Inland Water Resources
Inland Water Resources Confluence of Rivers' Bangladesh lay on the confluence of three major rivers of Asia, named the Ganga, the Brahmaputra and the Meghna. These rivers fall in the Bay of Bengal, through this country. The watershed basins of these rivers are adjacent to each other, covering the center, north and northeastern part of South Asia, and the Great Himalayan Range. These basins altogether is around 10,87,000 square kilometers in area. The Ganga River originates from the southern slopes of the Himalayas, but its tributaries originate from both the Himalayas and the central highlands of India. The Brahmaputra River originates from the northern slopes of the Himalayas, in Tibet of China. Its tributaries originate from the Kailash Range, Tibetan Plateau, the Himalayan Range, the Naga Hills, the Lushai Hills and the Garo and Jaintia Hills. The Meghna River originates from the Naga Hills. Its tributaries originate from the Naga, the Lushai, and the Garo and Jaintia Hills.
Most of the rivers in Bangladesh are either tributaries or distributaries of the Ganga, or the Brahmaputra, or the Meghna River. An imaginary line from the Chhotonagpur Highlands to the Garo Hills, through the North Bengal, separates the entire Bengal Plain into two, the upper one and the lower one. The upper one to the northwest and north has all the tributaries of Ganga and Brahmaputra rivers. The lower one towards south has the distributaries of those two rivers. But towards east of this lower plain has the tributaries of Meghna River. The Bengal plain has some independent rivers to the west originating from the Chhotonagpur Highlands, and to the east originating from the Lushai Hills, and falling directly to the sea.
The Bengal plain is divided mainly into two countries, India and Bangladesh, and partly to Bhutan and Nepal. These divisions did not follow any natural barrier but was made politically. Thus, we can find the Dharla river originating in Bhutan travels through India then enters into Bangladesh. The Mechi River originates in Nepal passes through India to fall in the Mahananda River. The Mahananda River enters Bangladesh to fall in the Ganga River. Some rivers e.g., the Atrai, and the Punarbhaba originate in Bangladesh travel through India then again enter Bangladesh. Thus, the water resources generated in Bangladesh is very much inter-dependant between these countries.
Surface Water The source of surface water in Bangladesh is the rainfall fallen over this country, and the stream flow coming in from outside this country. With an average annual rainfall of about 2300mm, it generates about 276 Million-Acre Feet (MAF) of water. Water coming from outside by stream flow is estimated at about 818 MAF (Bangladesh Water Vision 2025, BWP) annually. Thus, the total water resource generated in this country is about 1094 MAF or 1350 Billion Cubic Meter (BCM) annually. However, most of this huge resource is drained down to the Bay of Bengal, and forms brackish and sweet water mixed ecosystems in the estuaries. The water brings down about 1400 million tons of silt annually, to deposit over the flood plains and in the Bay of Bengal. The surface water resource of this country is very much essential for its human and animal living, aquatic flora and fauna, navigation, agriculture, etc. It is also necessary for keeping alive the distributaries in the delta, and maintaining the brackish water ecosystem along the sea, on an annual cycle. The annual average open water evaporation from this country is about 1250 mm.
Surface Water Fluctuation The Bangladesh Water Development Board Act of 2000 gives sole authority to this Board to monitor and keep the water resources records all over the country. BWDB does it by its Department of Hydrology. It has 343 river-stage monitoring stations. These stations take readings on hourly basis round the year. There are 110 river-discharge measurement stations. These stations take readings on weekly or biweekly basis round the year. 'In addition, this Department has 269 rainfall and 39 open-water-evaporation measuring stations.
In Bangladesh, surface water fluctuations are very common in the rivers, as they vary on seasonal precipitation and remain dependant on the inflow coming from upland sources. Precipitation is significantly less during winter and summer. At these times, water demand for irrigation is high. But, India controls and withdraws water from all trans-boundary rivers. Withdrawal of the remaining surface and groundwater resource for irrigation in Bangladesh leads the rivers deplete and the wetlands vanish. Surface water monitoring is very much essential for water resources management and flood forecasting. Some 54 rivers are classified as trans-boundary between India and Bangladesh. These two countries have a Joint Rivers Commission (JRC) to monitor the river flows, and look into interventions on the rivers encroaching the international boundary. The JRC also works for amicable settlement of disputes over the use of trans-boundary water resources. One of its significant out-come is the Ganges Water Sharing Treaty of 1996.
Surface Water Storage The rivers and wetlands in Bangladesh are the places of storage of its large quantity of surface water. The wetlands, called the 'beel's and 'haor's are depressions caused by shifting of river courses, tectonic activities and land subsidence over ages. The open water bodies keep the dry season storage in winter, and through their drainage outlets contribute to the base flows of the outfall rivers. Many rivers have flood plains over the countryside, those go under water during the monsoon and the floods. These water storages fluctuate on the fluctuations of their linking rivers' water stages.
Among the closed water bodies, the 'baors' lay mostly in southwestern Bangladesh. These are horseshoe lakes formed by the dead courses of rivers. However, the traditional surface water storages all over the country are the ponds. The village people excavate these ponds to meet their day-to-day water demand for living. In saline coastal areas, embanked lowlands store sweet rainwater for crops and public use. These closed water storage fluctuates with the fluctuation of the ground water table in the area. Bangladesh has a dam built over the river Karnafuli at Kaptai creating an artificial reservoir of 8.25 million acre-feet capacity. This dam is used for producing hydro-electricity and flood control in the Karnafuli River basin. A barrage built over the Feni River at Sonagazi has created sweet water storage in its upstream of about 1.2 million acre-feet capacity.
Surface Water Diversion Surface water resource in Bangladesh is severely affected by the diversion of water from two major trans-boundary rivers, in India. The Ganga River water flow is intervened by a barrage at Farakka, and through a link canal, India diverts a large quantity of its flow to the Bhagirathi River in West Bengal. Similarly, the Tista River water is intervened by a barrage at Gazaldoba and diverted to the irrigation projects in Bihar and West Bengal. India also diverts water from other smaller trans-boundary rivers. Bangladesh and India signed a Treaty on the Ganga River Water Sharing in 1996, but the dispute of Tista River water sharing is yet to be resolved. India in West Bengal withdraws water by putting control structures on the Atrai and Punarbhaba Rivers in South Dinajpur, the Bhairab in Nadia, and the Betna and the Kodla in North 24 Pargana. India holds and diverts water flow of the Feni, Gomti, Bijni, Khowai, Manu and some tributaries of the Barak River by putting control structures over them. Thus, sharing of water of all trans-boundary rivers remains a regular agenda put forward by the Bangladesh side to the India Bangladesh Joint Rivers Commission.
Inside Bangladesh, several barrages are built to divert river water for irrigation and storage. The Tista River in North Bangladesh has a barrage over it, with a diversion capacity of 10,000 cusec of water for the Tista Irrigation Project. A similar barrage structure is there on the Manu River to the northeastern Bangladesh. This has a diversion capacity of 530 cusec of water for the Manu Irrigation Project. There are barrages over the Tangon River in Thakurgaon District and Buri Tista River in Nilfamari District for irrigation and storage. On the right bank of Ganga River at Bheramara, a Pump House can lift about 4400 cusec of water for the G.K. Irrigation Project spread over Kushtia, Chuadanga, Jhenidah and Magura districts.
There were many historical cuts made in Bangladesh to divert river water and facilitate navigation. Many of those cuts changed the main river courses to different directions. In North Bangladesh, the Kata Khal diversion shifted the main flows of Karotoa River towards the Bangali River. In southwest Bangladesh, the Mathabhanga River was the old diversion canal of the Kumar/Nabaganga River to rejuvenate the dying Bhairab River. This diversion work put death to the Nabaganga River. A Gaznabi Cut to link the off-take of Nabaganga River could not reinstall the flow path of the river. To the south, the Halifax Cut for navigation towards the Khulna City diverted 80% of the Madhumati River water to the Nabaganga River. The Madaripur Beel Route cut for navigation diverted the Kumar River flows in total towards the Madhumati River. The Barrah Navigation cut in Nababganj diverted the main flows of Ichhamoti River towards the Bhangabhita River. The navigation cut linking the bazaars of Barmi and Toke diverted the main flow of Old Brahmaputra River towards the Banar River.
Surface Water Augmentation Change of river courses over recent years made huge surface water loss, particularly in the southwestern region of Bangladesh. The Chitra River in Chuadanga and Jhenidah, the Kobadak and Betna Rivers in Jessore, the Harihar, Bhadra and Mukteswari Rivers in Khulna, and the Marichap River in Satkhira districts dried up due to the change of' Bhairab River course. The bad effects of these changes were drought in the western Bangladesh, and intrusion of salinity deep inland. At the center, a major change of the Dhaleswari River course left its old bed dry in Manikganj and Dhaka districts. Resuscitation of dead rivers and link them to perennial sources were attempted, during sixties and early seventies, of the last century. Projects were launched, but many of the projects aggravated the situation further. They over-drained the basin, exposed the subsurface flow and increased the open water evaporation.
The 2nd phase of resuscitation of dead rivers started with the popular demand for irrigation. During the end of seventies massive canal digging programs started all over the country. Water lifted by pumps from perennial sources flown through the dug up canals, for a second lift to the fields. However, many of the sources and the canals dried up, due to over exploitation, increased open water evaporation and deep percolation through the sandy soils.
It is believed that, upstream withdrawal of water by India from the major rivers, particularly the Ganga River, lowered the river flows in Bangladesh. And it is thought that, if water could be stored in the reservoirs further upstream in Nepal, it could augment the Ganga downstream flow during drought. However, these ideas from Bangladesh are not accepted by India, the mightiest among the co-riparian countries. India on the other hand wanted to divert water from Brahmaputra, Sankosh, Dudkumar and Dharla Rivers to the Farakka Barrage point for augmentation, by an overland dug up canal through Bangladesh. This proposal if implemented could devour a vast amount of arable land to the north, so was not accepted by Bangladesh. Now India has a new proposal with Rivers Interlink Project. This project envisages linking between water hungry western and southern regions of India with the water rich northeastern regions. Bangladesh seriously opposes this project as its effect shall damage the age old ecosystem of this country totally.
Bangladesh has only one inland reservoir over the Karnafuli River at Kaptai. This reservoir capacity is essential for hydroelectricity production, so cannot augment its lower reaches. Barrages constructed over the Tista and the Tangon Rivers to the north, over the Monu River to the northeast, and over the Nabaganga River to the southwest cannot augment their downstream flows, because of low capacity. Recently Rubber Dams are constructed over the Matamuhuri, the Bhogai-Kangsa and some other smaller rivers. The storages created by those dams are for irrigation, not for augmentation.
Coastal Water The Ganga and Brahmaputra rivers of Bangladesh have hundreds distributaries towards the sea, forming series of estuaries big and small. These estuaries have varying degree of salinity in their water, depending on varying quantity of sweet water coming from the upland source. These are open water bodies, cover about 500,000 hectares of area. The water resources qualities in the estuaries change at different times of the year because of changes in the composition of silt and concentration of salinity. During monsoon and floods, the salinity remains near the sea, but the water becomes full of silt carried from the uplands. In autumn and winter, river water becomes clean of silt; but because of reduction of flow, salinity starts traveling inland and affects the sweet water agriculture. In Satkhira and western Khulna areas, no sweet water flow arrives from the uplands during this time. In spring and summer, high tidal fluctuations erode banks and beds of the estuaries in these areas, and make their water muddy.
The coastal waters in the estuaries of Bangladesh are very rich of fish, in all seasons of the year. The main estuaries are the breeding ground of Hilsa fish, an indigenous variety famous for its taste. The coastal area in Southwest Bangladesh has the largest brackish water mangrove forest in the world, called the Sundarbans. Some fish species found in the Sundarban areas are unique of their variety. The coastal lowlands in polder areas are the major source of sweet water for the people living there and their agriculture. These areas are about 1-2 meters above mean sea level, vulnerable to submersion by high tide and surge. Thus, the coastal embankments along the rivers are vital to stop saline water and surge to flow into the countryside. The embankments have flushing sluices to allow the required drainage from the countryside. People also dig ponds inside the polders to store rainwater for drinking and living purposes, useable round the year. Thus in the coastal areas, two separate eco-systems are maintained, one with the brackish water outside the embankment in the rivers, and the other with sweet water inside the embanked polder areas. [M Inamul Haque]