Surface Water
Surface Water made up of rivers, streams, lakes, beels and ponds. In other words, all waters on the surface of the earth including fresh and salt water, ice and snow. In Bangladesh rainfall and trans-boundary rivers flows are the main sources of surface water. Bangladesh has an average annual surface flow of about 1,073 million acre feet (MAF), of which about 870 MAF (93%) are received from India as inflow and rest 203 MAF (7%) as rainfall. This water is enough to cover the entire country to a depth of 9.14m. About 132 MAF (65% of rainfall and 12% of total) are lost in evaporation (114.30 cm) and the rest flows to the bay of bengal.
River water Bangladesh has about 700 rivers including tributaries and distributaries, which criss-cross the landscape and create about 98,000 ha of inland water bodies and more than 24,000 km streams or water channels. Of these, 54 rivers, including the ganges and the brahmaputra, originate from India and 3 originate from Myanmar. About 93% catchment areas of these 58 rivers are beyond Bangladesh, while only 7% is in Bangladesh. During the dry season when irrigation is necessary, these rivers either flow at their lowest levels or become dry in the Bangladesh portion, due to upstream barrages, built in the upper riparian region.
The river system that flows through Bangladesh comprises the third largest source of fresh water discharge to the world's oceans. Only the Amazon in Brazil and the Congo in Africa have larger discharges than the Ganges-Brahmaputra-Meghna river system. The annual volume of flow past Baruria, just below the confluence of the Brahmaputra and Ganges, is 795,000 million cu m, which is equivalent to 5.52 m of depth over the 14.40 million ha of the land area of Bangladesh.
The surface water resources available in Bangladesh can be expressed in terms of monthly streamflow, dry season static water (or standing water) and in-stream storage potential. Total available streamflow in the country, which is total outflow to the Bay of Bengal, varies from 174,000 cumec in August to 5,63 cumec in 1990 water use conditions. Total inflow to Bangladesh from India is approximately 90% of the total available streamflow in the country. About 85% of dry season (January-April) streamflow are found in the main river systems (Ganges-Brahmaputra-Meghna). Streamflow in the individual catchment region, excluding the streamflow in the main rivers, is extremely limited, with the exception of the augmented flow in the karnafuli river in the southeast region and streamflow in the Barisal-Patuakhali area in the mid-south region.
Total in-stream storage potential in the country during the dry season is estimated to be about 65 cumec. This estimation is based on a depth of one metre abstraction for the rivers having a width less than or equal to 100m. The mean monthly available stream outflow in the northwest region varies from about 417 cumec in March to about 7,970 cumec in August. Mean monthly inflow from India to the region in the driest month (March) is about 406 million cumec. The major shares of streamflow during March are concentrated in the tista (37%), dudhkumar (30%), dharla (19%) and mahananda (19%). Available static water is about 167 million cu m, and in-stream storage potential is about 103 million cu m.
In the northeastern region, the mean monthly streamflow varies from about l00 cumec in February to about 18,700 cumec in July. Mean monthly inflow from India to the region in February is about 223 cumec. In addition, the region receives about 80 cumec from the Brahmaputra in February. Inflow exceeds outflow in February by about 203 cumec. Major sources of streamflow during February are the Manu-Kushiyara (75%), surma (25%), dhaleshwari (17%) and old brahmaputra (17%). Available static water is about 374 million cu m and interstream potential is estimated to be about 275 million cu m.
The mean monthly available streamflow in the southeast region varies from 377 cumec in March to 4,870 cumec in July. About 89% of the streamflow during the driest month (March) is the augmented flow in the Karnafuli from kaptai lake. Other significant sources of streamflow for March are the gumti (3%), feni (3%), sangu (3%) and matamuhuri (2%).
In south-central region the mean monthly streamflow varies from 1,120 cumec in March to 21,500 cumec in August. Over 80% of the available dry season flow enters the region through the Lower Meghna offtakes - the Abupur, Hizla and Ramdaspur. Major sources during March are the bishkhali (41%), burishwar (28%) and arial khan (5%). Available static water is 9 million cu m and in-stream storage potential is 3.9 million cu m.
The mean monthly available streamflow in the south-western region varies from 190 cumec in March to 7,650 cumec in August - the cross border flow from India through the Mathabhanga river varies from 1.7 cumec in April to 124 cumec in September. Significant sources during March is the gorai (57%) a distributary of the Ganges, the Ganges-Kobadak intakes (27%) and the Mathabhanga (1.6%). Available static water is 62 million cubic m, and interstream storage potential is 65 million cu m. Streamflow in about 45% of the region have salinity greater than 2,000 B5-mohs.
Water in ponds and beels there are about 1.3 million ponds in Bangladesh covering about 1,47,000 ha area. There are about 10,000 haors, baors and beels. Most of these become waterless during the winter season. These ponds can be converted to potential mini reservoirs for irrigation water and/or fish cultivation through planned excavation and utilisation. Total available static water (water contained in the topographical depressions including haors, baors and beels) in the country during the dry season is about 79 cumec (611 million cu m) under water use condition in 1990. This figure was based on depths of 0.5, 0.5, 0.0, and 0.5m abstraction for the northwest, northeast, southeast and southwest regions, respectively, at 24-hour/day continuous pumping for a 90-day period.
Flood water almost every year, floods in Bangladesh cover almost one-third of the land area, and in years of severe flood, almost half of the country. Large areas of central and northeastern Bangladesh are normally flooded each year. In general, seasonal flooding is shallow in the northwest, west and east, but deep in the centre and northeast. Flooding depths vary within small areas because of differences in topography and man-made modifications of the land surface.
Although an immense quantity of surface water flows through Bangladesh, the development potential is constrained for a number of reasons. Most importantly, there are very few opportunities for either gravity diversions or surface storage. Under existing conditions, annual outflows from the major rivers to the Bay of Bengal are essentially equal to inflows from India. A water balance study for the critical dry month of March indicates that even in this month of relatively high diversions for irrigation and low base flow, the net diversions from the entire system are only about five percent of the inflows. Under future development conditions, diversions may possibly increase to 15% without major barrages and up to perhaps 35% with barrages. [Md Sazzad Hossain and Md Harun-ur-Rashid]
