the Chuangning Basin; Deep groundwater was pumped by 12 million m³/a in the Minqin Basin and 25 million m³/a in the Chuangning Basin. Water pumped from the irrigation wells was generally from the aquifers 60 to 80 meters underground.

In the same year, the total use of water was 2,399 million m³, among which surface water accounted for 38% and groundwater accounted for 62%. The use of water in the Wuwei Basin was 1,432 million m³, of which surface water was 12% and groundwater was 88%. The use of water in the Chuangning Basin was 332 million m³, of which surface water was 51% and groundwater was 49%.

Comparing the pumping yield with exploitable resource, we can find that 766 million m³/a of groundwater has been overexploited. In the Wuwei Basin 405 million m³/a of groundwater has been overexploited and its exploitation degree is 223.51%, in the Minqin Basin 220 million m³/a of groundwater overexploited and its exploitation degree 162.55%, and in the Chuangning Basin 141 million m³/a of groundwater overexploited and its exploitation degree 409.10% (Table 1).

Table 1 The utilization of groundwater in the Plain of Shiyang River Basin

4 Existing Problems of Groundwater Resources Utilization

Although the problems in the study area have been researched again and again for many years, many scholars, such as Chen Mengxiong (2002) et al_., have indicated these problems of water resources utilization and have brought forward many valuable treatment and management ways, yet in the actual process of groundwater utilization, the mode and tactic of water resources utilization haven't largely changed, and there still exist main problems as follows:

As strengthening of human being's activity, the artificial surface water system has replaced the natural system gradually, and the surface water system has been changed largely. Since 1957, different-sized reservoirs have been built at the upward reaches of all rivers, almost controlling the coming water from the mountain area. The Hongyashan Reservoir has been built in the middle reaches of the Shiyang River, and becomes only supplied source of the Minqin Basin. Heretofore, 7 medium-sized and 6 small-sized reservoirs, with the storage capacity of 218 million m³ have been built in the river basin, there are 245 km of the first main channel, 1,208 km of the second main channel and 2,509 km of the branch canal, 70%, 83% and 48% of which have been built by stone. In the Shiyang River system, the mean total flux of Xida River, Dongda River, Xiying River, Jinta River, Zamu River, Huangyang River and Gulang River is 1,481 million m³/a, and there is 1,128 million m³/a of surface water (including spring water) imported into channel for irrigation, the ratio of surface water imported makes up 76.43%. 699 million m³/a imported in the Wuwei Basin, 64 million m³/a imported in the Minqin Basin and 51 million m³/a in the Chuangning Basin. The utilization efficiency of ditches has been improved to 0.5- around 0.7 at present from 0.3-0.4 in 1960s. The utilization efficiency of surface water has reached 73%, above the mean level of China (15%), and approaching the maximum value. Due to all above reasons, the groundwater recharged from surface water is reducing year after year.

In recent 50 years, the groundwater recharge of study area has reduced markedly. The groundwater resources were 1,587 million m³/a in 1950s, 1,201 million m³/a in 1960s, 977 million m³/a in 1970s, 925 million m³/a in 1980s and 617 million m³ in 2000. Compared with that in 1950s, the groundwater resources have been decreased by 970 million m³/a in 2000, and the reduction ratio is 61%. With the successively increase of the economic and social needs year by year, the pumping yield of groundwater has becoming larger and larger, and groundwater resources have been overexploited seriously. In 1995, the total water need was 2,044 million m³ and the pumping yield was 1,116 million m³; and in 2000, the figures were 2,399 million m³ and 1,491 million m³ respectively.

The relationship between the spring flux and the groundwater level is very close, and they have the same dynamic process. As surface water has been imported for irrigation and groundwater exploited in the Wuwei Irrigation area, the groundwater level has declined and the spring flux has reduced, the springs have disappeared and the overflow zone has moved to the downriver area. For example, the spring 549, since 1986, its flux decreased about 1/3 and the groundwater level in the period of 1986-1988 was one meter lower than that seven years ago (Fig.3). According to the recent dynamic monitoring data, the groundwater level has declined successively with the speed of 0.5-0.9 m per year, and the burial depth of groundwater level in the old overflow zone has increased to more than ten meters. Springs have almost disappeared, the proportion of the overflow zone has decreased, and the incoming water of Hongyashan Reservoir has reduced rapidly. Therefore, the contradiction of water utilization between the upriver area and the downriver area is very outstanding, and the status has been formed that the upriver area occupies the water used for the downriver area and the downriver area occupies the water used for eco-environment.

Fig.3 Relationships between the spring flux and the groundwater level

The Hongyashan Reservoir is an important source of groundwater in the Minqin Basin. However, its incoming water has decreased gradually since 1960s. There were 449 million m³/a in 1960s, 323 million m³/a in 1970s, 248 million m³/a in 1980s, 167 million m³/a in the period of 1990-1995 and 103 million m³ in 1999. Because of the spring flux reduction, the Shiyang River runoff measured at the observation station on the Xiangjiawan River has reduced about 20 million m³/a in the period of 1980-1988 than that in the period of 1968-1980 (Fig.4).

Fig.4 The flux Relationships between spring and the Shiyang River measured at the Xiangjiawan observation station(^①The Second Hydrogeology and Engineering Geology Team of Gansu Bureau of Geology Exploration, et al. 1991. Study on the Management Modeling of Water Resources in the Plain of Shiyang River Basin of Gansu Province (in Chinese). Internal report, 29.)

Further, as the intensively pumping of groundwater, in the center zone where groundwater was pumped more, the TDS reported has increased rapidly, and the depravation of groundwater quality has been very distinct. The quality of groundwater environment has been relatively wicked. The groundwater was polluted to the serious degree in most regions, and especially, in the northern basin, the groundwater quality was depraved rapidly, belonging to the polluted zone of serious degree. It not only is undrinkable, but also is very dangerous for irrigation (Li Youlin, 2000). Hence, the environment problems, such as the desertification of land, the degradation of vegetation and so on, have become serious day by day. In the lake area of Minqin County, the irrigated oasis has decreased about 10 thousand hm², the desert vegetation is becoming less and less, the meadow vegetation has died to a large extent, the man-planted woods has died in the downriver area etc. According to the statistical data, near the Minqin oasis, there have been 2,800 hm² of Calligonum (about 21,520 thousand trees) died completely, and 5,800 hm² of Calligonum (about 7,000 thousand trees) have fallen off, accounting for 9% and 21% of the total area (Wang Qi et al., 2003).

5 Suggestions for Reasonable Utilization of Groundwater Resources

The Shiyang River Basin is the region with the highest exploitation degree of groundwater and with the most serious environment problem in the Hexi Corridor, and these environment problems have directly intimidated the human being's existence and the sustainable development of society. The weak eco-environment in study area plays an important role to resist the land desertification, and if destroyed, it needs huge engineering and long time to recover. Therefore, we should utilize groundwater to adjust eco-environment, to exploit water resources and to protect the eco-environment at the same time, according to the rule of the transformation among surface water and groundwater several times, to increase the total efficiency of water resources utilization, and to combine the utilization and recharge process of water resources (Guo Zhanrong et al., 2001). And at last, we should reach the goal to unify the benefits of economy, society and environment really.

(1) Because the utilization degree of surface water has been reached the maximum value and the groundwater has been over-exploited seriously, on the one hand, we should reduce the water consume for the production value of 10 thousand yuan RMB by adjusting the industrial structure. And on the other hand, we should save the water in agriculture actively to reduce the irrigation ratio. Due to water resources have been wasted very seriously in study area, it is very potential to save water in agriculture. For example, the irrigation ratio was 18,000 m³/hm² in Minqin Basin and 9,750 m³/hm² in Chuangning Basin.

(2) We should understand the advantages and disadvantages of reservoirs fully. The advantageous aspect is in favor of the artificial control and management, consequently to establish the highly efficient irrigation system to benefit the water resources utilization. The disadvantageous aspect is that evaporation is very huge, the recharge of groundwater will be reduced and the eco-environment will be depraved. Hence, when the water use of city, industry and living could be ensured, we should to release water from reservoirs to recharge groundwater and to maintain the eco-environmental development.

(3) Based on the principle to plan the water resources utilization and management in the whole river basin, we should control the well building and the groundwater exploitation to ensure the water need of the Minqin Basin.

(4) The research on the relationships between the evolution of eco-environment and the utilization of water resources, especially the eco-environmental sense study of soil water and coagulated water, should be carried out to bring forward the eco-environmental water requirement and the eco-environmental carrying capacity so as to satisfy the eco-environmental need firstly to remain the sustainable development of it.

(5) To implement the national policy that woods and grass are encouraged to plant instead of farming, to enlarge the area to resist the wind and the desertification. On one hand, we should reduce irrigation water; on the other hand, we should decrease the useless evaporation of water resources to enrich groundwater, consequently to improve the eco-environment.