Hydrology in Mountainous Regions. I - Hydrological Measurements; the Water Cycle (Proceedings of two Lausanne Symposia, August 1990). IAHS Publ. no. 193, 1990. Hydrology and development of the Arun River, Nepal RICHARD KATTELMANN Center for Remote Sensing and Environmental Optics Computer Systems Lab, 1140 Girvetz Hall University of California, Santa Barbara, CA 93106.USA ABSTRACT The Arun River drains a large area of the Tibetan Plateau before crossing the Himalaya into Nepal where its discharge increases dramatically. The steep gradient and relatively high dry-season flow of the Arun have led to plans for major hydroelectric development. Little information about the hydrology of the Arun basin was available to guide the planning efforts. This review of the limited knowledge concerning this river basin illustrates the unique features of Himalayan hydrology that must be considered when assessing the potential for water resources development in this region. INTRODUCTION Water resources of the Himalaya are often viewed as the key to successful development of much of the Indian subcontinent. Year-round flow in the mountain rivers provides water for agriculture during the long dry-season and represents enormous hydroelectric potential. Hydroelectric generation in the Himalaya began in 1897 in Darjeeling. Large projects began to be built after Partition of India and Pakistan. In the past decade, much of the hydropower attention has been directed at Nepal, which is seeking rapid economic development, a product for export, and an alternative energy source to fuelwood. The combination of high precipitation and steep terrain provide Nepal with a theoretical potential for hydroelectric production of 83,000 MW, although only a fraction of this amount can be feasibly developed (Shrestha, 1983). Although many large-scale projects are under consideration in Nepal, hydroelectric development in the Arun Valley in the eastern part of the country appears to be the best prospect for the 1990's. Initially, a cascade of power projects was envisaged for the Arun in a broad plan for the development of the Kosi River basin (Japan International Co-operation Agency [JICA], 1984). Subsequent planning has concentrated on two sites where the river channel has dramatic bends (Figure 1), which minimize the tunnel length for a substantial head loss. These two proposed projects are called Arun-3 and Upper Arun. Pledges of US$550 million in international financing for Arun-3 have already been obtained (Bhattarai, 1989). Planning for these projects has necessarily proceeded with little hydrologie and climatic information. The Arun River example provides a case study of water resource assessment in a mountain region where data is lacking and the hydrology is poorly understood. GENERAL GEOGRAPHY The Arun is the largest trans-Himalayan river passing through Nepal and also has the greatest snow- and ice-covered area of any Nepalese river basin. The Arun drains more than half of the area contributing to the Sapt Kosi river system but provides only about a quarter of the total discharge. This apparent contradiction is caused by the location of more than 80 percent of the Arun's drainage area of about 30,000 km2 in the rain shadow of the Himalaya. Average annual precipitation in Tibet is about 300 mm (Liu, 1989). 777 Richard Kattelmann 778 ' *»_/ -border (China-Nepal) Upper Arun site Leguvaghat ®É^ j ^ of rQad (1M()) FIG. 1 More than 80 percent of the Aran's drainage area is in Tibet. In Tibet, the river is known as the Men Qu (Moinqu) in its upper reaches north of Xixabangma and then as the Peng Qu (Pumqu) for most of its course north of the Himalayan crest. After progressing eastward through arid grasslands, the Peng Qu turns south at the 4050 m elevation confluence with the Yarn Qu (Yeyuzangbu). The Peng Qu then flows through the narrow Yo Ri gorge and a broad valley before entering the Longdui gorge at 3500 m at a point below Kharta. The climate changes abruptly in this area from rain-shadow to monsoon-soaked (Howard-Bury, 1922). This portion of the Peng Qu basin may generate much of the streamflow that crosses the border. The Peng Qu crosses the Himalayan crest at an elevation of about 2175 m and becomes known as the Arun in Nepal. South of the Himalayan crest, the flow of the Arun increases rapidly downstream in the seasonally-humid environment of east Nepal. The 5000 km2 of land contributing water to the Arun inside Nepal is only 17 percent of the total basin area, but it provides more than 70 percent of the Aran's total flow at its confluence with the Sapt Kosi (JICA, 1984). The landscape south of the border tends to be steep with less than 15 percent of the area having a sustained slope of less than 15° and is strongly dissected by stream channels. Many of the hillslopes are structurally unstable, and the region is seismically active (Kansaker, 1988). An earthquake in August 1988 with an epicenter more than 50 km south of the Aran basin had a magnitude of 6.7 on the Richter scale and resulted in more than 100 deaths in the Aran basin alone (Dunsmore, 1988). Soils tend to be shallow (generally less than 20 cm deep) and stony (Goldsmith, 1981). The alpine zone above 4000 m covers about 5-10 percent of the lower Aran basin (Shrestha, 1988). Several large glaciers are found in the Baran River tributary near the 8000 m peak, Makalu. 779 Hydrology and development of the Aran River, Nepal The northern third of the Nepalese portion of the Arun basin supports a rich, though human-modified, forest of mixed hardwoods, Chir pine, fir, and rhododendron at elevations of 1000 to 4000 m (Cronin, 1979; Shrestha, 1988). The vegetation in the southern two-thirds of the area has been extensively modified for subsistence agriculture. Most of the half-million people in the Arun basin live in this southern area between 300 and 2200 m in widely scattered villages near the slopes they farm (Dunsmore, 1988). None of the four towns in the basin had more than 14,000 people in 1988. Less than 80 km of motorable road has been built in the low-elevation southeast comer of the basin. The potential for the basin to support either the existing or a growing population under subsistence agriculture is problematic and depends on active conservation of soil and forests (Dunsmore, 1988). HYDROELECTRIC DEVELOPMENT PLANS The immense hydropower potential of the Arun River has long been recognized from considerations of the large discharge and steep gradient. The first detailed assessment of the basin estimated that more than 1100 MW of capacity could be developed in a cascade of six generating stations (JICA, 1984). One proposed site, known as Arun-3, was particularly attractive because of a great S-shaped curve in the channel around a ridge of resistant rock. Because a tunnel could cut off the bend and drop more than 200 m in 11 km, this project was judged to be the most efficient development (Nepal Electricity Authority, 1986). Arun-3 would involve diverting up to 150 m3 s_1 from a dam across the Arun upstream of Num to a powerhouse of 400 MW capacity at Pikuwa. As detailed design work progressed, the estimated construction cost more than doubled from US$240 million (JICA, 1984) to US$550 million (Bhattarai, 1988). An access road of at least 170 km length is necessary to begin construction of the dam, tunnel, powerhouse and transmission line. A minimal road could cost about US$35 million (US$200,000 per km) and take 2 to 3 years to complete (Dunsmore, 1988). However, the planned route of the road has been extended to 193 km to serve as many towns as possible. This alternative alignment has delayed funding and extended construction time to at least 4 years. Consequently, the Arun-3 project is unlikely to be completed before the year 2000. Another site with characteristics similar to Arun-3 was identified in 1985 near the border with China. This so-called Upper Arun site also takes advantage of a dramatic curve in the channel to minimize the length of the headrace tunnel. This project as currently proposed would divert up to 120 m3 s-1 from a dam no more than 12 m high through a 7 km tunnel to an underground powerhouse (Nepal Electricity Authority, 1987). The Upper Arun project would have an installed capacity of 350 MW and could generate more than 3000 GWH annually. The total project cost including access road and transmission line is about US$400 million (Nepal Electricity Authority, 1987). All planning and design work for these potential hydropower developments on the Arun has been conducted with far less information about basic hydrology than is available for projects in North America or Europe. For example, the minimal information that was available at the time of the Kosi Basin Master Plan gave the appearance that streamflow was greater than precipitation in some basins. The critical lack of data and knowledge about the Himalayan hydrologie system has been a persistent difficulty in water resources development throughout the region (Kattelmann, 1987; Gyawali, 1989). The two principal questions that must be asked early in any planning efforts are: (a) How much water is reliably available for hydroelectric generation? (b) Is the environment conducive to construction and operation of generating facilities? Two recent hydroelectric projects elsewhere in Nepal illustrate the risks of constructing projects in the Himalaya with inadequate hydrologie information: (a) in August 1985, a nearly-completed small facility on the upper Dudh Kosi in the Mt. Richard Kattelmann 780 Everest region was destroyed by flood waters from a glacial lake outburst only 12 km upstream; (b) the reservoir that supplies electricity to the capital city of Kathmandu proved to have insufficient storage to operate continuously following below-average precipitation during the 1988 monsoon.