Analysis of the hydrological system of Hexi Corridor, Gansu Province. CHEN MENGXIONG Ministry of Geology and Mineral Resources, Beijing , China

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1 The Hydrological Basis for Water Resources Management (Proceedings of the Beijing Symposium, October 1990). IAHS Publ. no. 197,1990. Analysis of the hydrological system of Hexi Corridor, Gansu Province INTRODUCTION CHEN MENGXIONG Ministry of Geology and Mineral Resources, Beijing , China Abstract There are three major drainage systems in the Hexi Corridor, known as the Shiyangho, the Heiho and the Suleho, running from east to west, respectively. Each drainage system usually involves two or three basins connected with each other from the upper reach to the lower reach, to form a complete hydrological system. This paper gives a detailed analysis of the configuration of the hydrological system, dealing particularly with the relationships between the surface water system, the groundwater system, the water balance and the variation of the groundwater regime, as well as with the interaction of surface water and groundwater in water resources exploitation. Analyses du système hydrologique du Corridor de Hexi, Gansu Résumé Dans le Corridor de Hexi existent respectivement, de l'est à l'ouest, trois systèmes importants de drainage nommés Shiyangho, Heiho et Suleho. Chacun de ces trois systèmes comporte généralement deux ou trois bassins interconnectés l'un à l'autre et de l'amont à l'aval pour former un système hydrologique complet. Cet article présente des analyses détaillées sur la configuration du système hydrologique, en particulier, sur les relations entre le système d'eau de surface et celui des eaux souterraines, le bilan hydrologique, les variations de régime de l'eau souterraine, ainsi que l'interaction des eaux de surface et des eaux souterraines dans l'exploitation des ressources en eau. The enormous inland basins in northwest China, such as the Zhungeer basin, Chaitamu basin and Hexi Corridor, are known as typical Gobi Desert areas wherein annual precipitation is extremely scarce, but in which streams, that originate from the rainfall and melting snows in high mountains, such as the Tienshan and Qilianshan Mountains on the border of the basins, form a large volume of surface runoff flowing into the piedmont plain. The surface water and groundwater transform into each other repeatedly in the entire catchment area. Therefore the problem of how to use the water resources reasonably is very important. Experience in the Hexi Corridor has taught important lessons in the development of water resources. It represents a typical example of such arid inland basins. 3

2 Chen Mengxiong 4 ORIGIN OF SURFACE RUNOFF The Hexi Corridor is a long narrow zone between the Qilienshan Mountains and the Beishan Mountains. It is elongated in the NWW-SEE direction, more than 1000 km in length, and covers an area of about 30 x 100 km 2. It is known as a typical arid Gobi Desert area, in which precipitation is only mm and is gradually reduced from east to west or from south to north, while strong potential evaporation reaches mm and increases from east to west. On the other hand, the famous Qilienshan mountain range at the southern border of the corridor, with an elevation of about m, is widely covered with glaciers and snows, especially in the western part of the range, and comprises an area of about 1335 km 2. All the streams of this area originate from the high mountain Qilienshan. The precipitation in the mountainous area reaches mm. Thus the rainfall and melting snows discharged into the streams form a great volume of surface runoff, including the discharged groundwater. In the debouches the runoff reaches x 10 8 m 3 year" 1, which is approximately equivalent to the total water resources of the whole region. Rainfall is the main constituent of the runoff and comprises more than 50% of the total runoff; but in the western part of the range, such as in the upper reach of the Suleho, melting snows and ice are predominant, and comprise 37.6% of the runoff, while rainfall is only 22.9%. The discharged groundwater is also one of the major constituents, representing 30-40% of the total runoff (Table 1). Table 1 Constituents of the surface runoff in mountainous areas Name of the Coverage of Amount of runoff Rainfall Melt-water Discharged stream glaciers, groundwater (km 1 ) x ic? m s year (%) (%) (%) Shiyangho Heiho Suleho Total CHARACTERISTICS OF THE HYDROLOGICAL SYSTEM As mentioned above, all the streams running out from the mountainous region flow into the piedmont plain and pass through two or three basins, known as the south basin and north basin or upper basin and lower basin; these are connected with each other, but separated by a rock gorge. The stream is finally discharged into an inland lake or dissipated in the desert area of the lower basin. All the basins belong to the Quaternary fault-basin. They have an accumulation of enormously thick unconsolidated gravel and other terrestrial sediments, which become thinner and finer, composed mainly of clays

3 5 Hydrological system of Hexi Corridor interbedded with sand layers in the green land. They originate from Cenozoic or Ceno-Mesozoic basins controlled by the neo-tectonics, especially the strong upthrust at the front of the mountain range. The Quaternary aquifer system in the basin includes the Yu-men formation (Q^ and the Jiu-quan formation (Q 2. 3 ), with a total thickness of about m, and even more than 1000 m in places. The basin can be divided into three distinct zones, based on either geological or hydrological aspects: (a) The zone of the Gobi plain This can also be called the zone of infiltration or the zone of groundwater recharge. The groundwater is deeply buried with a high hydraulic gradient and strong permeability. About 90% of the running water penetrates into the underground, including seepage loss of the canals. The great thickness of the gravel beds and the deep groundwater level combine to serve as a large natural subsurface reservoir for the storage of a large volume of groundwater. (b) The zone of green land Groundwater emerges on the frontal part of the alluvial fan in the form of spring clusters, and flows into the green land, the so-called "Oasis", the main cultivated area in the basin. The amount of outflow reaches 45% of the total recharge, while the rest remains as underground runoff in the aquifers, including confined and unconfined. Part of the surface water, as emerged springs and irrigated water, infiltrates again into the underground, while a part of the groundwater is extracted for well irrigation. (c) The zone of saline soil This zone is usually a transition zone to desert land. The groundwater level becomes very shallow, showing strong evaporation, and the water quality gradually worsens, from fresh water to brackish or saline water; thus it can be also called the zone of evaporation or the zone of groundwater discharge. When the surplus water of the upper basin flows into the next basin in the form of surface runoff, it starts a new cycle of transformation. The mutual transformation of surface water and groundwater in different zones is actually very complex, as shown in the schematic diagram (Fig. 1). In fact, the present hydrological situation is already strongly affected by human activities, e.g. the construction of many reservoirs and the replacement of spring irrigation by well irrigation, because of the diminution of spring outflow. In short, a complete hydrological system usually involves two or three sub-systems (Fig. 2), represented by the connected basins of a drainage system, where each sub-system is formed by the combination of the surface water system and the groundwater system inter-transforming into a unified body. SYSTEMATIC ANALYSIS OF THE WATER RESOURCES As shown in Table 1, the amount of surface runoff reaches x 10 8 m 3 year" 1, but the underground flow passing through the alluvial aquifer

4 Chen Mengxiong 6 RE, => C2 SWS : s, S3 s 4 RE 2 P + M SP, F=mSi- 4B- -US CWS : -» G2.JL -» G 3 G< JL1, G 5 - Fig. 1 Schematic diagram showing the mutual transformation between surface water and groundwater in a hydrological system. SWS = surface water system; S = streams; C = channels; GWS = groundwater system; G = groundwater; SP = springs; RE = reservoir; I = infiltration of irrigation water; T = evaporation; P = precipitation; M = meltwater; direction of water flow; -v' direction of main water flow; ÏÏ well-extraction. p / V K,» M > s, SWS 1 ' GWS SU > S 2 SWS k ^ GWS su ^ ^ 1 S3 SWS ^ \ GWS su " LK M SU Fig. 2 Schematic diagram showing the configuration of a complete hydrological system. P = precipitation; M = meltwater; R = surface runoff; S = sub-system; SWS = surface water system; GWS = groundwater system; SU = surplus; LK = inland lake. of the valley plain is only 2.52 x 10 8 m 3 year" 1 ; i.e. the total inflow to the basin is about 70 x 10 8 m 3 year". In the plains area, the infiltration of local precipitation, including condensation water, is only 2.42 x 10 8 m 3 year" 1 (Table 2). This means that of the total amount of water resources in the entire region, i.e. the combination of these three major components, the surface runoff from the mountain area comprise 93.22% of the total resources. However, when considering the repeated infiltration of the surface water, particularly the infiltration of the irrigation water and the re-use of the so-called return flow, the maximum water yield is much larger than the natural water resources mentioned above. Statistically speaking, the total permissible yield reaches x 10 8 m 3 year" 1 (Table 3), while the average rate of water re-use is about 40%.

5 7 Hydrological system of Hexi Corridor Table 2 The major components of total water resources (10 8 m 3 year 1 ) Surface runoff Subsurface runoff Precipitation infiltration Total water resource % 3.50% 3.28% 100% Table 3 Statistics of the total permissible yield (10 8 m 3 year' 1 ) Drainage system Shiyangho Heiho Suleho Total Maximum water yield Rate of water re-use (%) Table 4 Main elements of groundwater balance (1977) Recharge (X la m 3 year' 1 ) % Surface runoff South basin North basin Subsurface runoff Precipitation Total % Discharge (X 1& m 3 year" 1 ) % Springs Evaporation Withdrawal Others 0.03 Total % Due to groundwater resources, the recharge of the surface water, including both stream infiltration and canal seepage loss, comprises more than 90% of the total recharge, while for discharge, the outflow of spring clusters is most dominant and reaches 45% of the total amount (Table 4). According to recent data, the exploitation of groundwater is also becoming one of the important factors; it already reaches 15 x 10 8 m 3 year" 1 (1980), 80% of which is concentrated in the Shiyangho basin. The relationship between the recharge and discharge is quite different in the upper and lower basins (sometimes including a middle basin), because of their different hydrogeological conditions. These relationships can be expressed

6 Chen Mengxiong 8 in the following simplified balance equations: upper basin middle basin lower basin I - S = W / - (S + Z + E) = W I - (Z + E) = W where / = surface water infiltration; S = overflowing springs; Z = evaporation; E = extraction; and W = variation of groundwater storage. HYDROLOGICAL EFFECTS DUE TO HUMAN ACnVTTIES During the past 30 years, many reservoirs and irrigation channels have been built for the development of water resources. These developments have resulted in the reduction of groundwater recharge and the depletion of spring outflow. The total recharge of 56 x 10 8 m 3 year" 1 in the 1950s decreased to 44 x 10 8 m 3 year" 1 in the 1970s (Table 5), while spring outflow decreased from 32.5 x 10 8 m 3 year" 1 in 1959 to 22.3 x 10 8 m 3 year" 1 in 1977 (Table 6). The rates of reduction of groundwater recharge and spring outflow are about 22% and 31%, respectively. The groundwater level widely and continuously declined about 5-10 m, reaching m. The rate of re-use decreased from 70% to 40% during the period The conditions in the Shiyangho basin are much more serious than in the other regions because of the higher degree of development of the surface water. The higher level of development of surface water, the greater is the loss, due to the following negative effects: (1) reservoirs and other waterworks suffer great evaporation loss; (b) spring irrigation of fields is being abandoned Table 5 Variation of groundwater recharge (10 s m 3 year' 1 ) Name of stream Shiyangho Heiho Suleho Total Table 6 Variation of spring outflow (10 m year' 1 ) Name of stream % Smyangho Heiho Suleho Total

7 9 Hydrological system of Hexi Corridor and replaced by well-irrigation due to the depletion of the overflowing springs; (c) the intake of surface water in the lower reaches gives rise to salinization of the soil, due to the rising of the groundwater level; (d) full utilization of the surface water in the upper reaches influences the inflow of the lower basin; (e.g. inflow of the Minqing basin in the lower reaches of the Shiyangho has been reduced from 5.7 x 10 8 m 3 year" 1 to * 10 8 m 3 year" 1 ; it has caused extensive decline of the groundwater table and has resulted in serious deterioration of the ecological environment, including the deterioration of water quality, drying up of springs and lakes, decay of vegetation, degeneration of pasture land and the extension of desertification); (e) waterworks require not only a large amount of investment for construction but also large funds for their maintenance; and (f) the entire region, especially that at the foot of the Qilienshan, is an active zone of neo-tectonics probably threatening the safety of the waterworks due to the probability of strong earthquakes. CONCLUSION AND DISCUSSION The drainage system in Hexi Corridor usually involves two or three basins, from the upper to the lower reaches, in the formation of a complete hydrological system. In the basins, the surface water and groundwater are inter-transformed as a closely unified body. The outflow of the surface runoff from the mountainous area is nearly equivalent to the total resources of the entire region. Therefore the problem of designing a management plan for the optimal allocation of the water resources and the integrated development for rational utilization of the surface water and groundwater is very important. From the point of view of social, economic and environmental advantages, much experience and many lessons learned in this region show that the full exploitation and use of groundwater resources, including the overflowing springs, is much more beneficial than exploitation and use of the surface water. Therefore, the building of water conservation constructions in the upper reaches should be limited, and a certain amount of seepage loss in channels should be permitted for the preservation of groundwater recharge in the gravel plain. This means that the original spring irrigation system of the green land must be protected as much as possible and the depletion of the springs kept at a lower, more reasonable degree. The best way to artificially regulate the water resources is to make use of the large natural underground storage capacity as an underground reservoir in the Gobi gravels, because there is absolutely no evaporation and the underground flow can be transferred naturally into the green land without any artificial waterworks. The intake of a certain quantity of surface water is certainly necessary for extending the irrigated area, but consumption in the upper reach must be limited and attention paid to preventing the salinization of soil in the middle or lower reaches due to the rising of the water table. Thus, in green lands, particularly in the zone of saline soil, the irrigation canal is more favourable

8 Chen Mengxiong 10 when combined with well irrigation and well discharges. On the other hand, the use of the favourable conditions of the repetition of infiltrations in irrigation fields is a good approach in stressing the ability of water yield. Optimal water distribution between upper and lower basins is another important problem area in a hydrological system. The main principle is to guarantee that the water surplus of the upper basin, i.e. the inflow to the lower basin, can satisfy the water requirements without any harmful side effects to the ecological environment in the lower basin. The degree of development and utilization of water resources in Heiho and Suleho drainages is still comparatively at a low level; there are some large water conservation projects, such as the Changma Reservoir of Suleho, now under consideration. The feasibility and the probable side effects of these projects, including the possible threat of earthquakes, must be carefully and thoroughly studied. Acknowledgements The author is grateful for help received from the hydrogeologists of the Geological Bureau of Gansu. The author would also like to mention that all the data used in this article are based on the research report of Professor S. P. Fan: "Research on the Distribution of Groundwater and its Rational Development in Hexi Corridor, Gansu Province," published by the Research Institute of the Geological Bureau of Gansu, 1984.