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Geography, population and climate


The Ganges-Brahmaputra-Meghna (GBM) river basin is a transboundary river basin with a total area of just over 1.7 million km2, distributed between India (64 percent), China (18 percent), Nepal (9 percent), Bangladesh (7 percent) and Bhutan (3 percent) (Table 1). Nepal is located entirely in the Ganges river basin and Bhutan is located entirely in the Brahmaputra river basin. The GBM river system is considered to be one transboundary river basin, even though the three rivers of this system have distinct characteristics and flow through very different regions for most of their lengths. They join only just a few hundred kilometres upstream of the mouth in the Bay of Bengal. Not only is each of these three individual rivers big, each of them also has tributaries that are important by themselves in social, economic and political terms, as well as for water availability and use. Many of these tributaries are also of a transboundary nature (Biswas, after 2006). The GBM river system is the third largest freshwater outlet to the world’s oceans, being exceeded only by the Amazon and the Congo river systems (Chowdhury and Ward, 2004).

The headwaters of both the Ganges river and the Brahmaputra river originate in the Himalayan mountain range in China. The Ganges river flows southwest into India and then turns southeast, being joined by many tributaries. After flowing into Bangladesh, the Ganges, Brahmaputra and Meghna rivers join and flow into the Bay of Bengal as the Meghna River. The Brahmaputra river (known as Yalung Zangbo in China) flows east through the southern area of China, then flows south into eastern India, turns southwest, then enters Bangladesh (where it is also called Jamuna) before merging with the Ganges and Meghna rivers. The tributaries of the Meghna river originate in the mountains of eastern India (the main one called Barak), flow southwest and join. The Meghna river flows southwest and joins the Ganges and Brahmaputra rivers before flowing into the Bay of Bengal (McEwen, 2008).

Bangladesh has been formed as the greatest deltaic plain at the confluence of the Ganges, Brahmaputra and Meghna rivers and their tributaries. About 80 percent of the country is made up of fertile alluvial lowland that becomes part of the Greater Bengal Plain. The country is flat with some hills in the northeast and southeast. About 7 percent of the total area of Bangladesh is covered with rivers and inland water bodies and the surrounding areas are routinely flooded during the monsoon.


It is estimated that at least 630 million people live in the GBM river basin. This is almost two-thirds of the population of Africa, while the size of Africa is about 18 times the size of the GBM river basin. In 2008, the total population in Bhutan, which is entirely located in the Brahmaputra river basin, was estimated at 687 000 inhabitants, of which 66 percent is rural. About 95 percent of the population lives in the southern subtropical zone or in the central mid-mountainous zone of Bhutan, mainly in the relatively gentle sloping areas of the river valleys. In Nepal, located entirely in the Ganges river basin, the total population was 28.8 million, of which almost 83 percent rural. The total population of Bangladesh is 160 million (73 percent rural) of which 122 million inhabitants live inside the GBM river basin approximately. The total territory of India has a population estimated at 1 181 million inhabitants (71 percent live in rural areas), of which 476 million inhabitants live inside the GBM river basin (WB, 2010). In the total territory of China, the population is about 1 345 million, of which 57 percent are living in rural areas. However, only 1.7 million inhabitants are estimated to be living in the GBM river basin (WB, 2010). Population density in the basin area ranges from 6 and 18 inhabitants/km2 in China and Bhutan respectively, to 195, 432 and 1 013 inhabitants/km2 in Nepal, India and Bangladesh respectively.

In 2008, access of population to improved drinking water sources reached 92, 88 and 80 percent in Bhutan, Nepal and Bangladesh respectively. In the total territory of India, 88 percent of the population had access to improved water sources and in the total territory of China this was 89 percent.

The GBM river basin contains the largest number of the world’s poor in any one region. The population is increasing steadily, population density is very high in a large part of the basin, and, unless the current development trends are broken, poverty will become even more pervasive. The region is endowed with considerable natural resources that could be used to foster sustainable economic development. Water could be successfully used as the engine to promote economic development in the region, which has been hindered because the most populous part of the basin is shared by three countries - Bangladesh, India, and Nepal - which have in the past been unable to agree to an integrated development plan (Biswas and Uitto, 2001).


The GBM river basin is unique in the world in terms of diversified climate. For example, the Ganges river basin is characterized by low precipitation in the northwest of its upper region and high precipitation in the areas along the coast. High precipitation zones and dry rain shadow areas are located in the Brahmaputra river basin, whereas the world’s highest precipitation area is situated in the Meghna river basin (Mirza et al., 2011).

Precipitation in the Ganges river basin accompanies the southwest monsoon winds from July to October, but it also comes with tropical cyclones that originate in the Bay of Bengal between June and October. Only a small amount of rainfall occurs in December and January. In the upper Gangetic Plain in Uttar Pradesh (India), annual rainfall averages 760-1 020 mm, in the Middle Ganges Plain of Bihar (India) 1 020-1 520 mm, and in the delta region 1 520-2 540 mm. The delta region experiences strong cyclonic storms, both before the commencement of the monsoon season - from March to May - and at the end of it - from September to October. Some of these storms result in much loss of life and the destruction of homes, crops and livestock (Ahmad and Lodrick).

Nepal, located entirely in the Ganges river basin, experiences tropical, meso-thermal, micro-thermal, taiga and tundra types of climate. Mean annual precipitation is 1 500 mm, with a maximum of 5 581 mm recorded in 1990 at Lumle in Kaski district at an elevation of 1 740 m and a minimum of 116 mm recorded in 1988 at Jomsom in Mustang district. There are two rainy seasons in Nepal: one in the summer from June to September, when the southwest monsoon brings more than 75 percent of the total rainfall, and the other in winter from December to February, accounting for less than 25 percent of the total. With the summer monsoon, rain first falls in the southeast of the country and gradually moves west with diminishing intensity.

During winter, rain first enters Nepal in the west and gradually moves eastward with diminishing intensity. Temperature increases from the high Himalayan region to the lowland terai (northern part of the Ganges plain). Extreme temperatures recorded are -14.6°C in 1987 in Lo Manthang (Mustang district), located at an elevation of 3 705 m, and 44°C in 1987 in Dhangadhi (Kailali district), located at an elevation of 170 m. Precipitation falls as snow at elevations above 5 100 m in summer and above 3 000 m in winter. Temperature is a constraint to crop production in the Himalayas and the mountain region where only a single crop per year can be grown. On the other hand, in the lowland terai three crops per year are common where the water supply is adequate. Single rice cropping is possible up to elevations of 2 300 m while double rice cropping is limited to areas below 800 m.

The climate in Bhutan, located entirely in the Brahmaputra river basin, is cold in the north, with year-round snow on the main Himalayan summits, temperate in the inner Himalayan valleys of the southern and central regions, and humid and subtropical in the southern plains and foothills. Bhutan’s generally dry spring starts in early March and lasts until mid-April. Summer weather starts in mid-April with occasional showers and continues through the early monsoon rains of late June. Autumn, from late September or early October to late November, follows the rainy season. From late November until March winter sets in, with frost throughout much of the country and snowfall common above elevations of 3 000 m.

Temperatures vary according to elevation. In the capital Thimphu (elevation 2 320 m), temperatures range from approximately 14°C to 25°C during the monsoon season of June through September but drop to between about -4°C and 14°C in January. Most of the central portion of the country experiences a cool, temperate climate year-round. In the south, a hot, humid climate helps maintain a fairly even temperature range of between 15°C and 30°C year-round, although temperatures sometimes reach beyond 35°C in the valleys during the summer. Average annual precipitation is estimated at 2 200 mm, varying from a low of 477 mm at Gidakhom in Thimpu district to as high as 20 761 mm at Dechenling in Samdrup Jhongkhar district. The climate of the north is severe and cold with only about 40 mm of annual precipitation, primarily snow. In the temperate central regions, a yearly average precipitation of around 1 000 mm is more common and 7 800 mm has been registered at some locations in the humid, subtropical south, giving rise to the thick tropical forest. Western Bhutan is particularly affected by monsoons that bring between 60 and 90 percent of the region’s precipitation. The winter northeast monsoon brings gale-force winds down through high mountain passes.

Bangladesh has a tropical monsoon climate with significant variations in rainfall and temperature throughout the country. There are four main seasons: i) the pre-monsoon during March-May, which has the highest temperatures and experiences the maximum intensity of cyclonic storms, especially in May; ii) the monsoon during June-September, when the bulk of rainfall occurs; iii) the post-monsoon during October-November which, like the pre-monsoon season, is marked by tropical cyclones on the coast; iv) the cool and sunny dry season during December-February. Average annual precipitation over the country is 2 320 mm, of which about 80 percent occurs in the monsoon. It varies from 1 110 mm in the extreme north-west to 5 690 mm in the northeast. The country is regularly subjected to drought, floods and cyclones. Mean annual lake evaporation is 1 040 mm, which is about 45 percent of the mean annual rainfall. Mean annual temperature is about 25°C, with extremes as low as 4°C and as high as 43°C. Humidity ranges between 60 percent in the dry season and 98 percent during the monsoon.


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       Quote as: FAO. 2016. AQUASTAT website. Food and Agriculture Organization of the United Nations (FAO). Website accessed on [yyyy/mm/dd].
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