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 CHEN MENGXIONG Ministry of Geology and Mineral Resources, Beijing 100812, 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 com­ porte 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 con­ figuration 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. INTRODUCTION 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 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 km2. It is known as a typical arid Gobi Desert area, in which precipitation is only 50-200 mm and is gradually reduced from east to west or from south to north, while strong potential evaporation reaches 2000-3000 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 4000-6000 m, is widely covered with glaciers and snows, especially in the western part of the range, and comprises an area of about 1335 km2. All the streams of this area originate from the high mountain Qilienshan. The precipitation in the mountainous area reaches 400-600 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 67.30 x 108 m3 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 1 s (km ) x ic? m year (%) (%) (%) Shiyangho 64.82 15.44 65.4 4.5 30.1 Heiho 436.65 37.22 54.1 11.5 34.4 Suleho 833.28 14.64 22.9 37.6 39.5 Total 1334.75 67.30 49.8 15.7 34.5 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 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 (Q2.3), with a total thickness of about 500-1000 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 67.30 x 108 m3 year"1, but the underground flow passing through the alluvial aquifer Chen Mengxiong 6 RE, => C2 SWS: RE2 s, S3 s4 P + M SP, F=mSi- 4B- -US CWS : .JL JL1, -» G2 G< G5 - -» G3 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 SWS SWS SWS su / V 1 k ^ " K —> s, SU —> S2 su ^^1 S3 LK , » M ' ^ \ M GWS GWS GWS 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 108 m3 year"1; i.e. the total inflow to the basin is about 70 x 108 m3 year" . In the plains area, the infiltration of local precipitation, including condensation water, is only 2.42 x 108 m3 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.