GBM Basin

Description of GBM Basin Hydrology

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Palash, W., Y. Jiang, A.S. Akanda, D.L. Small, A. Nozari, and S. Islam2018A Streamflow and Water Level Forecasting Model for the Ganges, Brahmaputra, and Meghna Rivers with Requisite Simplicity. J. Hydrometeor., 19201–225, https://doi.org/10.1175/JHM-D-16-0202.1

Physical Setup

The GBM river basin system, with a combined drainage area of approximately 1.5 million km2, is the third largest freshwater outlet to the ocean (Chowdhury and Ward, 2004). Although the GBM system is often referred to be one large river basin, three river basins of this system feature unique geomorphology and flow regime, and the tributaries of these river basins flow through different ecological, social, economic and political realm (Biswas, 2008).

The Ganges Basin

The Ganges River – draining about 907×103 km2 of land area (Biancamaria et al. 2011) across India, Nepal, China, and Bangladesh – originates in the Gangotri Glacier of the northeastern Himalaya, India and travels about 2000 km towards southeast before entering Bangladesh. In Bangladesh, the river continues to travel 225 km southeast further and joins with the Brahmaputra River near Aricha. The larger part of the Ganges basin falls inside India, about 79% while 14% drains in Nepal, 4% in Bangladesh, and the remaining 3% in China.

The Brahmaputra Basin

The Brahmaputra River – draining about 573×103 km2 (Sarkar et al., 2003) of land area across China, India, Bhutan, and Bangladesh – originates in the Kailas range of southwestern Tibet, China. The river with the name the Yarlung Tsangpo travels about 2000 km from west to east across the Tibetan Plateau first, then travels another 1000 km from northeast to southwest through Assam Valley, India and enters into Bangladesh. The river then travels another 230 km south and joins with the Ganges River near Aricha. About 50.5% of the basin area is located in China, 7.8% in Bhutan, 33.6% in India and the remaining 8.1% in Bangladesh (IWM, 2013).

The Meghna Basin

The Meghna basin, compared to other two basins of the GBM system, is much smaller in size and draining about 82×103 km2 area. The Barak is the most important upstream tributary of the basin that originates from Japvo peak at the border of Nagaland and Manipur, India. The Barak basin drains 30% of the Meghna (Singh et al., 2013). After entering Bangladesh, the river splits into two branches at Amalshid – the Surma on the north and Kushiyara on the south – join again near Bhairab Bazar, travels towards south and meets with the Padma River. About 57% of the Meghna basin area falls in India and remaining 43% in Bangladesh.

The combined flow of the Ganges and Brahmaputra travels about 100 km towards southeast with name the Padma, joins with the Meghna near Chandpur, and travels 140 km further south with name the Lower Meghna before empties into the Bay of Bengal (Palash, 2008)..

Hydrology

The hydroclimatology of the GBM system is diverse and nonhomogeneous but similar in seasonal patterns of rainfall occurrence and streamflow generation (Rasid and Paul 1987). For example, the southwest monsoon originating from the Bay of Bengal brings more than 70% of average annual rainfall during June–September over the GBM. The western Ganges receives relatively less annual rainfall than the rest, about 760–1020 mm, while the middle and east receive 1020–1520 mm and 1520–2540 mm, respectively (FAO 2016). The Brahmaputra basin shows a large north– south gradient in annual precipitation. The upper Brahmaputra in the Tibetan Plateau receives 300mm of annual rain, whereas it is 1200 mm in the east of the lower Brahmaputra and 6000mm in the south over the Meghalaya Plateau (IWM 2013; Bajracharya et al. 2015). The annual rainfall in the western Meghna basin ranges from 2150 mm over the northeast haor region of Bangladesh to 6000mm over the southern foothills of the Meghalaya in India. The UM (Barak) receives the highest annual rainfall, from 1700mmin the east to 3000 mm in the west. Because of the seasonal hydroclimatology, there is a strong difference in average annual and peak discharges of these rivers. Table 4.1 shows the key physical and hydrologic features of the GBM basins while Figure 4.3 shows the seasonality in the hydrology of these three river basins, with large variations between the monsoon and nonmonsoon months’ rainfall and river flow.

Figure 1: (left) The seasonality of the average monthly discharge and (right) rainfall–discharge relationship of the GBM basin at HB, Bahadurabad, and Amalshid. The box-and-whisker plots in the left panels show the upper and lower quartiles deviated from the median as a box. The whiskers, the lines extending vertically from the boxes, indicate variability (maximum and minimum) outside the upper and lower quartiles of monthly average flow. In the right panels, monthly rainfall is plotted on the primary y axis, with values in reverse order.

Supply & Demand

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Uncertainty

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Water Management

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