Nepal is rich in water resources. There are about 6000 rivers in Nepal having a drainage area of 191000 sq. km, 74% of which lies in Nepal alone. There are 33 rivers having drainage areas exceeding 1000 sq. km. Drainage density expressing the closeness of spacing of channels is about 0.3 km/sq. km. the surface water available in the country is estimated to be about 225 billion m3 per annum, equivalent to an average flow of 7,125 m3/s (WECS, 2003). The cumulative length of rivers is 45,000 km. There are 1000 rivers longer than 10 km and about 24 of them are more than 100 km. These rivers are reliable sources of water and also provide potential opportunities for hydropower generation and irrigation downstream.
Their flow regimes are depending on the melting of snows and glaciers, monsoon precipitation and groundwater. Contribution from groundwater yield maintains the minimum flow level and prevents drying during non-monsoon periods. The rainfall varies greatly from place to place due to sharp topographical variations. Most of the rainfall falls over the foothills of the Lower Himalayas and this rainfall increase with altitude on the windward side and sharply decreases on the leeward side.
There are many snowfalls in the Himalayas during pre-monsoon season. Pre-monsoon rain in Nepal is associated with thermal convection combined with orographic effects, resulting in strong thunderstorm activities leading to heavy precipitation over narrow bands within the region. Monsoon rainfall amounts more in eastern Nepal whereas precipitation from westerly disturbances brings more in the western part. The pre-and post-monsoon rains are more intense at higher elevation.
Nepal has an agriculture-based economy, which relies heavily on stream flows and seasonal monsoon rains, which could be affected by potential long-term trends in streamflow or river flow. Information about river basins facilitates to analyze and inform basin management, information on river flow volumes, condition of freshwater habitats and species, as well as the degree of impact on rivers and freshwater biodiversity are still lacking for many parts of the world. The assessment of hydrologic water balance from large basin areas is difficult as it requires reliable data on river discharges, actual evapotranspiration, irrigation water requirement, and recharge to aquifers, groundwater abstraction. The process is complicated due to uncertainties in the hydro-geological evaluation and inters basin groundwater flows.
Literature Review
The Himalayan region, also known as the water tower of Asia, plays an important role in regional climate, most particularly with respect to monsoon circulation. Rivers form a hydrological mosaic, with an estimated 263 international river basins covering 45.3% of the land surface area of the earth, excluding Antarctica.
In the Hindu Kush-Himalayan region, there have been a few studies of river basins in China and India (e.g. Yang et al. 2005, Zhang et al. 2006, Chen et al. 2007, Lin et al. 2008, Bhutiyani et al. 2008). Lin et al. (2008) reported an upward trend for annual and seasonal averages of streamflow
in the Lhasa River basin on the Tibetan Plateau, which was paralleled by an upward trend in temperature and precipitation. In contrast, Bhutiyani et al. (2008) reported a statistically significant downward trend for total annual and monsoon streamflow in the Beas and Sutlej basins and an upward trend of winter streamflow in the Chenab basin of the northwestern Himalayan region in India.
Nepal is bordered by the Himalayas in the north and the Indian plains to the south (Shrestha et al., 2008). The country has a diverse topography, which can be classified into five physiographic zones extending from the east to the west of the country (NCVST, 2009). DHM indicates that 70–80% of the annual total precipitation and river flow occurs during the monsoon (Gautam and Phaiju, 2013; Shrestha et al., 2014). Snowmelt contributes to pre-monsoon and post-monsoon streamflow. The concentrated period of high rainfall results in dramatic changes in flow and water level from summer to winter.
Currently, about 10% of total precipitation in Nepal falls as snow, about 23% of Nepal’s total area lies above the permanent snowline of 5000 m., about 3.6% of Nepal’s total areas are covered by glaciers. There are 3,252 glaciers covering an area of 5,323 sq. km with an estimated ice reserve of 481 km3. There are 2323 glacial lakes in Nepal covering an area of 75 sq. km.
On the basis of their morphological and sediment characteristics, the rivers in Nepal can further be classified into:
Antecedent rivers:
These river valleys are narrow and deep in the higher Himalayan Ranges. The exceptions are those in the glacial areas where they mostly are U-shaped; waterfalls, cascades, and rapids are very common; and the landscape is dotted with numerous glacial lakes and pools
Mahabharat rivers:
The rivers originating from the northern face of the Mahabharat mostly have a north-south flow; the gradient is high, and the valleys are deep and narrow.
Churia Hill rivers:
The rivers at the northern face join with the Mahabharat rivers and are steep and straight. But, those at the southern face rarely follow an east-west course but go straight to the Terai instead.
Terai rivers:
These rivers mostly originate from the spring line of the Terai. They follow a fairly straight path and join either with the Mahabharat or the Churia rivers near or at the Indo-Nepal border. (WCES, 2011).
There are major snow-fed rivers, such as the Koshi, Gandaki, Karnali and Mahakali, along with mid-size rain-fed rivers such as Mechi, Kankai, Bagmati, East Rapti, West Rapti and Babai. In addition, there are intermittent rivers originating from the Siwalik Hills that are subject to frequent flash floods and carry high sediment loads (Sharma, 1997), Identification of any trend in the streamflow in a basin can thus be important for understanding the impact of climatic variability and changes in the region. The monthly flow trend of some of the rivers indicates that the contribution of snowmelt in runoff is in an increasing trend for snow-fed rivers, similarly, for non-snow-fed rivers, dry season flows are decreasing and wet season flows are increasing.
Snowmelt is the primary source of pre-monsoon flow in basins lying above the snowline. Although most of the flow peaks occur during July and August, some stations show late peaks during August and September. Snowmelt contributes pre-monsoon streamflow (March–May) and is not significant in other months (Sharma, 1997). Western Nepal spreads over 50,000 km2 of the headwaters of the Ganges basin in South Asia. Any impacts in the headwater will have implications in the downstream communities and ecosystems in the Ganges basin as well.
Climate change is having major impacts on water resources, affecting the quantity, quality and timing of water flow in many places. These changes are likely to increase as climate change advances. Taking a holistic river basin approach to climate adaptation can bring many advantages when building resilience in natural and human systems and addressing conflicts that will increasingly arise as climate change advances (Oglethorpe et al., 2015).
Discussion
River basins of Nepal Nepalese can be grouped broadly into (i) High Himalayan river basins, (ii) High mountain river basins (iii) Middle Mountain river basins (iv) Siwalik river basins and (v) Tarai river basins. High Himal region has more snow and ice melt contribution whereas High mountains and the Middle mountains find base flow sources with less frozen winter melt. Rivers originating from Siwalik region are mostly ephemeral. Rivers in Tarai mostly are fed by groundwater flow. (Bhusal et all., 2008)
Mainly there are four major basins :
Mahakali Basin
The river Mahakali originating from Api Himal and Trans Himalayan region and is called by various names viz. Kali, Mahakali, Sarda along its course falling in India and Nepal. It has total catchment area of 15,260 sq.km at Banbasa of which 35.4% lies on Nepal.
Before the river encounters Tanakpur barrage (Sarda Ghat) and later Banbasa Barrage, the Kali river exhibits a free riverine character comprising four districts of Baitadi, Dadheldhura, Kanchanpur and Darchula.(DSCWM)
Two major sub basins of mahakali on Nepalese side are
a. Chamelia
b. Surnagad
The Mahakali River descends from 3600 m at Kalapani in Nepal to 200 m as it enters the Tarai plains. The river flows through Uttaranchal in India, boarders between India and Nepal and continues to flow down India. Being largely snow-fed basins, they are also vulnerable to climate change impacts. These rivers see a short-term increase in water availability during the dry season followed by long bouts of dwindling water availability.
Karnali Basin:
The Karnali is the longest river in Nepal and one of the principal resources of water for the western Nepal. It originates from the Mansarovar and Rakas lake and receives many snowfed rivers such as Mugu Karnali and Humla Karnali at the Himalayan belt. The basin formed by the river has a total catchment area of 127,950 sq.km, of which 45 percent is in India.
The Karnali River basin lies in western Nepal. It is a perennial transboundary river that originates in the Himalayas. It has three main tributaries (West Seti, Bheri and Karnali), which drain an area of 45 000 km2 above the Chisapani gauge station (Zurich, 2015). The Chisapani gauge station marks the boundary between the upper catchment dominated by steep-sided mountainous valleys and the lower catchment consisting of a flood plain that runs to the Indian border.
The climatic regime of the Karnali River basin is affected by the monsoon (June–September) and westerly circulation systems, with the former being dominant (Gautam and Phaiju, 2013). Interestingly, unlike most of the rivers in Nepal, which generally flow from north to south, Mugu Karnali flows from east to west, and the Humla Karnali from west to east. The Karnali River flows through the western part of Nepal and joins the Mahakali River in India, which is called the Ghaghara in lower reaches of India.
The KRB is mostly susceptible to drought and higher food prices; it will take longer for households in this region to recover from climatic shocks. The six sub basins of the Karnali are:
a. Sani Bheri
b. Thuli Bheri
c. Mugu Karnali
d. Humla Karnali
e. SudhiGanga
f. Seti
Gandaki River Basin:
The Gandaki basin in Nepal covers 32,057 square kilometers, and encompasses a varied topography from the trans-Himalayan desert and the snowcapped high Himalaya mountains in the north, down through the mid-hills to the Churia (Siwalik) range and the low-lying plains of the Terai in the south.
Elevation ranges from 8,091 m at the peak of Annapurna I, the tenth highest mountain in the world, to around 200 meters above sea level in the Terai. A small part of the basin lies in China to the north; the basin drains through the Narayani River in the south to India (Oglethorpe et al., 2015).
The basin is inhabited by over 4.5 million people of diverse ethnicities. In rural areas people are still heavily dependent upon forests and ecosystem services for their livelihoods and wellbeing; the basin has about 35% forest cover (Ministry of Forests and Soil Conservation, 2015). The Gandaki river is known as the Narayani in the plains of Nepal and as the Gandak in India, where it joins the Ganges (Ganga) near Patna. It’s also called ‘Saptagandaki’ because of its seven sub basins, which are as follows:
a. Trishuli
b. Kaligandaki
c. Buddhi Gandaki
d. Daraundi
e. Seti Gandaki
f. Madi
g. Marshyangdi
Koshi River Basin
The Koshi flows inside the eastern part of the Nepal. It is the biggest river in the Nepal. About 43% of this totalarea lies in China, 42% in Nepal and the remaining 15% in India (Sharma, 1997; FMIS, 2012). The Koshi River drains most of the eastern part of Nepal. The Koshi is 720 km long and drains an area of about 74,500 sq.km in Tibet, Nepal and Bihar. It has been known as ‘Saptakoshi’.
The Koshi River Basin covers three major ecological zones of Nepal with a transverse length (north-south) of about150 km. These zones are: (i) Snow covered Himalaya in the north, (ii) hilly region in the middle and (iii) plain region of Terai in the south. The variation of altitude in this short north–south reach is quite sharp ranging from 95 m to 8848 m. The High Himalayan region of the Koshi basin within Nepal is about 8220 km2(>3000 m) where glacial lakes are common. ICIMOD (2011) mapped 599 glacial lakes in the Koshi Basin covering an area of 26 km2.
The upstream Himalaya part of the Koshi Basin covers an area of about 17,620 km2, mainly covered with forests and agricultural land. This region is the high rainfall receiving zone of the basin. The downstream part in the Terai region of Nepal covers an area of 2000 km2 before it enters into Indian Territory.
Livelihood of hundreds of millions of people of Nepal and India is dependent on the water availability in the Koshi River and, at the same time, frequent floods and drought have rendered millions vulnerable to their impacts (Chen et al., 2013). The seven sub basin of the Koshi are:
a. Arun
b. Tamur
c. Dudhkoshi
d. Likhu
e. Tamakoshi
f. Bhotekoshi
g. Sunkoshi
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| River basins of Nepal |
It is seen that around 78% of the average flow in the country is available in the four major basins, 9% in the medium basins and 13% in the numerous small southern rivers of the Terai. As the southern slopes of the Mahabharat Range, the Himalayan Range and the eastern two third of the country receive the maximum precipitation, there is more contribution from the flow of these catchments.
According to the precipitation trend analysis, the annual average precipitation over Nepal is decreasing at the rate of 9.8 mm/decade, however the Koshi basin shows increasing trend. Trend of the annual discharge of three major River basins Koshi, Gandaki and Karnali indicates that the discharges in these major basins are decreasing annually but, the annual discharges in southern basins were in increasing trend.
All the four major river systems predate the uplift of the main Himalayan Ranges and have kept pace with the uplift so that they now cut through the ranges in deep valleys. In the mountain and midland zones, the geology tends to promote rapid run-off resulting in a dense network of small, steep streams draining into the major rivers. In contrast, the geology promotes groundwater storage in the Siwalik Hills, on the Terai margins and the inner Terai.
Conclusion
There are about 6000 rivers flowing across different parts of the country. These rivers are mainly fed from the water of the snow, ice, glaciers, monsoon precipitation and ground water.
All these rivers are categorized into four major river basins namely Mahakali, Karnali, Gandaki and koshi river basin. Each river basin is composed of different sub-basins.
The estimated runoff from different basin is about 7125 m3/sec and from Nepal is 5479m3/sec with drainage area of 147181 km2.
Each basin is characterized by large populations with varying needs for agriculture, drinking water and energy and on the same hand the communities also face floods and many a times extreme events thus making them integral to the coping strategy.
The water resources development should be included in the top priority in the national development policy among these basins.
There is also need of multi-sectoral and multi-scale adaptation approaches in river basins, including the need for appropriate institutional structures and policy frameworks.
Reference
Bhutiyani, M. R., Kale, V. S., & Pawar, N. J. (2008). Changing streamflow patterns in the rivers of northwestern Himalaya: implications of global warming in the 20th century. Current Science, 618-626.
DHM, (1999)f. Daily precipitation records of Gandaki and Narayani Zone through 1996, Department of Hydrology and Meteorology, WB/HMG, Kathmandu, Nepal.
DHM, (1999)h. Daily precipitation records of Mahakali, Seti, and Karnali Zone through 1996, Department of Hydrology and Meteorology, WB/HMG, Kathmandu, Nepal.
DHM, (2000). Daily precipitation records of Bheri, Rapti and Dhaulagiri zone through 1996, Department of Hydrology and Meteorology, WB/HMG, Kathmandu, Nepal.
DHM, (2000). NISP (Nepal Irrigation Sector Project), Year Book, November, Institutional Development of the Department of Hydrology and Meteorology, WB/HMG, Nepal.
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FMIS, (2012). Flood Management Information System. Water Resources Department, Bihar, http://fmis.bih.nic.in/Riverbasin.html#kosi (accessed December,2013) Gautam, D. K., & Phaiju, A. G. (2013). Community based approach to flood early warning in West Rapti River Basin of Nepal. IDRiM Journal, 3(1), 155-169.
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ICIMOD, (2011). Glacial Lakes and Glacial Lake Outburst Floods in Nepal. International Centre for Integrated Mountain Development (ICIMOD), Kathmandu,Nepal Khatiwada, K. R., Panthi, J., Shrestha, M. L., & Nepal, S. (2016). Hydro-climatic variability in the Karnali River basin of Nepal Himalaya. Climate, 4(2), 17.
Lin, X., Zhang, Y., Yao, Z., Gong, T., Wang, H., Chu, D., ... & Zhang, F. (2008). The trend on runoff variations in the Lhasa River Basin. Journal of Geographical Sciences, 18(1), 95-106. M. R. Gautam & K. Acharya (2012) Streamflow trends in Nepal, Hydrological Sciences Journal, 57:2, 344-357, DOI: 10.1080/02626667.2011.637042
NCVST: Vulnerability through the eyes of the vulnerable: climate change induced uncertainties and Nepal’s development predicaments, Institute for Social and Environmental Transition – Nepal (ISET-N) & Institute for Social and Environmental Transition, Kathmandu & Boulder, CO, available at: http://i-s-e-t.org/resources/major-program-reports/ vulnerability-through-the-eyes-of-vulnerable.html, last access:10 January 2017, 2009. Oglethorpe, J., Regmi, S., Bartlett, R., Dongol, B. S., Wikramanayake, E., & Freeman, S. (2015). The value of a river basin approach in climate adaptation. Organizers, 57. Sharma, C. K. (1997). A Treatise on water resources of Nepal. Sangeeta Sharma. Smith, P. J., Brown, S., & Dugar, S. (2017). Community-based early warning systems for flood risk mitigation in Nepal. Natural Hazards and Earth System Sciences, 17(3), 423-437
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1 Comments
Excellent summary of the river basins of Nepal.
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