1 Introduction

The Heihe River originates from Qilian Mountain, flowing through Qinghai, Gansu and Inner Mongolia. The river covers an area of about 140,000 km², which is high in the southwest and low in the northeast. The zoning is vertically apparent from landform，which is 2500～5826.6m at the altitude in the Qilian Mountains in the south with modern glacier above 4000m, and l000～1500m at the Hexi Corridor in the middle, 800～1500m in the Alashan plateau in the north. The upstream of the Heihe River is above theYingluo Gorge Station, and the middle stream from Yingluo Gorge Station to Zhengyi Gorge Station and the rest is downstream.

The basin is a typical continental monsoon

climate. The rainfall decreases gradually from east to west, north to south and high to low. In the southern mountains the average perennial temperature is -5～4℃, the rainfall 300～400mm. Snow is the important recharge to the river. In the corridor, the climate is dry, the average perennial rainfall 88.8～171.7 mm, the average perennial surface evaporation l708.5～2503.5mm. Ejinaqi basin is extremely dry in the downstream. The average perennial rainfall is 42.0mm, evaporation amounting to 4135.8mm. The climate is the long and cold winter with little snow in this area, but the summer is hot with concentrated rainfall, and the temperature changes quickly in spring and autumn. Heihe river system is composed of over 30 streams that can be divided into two tributaries, of which the east branch the Heihe River is the main river about 821km long with the average perennial flow 1.591 billion m³. The west branch is Taolai River with the average perennial flow 0.638 billion m³. The runoff is large in the mountains and becomes small in the hilly area and plains, and even no runoff at the end.

Zhangye named "golden Zhangye" is an important corn base in Gansu Province in the midstream. In recent decades, the development of economy with the impact of human activities is growing in the mid-stream area. A dramatic increase of land cultivating and overuse of water resources have led to not only the changes of water quality and quantities in the plain, but the Ejina Lake dying out, vegetations faded in the downstream in which the Ejina oasis used to be an ecological barrier in the north. The sandstorms become intensive and frequent in the Ejina basin, which has become one of the four major sources of sandstorm in the north China (Shengchun Xiao, 2003; Tao Wang, 2001; Jian’an Tan, 1964). Therefore, the evolution mode effected by human activities in the Heihe basin become so important to understand reasonable development of water resources. The study would be significance for relations between social economy and ecological environmental protection in order to keep sustainable development.

2 Water Exploitation and Evolution in the Heihe River Basin

2.1 Utilization of water resources

Since 2000 years ago, the land had been cultivated with opening dyke, irrigated in the Heihe basin. During that time, the local people were not more than 100,000 there in the Hexi Corridor. Gradually, the evolution turned from the natural oasis to the artificial in Juyan area. In the recent 500 year or so, the ancient oasis was dying out and the natural Ejina oasis grew. In the past 30 years, especially since the 1980s, the agriculture has been growing rapidly with the irrigation area increasing from 50,000 hm² in the 1950s to 213,000 hm² now in the middle stream area, including the new cultivated land more than 3300 hm² paddy fields. In 1999, the total water amount of the Heihe River is 3.778 billion m³, the pumping from the river came to 3.26 billion m³ in the middle basin, and 369.3 million m³ water remained to the downstream. The middle stream area compumed 86% of the total volume of river and the downstream only 9.8%. The recharge to groundwater was 2.967 billion m³/a, the exploitable water amount 1.17 billion m³/a, the pumping amount 694 million m³/a, which amounted to 60% of the total exploitable amount.

2.2 Evolution of surface water

The Zhengyi Gorge and the Yingluo Gorge Stations were built on the Heihe River (Fig. 1 and Fig. 2). T data show the natural changes of the water flow at the Yingluo Gorge station for years (Lan Yongchao et al, 1999, Ding Yongjian et al, 1999; Wang Jin-ye et al, 1999). The runoff at the Zhengyi Gorge indicates the flow reduction effected by both natural conditions and man activities as shown in Table 1.

Man activities leads to 8 times changes of runoff in the Heihe River far away from the normal runoff. The basin at the middle reaches of the Heihe River is just the area to change the runoff of the river. From Table 2, the change of runoff at the two stations tells us the fact during the 1950s till the 1990s.

Overall, the data show theat the flowing amount was normal at the Yingluo Gorge station from the 1950s to the 1990s as smooth changes, which was effected mainly by nature. The data show that the flowing amount at the Zhengyi Gorge Station was fairly rich in the 1950s, normal during 1960 and 1989, but the runoff was reduced dearly in the 1990s, which was taken as the fluctuation changes. The main reason is that man activities caused the reduction of runoff during the 1990s.

The changes of runoff at the Zhengyi Gorge rely not only on the water amount from the Yingluo Gorge, but also on the human activities with consumption of water between the two gorges (Fig. 3 and 4).

Figure 3 shows that the flow through the two stations have decreased in recent years and Figure 4 gives the data from the mid-stream river basin that the consumption of the river water was stable with 460 million m³/a, representing 29% of total runoff before 1979. The increase of river water consumption started in the period of 1980~1984 with 590 million m³/a, accounting for 37.3% of total runoff and during 1985～1989 the average 683 million m³/a, accounting for 43.2% of the total runoff. The difference began from the natural dynamic runoff to the artificial dynamic process. Since 1990, the exhausted period of the natural runoff started by consuming 812 million m³/a, accounting for 51.4% of total runoff that has exceeded 50% over years that clearly shows the fact of the artificial control.

                 Fig.1 Runoff at the Yingluoxia Station                   

 Fig.2 Runoff at the Zhengyixia Station

Table 1 Characteristics of perennial runoff at Yingluoxia and Zhengyixia stations

Table 2 Comparison of runoff in different periods of the Heihe River area billion m³ /a

Fig.3 Yingluoxia and Zhengyixia’s runoff and difference value       

       Fig.4 Changes of Yingluoxia and Zhengyixia’s  residual mass of runoff difference value

1-Yingluoxia; 2- Zhengyixia; 3- runoff difference value 1-residual value of runoff difference value;

4-runoff difference value trend 2-residual mass of runoff difference value

2.3 Groundwater evolution

Groundwater occurs mainly in the middle and downstream basin where the giant thick Quaternary sediment provides a good condition for the aquifers which can be divided into the unconfined and confined groundwater systems. The unconfined aquifer is spreading over Zhangye and Jiuquan basins and the north part of the Dingxin basin and eastern part of the Ejina basin. The yield from most of the wells only 5m deep is about 1000~5000m³/d in the middle reaches area, the downstream is about 100~1000m³/d. With the construction of water projects groundwater evolution in the basin is as follows:

2.3.1 The poorer groundwater recharge conditions, the less quantity recharged

One is the flow in the river course decreases year after year and another is the low seepage from ditches to recharge groundwater, which has taken place of the high infiltration from the river course, mainly from Gobi changed to the fine soil. A third, groundwater extraction caused the water level lowered and the effective recharge to groundwater decreased. At last, the river course becomes dry and no source recharges to groundwater. The calculation shows the result that groundwater recharge was reduced in the late 1990s of 0.99billion m³/a in the midstream area. The recharge now in the downstream area has decreased of 51.5% than the early 1980s (Table 3).

2.3.2 Springs disappearing, efficiency of water reuse reducing

Spring is kind of water resources which come from other regions and overflow in this area. The construction of water conservancy and over-exploitation of groundwater caused springs dramatically decreased. The statistics shows the discharges were 0.182 billion m³/a, 0.16 billion m³/a and 0.14 billion m³/a in the 1960s, in the 1970s and in the late 1990s respectively. The latter is reduced 0.84 billion m³/a of that in the early 1960s.

2.3.3 Dropdown of the regional groundwater level with local water quality deteriorated

In the midstream of the Heihe River, groundwater level drops down obviously close to the upstream (Fig. 5) but the water level keeps stable in some local irrigation areas (Fig. 6). The problem comes from the high evaporation which cause the salt accumulation and deterioration of the water quality in some irrigated land.

Table 3 Groundwater recharge in the middle reaches of Heihe River in recent decades

Fig.5 Groundwater dynamics from No. 65 well at Minle

  Fig.6 Groundwater dynamics from No. 7 well at Gaotai

3 Analysis of Evolution Mode of the Heihe River

In view of the Heihe River cycle, the Heihe River flows through the mountain area, the midstream basin and the downstream basin, which has experienced surface and underground several times that attribute to the multiple cycling mode. The conversion and transformation of water resources covers 50% of groundwater recharge for irrigation system in the mid-stream basin in 1999 (Fig.7). The drainage and exploitation became prior to evaporation. In the downstream basin, the recharge to groundwater relies on the river course seepage and the discharge is by evaporation. Both the recharge and discharge of the river system are strictly controlled by human activities. River recharge and evaporation are always the most important consumption, so that it is of great importance to increase the recharge to groundwater and reduce evaporation.

¹⁴C dating shows that there exist not only groundwater transformation which lasts over 40 to 55 years in the basin, also the water transformation less than 35 years in shallow groundwater in the local areas of the basin.

All in all, the man controlled development of water resources in the midstream area not only impacts the runoff downstream in the Heihe basin, but also changes completely the recharge to groundwater, the water level dynamics and spring occurrence downstream. All these have led to serious ecological problems. The Heihe water cycle evolution can be summarized as "natural water from mountain- man-control in the plain complex cycle evolution mode". It can be also divided as the upstream mountainous natural flow, off-composite artificial landscape (Yongjian Ding, 1999; Jinye Wang, 1999), the midstream natural-runoff evolution impacted by man activities of direct consumption and utilization above 86%, and the downstream passive accepting water, which is called the man controlled complex process.

Groundwater evolution system in the Heihe basin becomes an important part of "natural water from mountain- man-control in the plain complex cycle evolution mode". Therefore, the analysis should be from many aspects such as the natural recharge to groundwater, runoff and natural drainage impacted by human activities.

Groundwater recharge derives mainly from the Heihe River. Large water diversion and impervious constructions make the river bed stop infiltrating to recharge groundwater, for example, the recharge amount was nearly 1.5 billion m³/a in the middle basin in the 1950s, now turned to 0.7 billion m3/a. It is about 50% reduction. During the 1980s, the grain production bases were developed in the mid-stream areas, and direct import river for irrigation used much water. Yet the fine lithology of irrigation areas limited storage space. As the result, groundwater got less recharge but consumed much more. The fact can be summarized from the natural recharge from river course to the farming seepage recharge, but the total recharge decrease.

Groundwater flow pattern, due to the man disturbance and reduction of the natural recharge from the river course, changed its lateral runoff in the alluvial-proluvial fans. At the same time，the expanding irrigation results in the alternation of vertical infiltration in small territories. Therefore, groundwater runoff can be summarized as runoff degradation evolution from the lateral runoff to the artificial vertical recharge and the runoff flowing to pumping wells in some local area.

Groundwater resources is needed in the total basin from about 1.5 billion m³/a 50 years ago to the current 2.62 billion m³/a, especially in the midstream area where the consumption of water resources is about 2.45 billion m³/a of which 95% percent is used for irrigation, but 70% of the total amount is extravagant. As a result, the water for ecological environment decreases. Groundwater drainage cannot be taken as the only local drainage. The pumping field cannot be out of view. Therefore, groundwater drainage can be regarded as the evolution from the natural drainage to the manual mining. This drainage evolution is a kind of primary use for groundwater resources in which the groundwater extraction is taken a main part. The problems remain as springs are not taken as reusing water recourse to be effectively protected, so that the water reuse efficiency is reduced; another is that groundwater is used without considering the effective conservation and keeping ecological balance. So it is unsustainable.

The result shows that the Heihe River system turns to the evolution for multifunctional distribution of natural, social, ecological, environmental aspects. This is just the evolutional mechanism of the present and the future water resources. Surely in the 21st century the principle will make the foundation and direction for future investigations of groundwater resources assessment, utilization and the reasonable protection of the ecological environment as the advance of science and technology and awareness of people.

4 Mode of Exploitation of the Water Resource in the Heihe River Basin

So far, water resource of the Heihe River is used just for water irrigation in the Mayinghe irrigaton area and the Hongshuihe irrigation area. Spring water storage for irrigation is used in Luocheng irrigation areas. Springs mixed with river is used in Yanuanqiao area. Groundwater mixed with surface water is used in Liyuan, Youlian irrigation areas. It is no doubt that there was reasonable in the past. But the lack of basin water resource governing and rational regulating is obvious. The new water resource usage pattern should be constructed to serve the balance of environment, such as the pattern considering the ecological water quantity and groundwater level that may impact the ecology, surface water, groundwater quantities and quality etc. The groundwater reasonable exploitation patterns should first follow the principles of sustainable development, the social-economic-environmental coordination. The result is sustainable exploitation. Moreover, sufficient consideration should be made in groundwater and surface water transformation in order to improve the water using efficiency and to protect the ecological environment. The groundwater exploitation pattern in the Heihe River Plain is as follows:

4.1 Surface water, groundwater storage-control exploitation in Gobi zone

The area lies near the mountainous area. Groundwater level is generally above 10m. The aquifer is thick and wide distributed in this area with large storage space, stable ability of water supply. It is uneasy polluted, little loss of evaporation, and taken as the main groundwater recharge area, which is suitable for groundwater regulation and exploitation. This area should be well protected for regulation of groundwater sustainability and stable development and anti-penetration measures should be avoided to keep surface and groundwater efficient transformation..

4.2 Exploitation and water-soil ecology environment integrative in Midstream oasis

This area is wide distribution of irrigation areas in the midstream of the Heihe River such as Zhangye basin, Jiuquan basin and Jinta Basin downstream. The principle of water exploitation should firstly prevent from desertification around the irrigation area and salinization in irrigation areas. The water resource rational exploitation in the oasis can be divided into three subzones:

The first subzone: water resource is not abundant, groundwater table 5～7m. the further desertification should be prevented. Surface water shoud be used for irrigation. It lies in the outlying oasis areas.

The second subzone: water resource is abundant for ecology, groundwater tasble is less than 3m. Soil further salinization should be prevente in this area. Integrate irrigation with drainage and reduction of evaporation loss should be taken.

The third subzone: Water resource is not abundant for ecology, groundwater table about 3-5m. Surface water with groundwater properly exploited can be taken for this area, and spring can be considered if the condition is acceptable.

4.3 Exploitation with avoiding land desertification in downstream area

It is mainly in Ejinaqi basin. The key point is that water resources exploitation and utilization should avoid the degradation of the ecological environment, based on the stable supply for the existing industrial-agricultural and domestic operation. Some engineering measures should appropriately be taken to improve natural environment. It can also be divided into three subzones:

The first subzone: water table is above 5 m, and river water should be used and natural leakage from river beds should be kept with good quantities to recharge groundwater, for groundwater is the most important to keep the ecological environment in Ejinaqi basin. Only in this way can vegetation coverage ratio be improved.

The second subzone: Water table is between 3-5 m, and this buried depth is good for most vegetation growth. The land quality is poorer due to the thin soil in Ejina basin. The degradation of vegetation is serious effected by a variety of disturbance. The way to be chosen is to plant suitable grasses in the area and resume the desertification vegetation in a short period.

The third subzone: Water table is less than 3m. Groundwater can be exploited to keep the water table stable. Appropriate ranching, farming and efficient ecology is the purpose in this area.

5 Conclusions

In the 1950s, the Heihe River was natural and stable above the Yingluo Gorge, and runoff consumption below the Yingluo Gorge. With time going on, man activities changed the evolution in the middle 1980s. Groundwater became decreasing, water table dropping down, and springs crocking up. As the result, the water environment turns bad, and the groundwater evolution turns to artificial control process. Substantially, the water evolution has changed to natural, social, ecological, environmental mixed functions. This will be the mechanism of water resource evolution at present and in the future.

Generally, the Heihe basin is compound pattern of natural water resources from the mountains and artificial control in the plain. The natural runoff comes from the upper mountainous area, and natural-artificial runoff pattern in the mid-downstream basin. The natural recharge reduced and the man-effected recharge increased is dominant now in the plain. The runoff changes from the lateral to the artificial vertical recharge in the fan area, and some local areas turn to flow to pumping wells. The discharge pattern has become the natural drainage decreasing and the artificial exploitation increasing. It is not a sustainable exploitation evolution.

The proposed reasonable exploitation pattern in the Heihe River basin is as follows: The compound ways of surface water in the Gobi area, groundwater storage and control exploitation in the upstream, water resource exploitation and water-soil ecological environment integrative in oasis, and water resource exploitation with avoiding land desertification in downstream should be taken and protective construction in upstream, and reduction of excessive water waste in midstream, basin groundwater protection in middle basin, river stability in downstream, reduction of evaporation losses, man exploitation regularity may play an important role in achieving effective protection of the ecological environment.