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Modelling and Management of Sustainable Basin-scale Water Resource Systems (Proceedings of a Boulder Symposium, July 1995). IAHS Publ. no. 231, 1995. 151
Research on the rational use of water resources on the Lhasa River, Tibet
SHEN DAJUN Department of Hydrology, Institute of Geography, Chinese Academy of Sciences, Beijing 100101, China
Abstract This paper is an effort to clarify the use of water resources for hydropower generation without negative environmental and social impacts in the Lhasa River basin. A water budget is formulated and strategies for the rational use of water resources of the Lhasa River, which include construction of the Pangduo and Yangjingshi Reservoirs and transferring water from the main stream to the Pengbo River basin, are also discussed.
INTRODUCTION
The Lhasa River, with an area of 32 471 km2, is the largest tributary in the middle reach of the Yaluzangbu River (Fig. 1). It is located in the southern valley and has an average altitude of about 4500 m. It has a semiarid monsoon climate, with distinct dry and wet seasons. The low average temperature is between 1.2-7.5°C, solar radiation is 6954.3-7984.2 MJ m~2 and there is sunshine time of 2367-5196 h. Potential evaporation is 1700-2200 mm. The geological conditions in the river basin are complex. The neotectonic movement is active, and is accompanied by frequent earthquakes.
The Lhasa River basin is the political, economic and cultural centre of Tibet. In 1990 there were 329 700 residents, about 15% of the whole population of Tibet. Of that number, 208 700 were farmers and 88% Tibetans. Agriculture is the main activity in the river basin. The agricultural output of the Lhasa region in 1990 was 81.29 M yuan ( 19 80 price level), with a pastoral output of 29.94 M yuan and forestry output of 1.23 M yuan. Industry includes mainly electric power, automobile maintenance, agricultural machine production and chemical building materials. The total industrial output in 1989 was 76.85 M yuan. However the development of the region is now limited by an energy shortage.
WATER RESOURCES
Water in the river basin comes mainly from the warm and humid air flow from the Indian Ocean. Typical precipitation is a monsoon rain. Based on data from the Lhasa Hydrological Station between 1952 and 1990, average annual precipitation is 466.3 mm. The highest concentrations occur from June to September, representing 85 % of the year, while no more than 6% occurs from November to February. Influenced by topography, there is more rainfall in the mountains, and less in the river valley; more in the upper
152 Shen Dajun
Fig. 1 Sketch map of the Lhasa River basin.
reach, less in the downstream reach; about 700 mm in the northeast, 400 mm in the southwest, and less than 400 mm in the Pengbo Basin.
According to survey results from the Lhasa Hydrological Station, the average annual runoff is 335 m3 s"1, amounting to 10.55 billion m3 annually with a runoff depth of 325.4 mm. The groundwater feeding storage is 2.612 billion m3. Due to the variation in precipitation and temperature, the runoff distribution during the year is very uneven. Flood season, from June to September, accounts for more than 75% of the yearly runoff, with more than 20% in July. Water quantity from January to March accounts for only 3.6%-5.3% of the year by runoff.
The river basin has a large hydropower potential of 2 560 000 kW. The exploitation conditions are good: hydropower concentrates around the middle reach and could be exploited either in steps or centrally. Water quality of the Lhasa River is very good too. There is very little sewage discharge and very little use of chemical pesticides and fertilizer.
So, water resources in the Lhasa River basin are sufficient. Annual water quantity per capita is 32 000 m3. Hydropower potential per capita is 7.76 kW. Both can satisfy the water and power needs in the future if rationally developed.
CURRENT USE OF WATER RESOURCES IN LHASA RIVER BASIN
There are about 35 258 ha of farmland in the river basin, with 17 710 ha under irrigation, another 27 768 ha awaiting irrigation, and 1 850 000 ha of usable grasslands without any irrigation. There is, as well, 100 000 ha of forest. In the main stream down Tangjia, there are 31 irrigation canals, with total transfer capacity of 115.5 m3 s"1. They
Research on the rational use of water resources on the Lhasa River, Tibet 153
control 10 000 ha of farmland, of which 8666 ha could be irrigated. The farmland in the Pengbo Basin is 14 924 ha, of which 9333 ha could be irrigated.
Fourteen reservoirs have been built with a total capacity of 26.37 M m3. The Hutuoshan Reservoir is the largest one in Tibet, with a planned total storage of 12 M m3. Up until 1989, 83 medium and small hydroelectric stations, with a total fixed capacity of 26 124 kW, have been built, four of them being key stations in the Lhasa Electric Net.
RATIONAL USE OF WATER RESOURCES
Water use in the future
According to the social and economic development strategies of Tibet and the irrigation plans in the Lhasa River basin, irrigation would reach 39 333 ha with the assured irrigation ratio (P) of 75%. The irrigation of grasslands and forest would reach 17 933 ha in 2000 and 31 633 ha in 2010, with an assured irrigation ratio of 50%. It is calculated that, in 2000: (a) net agricultural water use will be 253 M m3; and (b) gross agricultural water use will be 460 M m3; and in 2010: (a) net agricultural water use will be 458 M m3; and (b) gross will be 961 M m3.
In the Lhasa River basin in the year 2000, there would be 1.8 M livestock. In 2010, livestock would number 2 M. Adding to household and industrial water use, the total water use, excluding that of agriculture, will be 40 M m3 in 2000 and 73 M m3 in 2010.
According to electricity planning for Tibet, one or more large hydroelectric stations will be constructed in order to provide energy for the middle of Tibet. At the same time, water resources utilization in the Lhasa River basin must provide for flood protection of Lhasa city, towns and farmland along the river banks.
Water budget
Considering the natural and socio-economic conditions in the river basin, we divided water use in the Lhasa River basin into three districts: Duilong District (Duilong River basin), Pengbo District (Pengbo River basin), Mainstream District. Afterwards, a water budget was done (Table 1).
The water budget analysis conclusions were that: (a) Considering a complete year and the total river basin, water resources are sufficient.
When P = 75%, water deficiency in the arid season was 68.7 M m3 in 2000 and 142 Mm3 in 2010 and there was plenty of water in the humid season. Through regulation, water resources could completely meet the demand.
(b) From the point of view of periods and districts, water deficiency in the Pengbo District was very high, especially from March to May during the plant growing period. When P = 15%, water deficiency in 2000 was 48.6 M m3, 46.2 M m3 from March to May; 80.1 M m3 in 2010 and 69.9 M m3 from March to May.
154 Shen Dajun
There will not be enough water in the Duilong District from March to May, but the shortage will not be severe. In the Mainstream District, runoff could meet the demand for irrigation, but would cause water use competition between irrigation and hydroelectric generation.
Water resources — rational utilization strategies
The building of the Pangduo Reservoir is a key measure for rational utilization of water resources in the Lhasa River basin The Pangduo Dam is located in the middle mainstream, about 100 km away from Lhasa city, and it controls the catchment of 10 706.5 km2, with annual average runoff of 188.8 m3 s"1. The form of the valley at the dam site is "U" and the altitude of the river bed is 4030 m. The geological condition is good. It is planned that the normal water level of the Pangduo Reservoir would be 4080 m and equivalent reservoir storage would be 687 M m3. The minimum water level would be 4060 m and the available storage would be 447 M m3; the fixed generator capacity of the hydroelectric station would be 120 000 kW, with a safe yield of 20 900 kW.
Runoff regulation of the Pangduo Reservoir would meet water demand for hydroelectric production and agricultural irrigation use at the developing level of 2010 downstream from the reservoir. It is estimated that after its construction, the irrigated farmland would increase to 27 600 ha, and irrigated forest and grasslands will be 7933 ha. Through economic analysis, the total investment would be 588 M yuan (1990 price level), the net present worth would be 758 M yuan (1990 price level), the ratio of total benefit to total cost, (/?), is 2.28. So the solution is economically effective.
The Pangduo Reservoir would not only control the large area of farm, forest and grasslands in the middle and low reaches of the Lhasa River, which could become an agricultural region in the future, but it would also be the key reservoir for compre-
Table 1 Water budget of the Lhasa River basin in the years 2000 and 2010 (scale: 10* m3).
Districts
Duilong
Pengbo
Main stream
Periods
March-May
Year
March-May
Year
March-May
Year
Shortage/ excess
Shortage Excess
Shortage Excess
Shortage Excess
Shortage Excess
Shortage Excess
Shortage Excess
2000
75%
- 1 669.67 847.77
- 2 004.14 78 060.88
- 4 619.35 221.23
- 4 860.50 17 171.98
0 29 432.13
0 767 318.8
50%
- 1 449.33 3 744.04
- 1 7 3 1 . 1 6 95 401.83
- 4 023.61 447.16
- 4 037.62 25 654.38
0 111534.54
0 953 512.9
2010
75%
- 2 339.16 1 224.06
- 2 937.33 76 943.58
-6 985.34 190.38
- 8 014.44 16 204.42
- 3 252.81 16 448.47
- 3 252.81 741 165.41
50%
-2002.19 2 535.09
- 2 547.22 94 172.40
- 6 249.21 275.92
- 6 630.75 24 126.01
-141 .44 95 182.09
-141 .44 909 698.24
Research on the rational use of water resources on the Lhasa River, Tibet 155
hensive hydropower exploitation. Its large capacity would be very advantageous for downstream hydropower development (the potential hydropower capacity is about 300 000 kW and the fixed generator capacity would be nearly 400 000 kW).
In the meantime, the Pangbo Hydroelectric Station would be considered as the compensating station for the Yangcuoyong Lake Pump Storage Station in order to protect the lake's ecological environment.
Building the Pangduo Reservoir will inundate 95 ha of farmland, 31 ha of forest and 639 ha of grassland. The number of people to be moved is 510. After the reservoir is built, the terraces and platforms in the reservoir region would collapse due to the increased water level. Downstream from the reservoir, the main river course will be narrowed and the mobile channel will change shape. Besides, the building of the reservoir will influence the frequency of earthquakes in the Lhasa River basin.
To transfer water from the main stream is the basic way to solve the water shortage in the Pengbo River basin The runoff in the Pengbo River basin is the least among the three districts. The annual runoff is only 371 M m3 when P — 50%, and only 239 M m3 when P = 75%, while the irrigated area is the most: the irrigated farmland, forest and grasslands would be 10 733 ha and 1333 ha, respectively, with gross irrigation water use of 107 M m3 at 2000; 14 061 ha and 3333 ha in 2010, respectively, with gross irrigation water use of 144 M m3. So unless large reservoirs are built for storing flood rains in the river basin, it is impossible to resolve the severe water shortage problem in the river basin.
The rational way to solve the problem is to transfer water from the main stream. The transfer site would be located at Zhikong, with a catchment area of 20 179 km2 and annual average runoff of 237 m3 s"1. The designed transfer capacity of the main canal would be 40 m3 s"1, including irrigation water used along the main canal on the right bank of the main stream, of which 10 m3 s"1 would transfer into the Pengbo River basin, supplying 6000 ha of farmland. The main canal would be 105 km long. At the same time, a power station could be built with a fixed capacity of about 30 000 kW at the outlet of the Pengbo River in order to use the water during the non-irrigation period.
The frequent cryogenic weathering in the river basin would be the main problem for the water transfer project. It could cause building material to fissure and large leakage of water from the canal.
To build the Yangjinshi Reservoir is the effective method to alleviate the water use conflict between irrigation and hydroelectric generation, downstream of the Duilong River Along the left bank of the Duilong River is Qinghai-Xizang Road, and in the middle reach is the Yangbajing geothermal field. Therefore, the Yangjingshi Reservoir could only be built at an upstream location. The dam site controls the catchment of 1580 km2 and has annual normal runoff of 394 M m3. If the normal water level of 4360 m above sea level is achieved, the reservoir capacity would be 81 M m3, of which available storage would be 19 M m3. A power station of 4500 kW would also be constructed. Runoff, after regulation, could completely solve the conflict between irrigation and power station water use downstream. It would increase irrigation to farmland, forest and grassland area to 6667 ha and 2667 ha, respectively.
156 Shen Dajun
Acknowledgements The author wishes to thank Professor Chen Chuan You and the Director of the Lhasa Hydraulic and Electric Bureau for providing a lot of data on the Lhasa River.
REFERENCES
Chen Chuan You (1990) Research on Water Resources Exploitation Strategy in Southwest China. Chinese Sciences and Technology Press,
Shen Junfang (1989) Hydraulic Engineering Economy. Hydraulic and Electric Press.
