Bangladesh is endowed with plenty of surface and groundwater resources. The surface water resources comprise water available from flowing rivers and static water bodies as ponds, beels and haors. Surface water inflows of the country vary from a maximum of about 140,000 m3/s in August to a minimum of about 7,000 m3/s in February. Two main rivers, the Brahmaputra and the Ganges account for more than 80% of streamflows. The highest flood discharge of the Ganges observed at hardinge bridge in1987 was 76,000 m3/s and that of the Brahmaputra observed at Bahadurabad in 1988 was 98,600 m3/s. the minimum discharges of the rivers are 261 m3/s and 2800 m3/s, respectively. The average daily flow of the Ganges is about 10,874 m3/s, which reduces to 1366 m3/s during season and increases to 32,00 m3/s. The highest flow is about 44,000 m3/s which is usually received in August. The annual average discharge of the Meghna at Bhairab Bazar is approximately 4,800 m3/s and the maximum flow occurs generally around mid August.Bangladesh also receives plenty of rainfall in the monsoon extending from June to October.The country receives plenty of rainfall and the amount of annual rainfall ranges from about 3200 mm in the northeast to about 1600 mm in the southwest region. Over the annual cycle rainfall exceeds evapotranspiration by about 10% in the northeast and southeast region whereas they are almost equal in northwest and in the southwest evapotranspiration exceeds rainfall by about 10%. During the seven-month dry season extending from November to May, evapotranspiration is greater than average rainfall except in the northeast region. Most of the winter crops and some summer crops can be grown under rain fed conditions. Generally it can be said that Bangladesh has plenty of water but its uneven distribution, overabundance in monsoon often causes catastrophic floods and scarcity in dry season causes severe drought conditions leading to loss of crops, livestock, public health problems and environmental degradation.
Water is needed for meeting consumptive demands, which include agricultural, domestic and industrial use and non-consumptive demands which comprise in-stream use (navigation, fisheries, salinity control, dilution of pollution) and water required for ecolological protection and wetland preservation. For proper planning, development and utilization of water resources, correct assessment of available water resources is essential but difficult. Water becomes really a scarce resource in Bangladesh during the dry months of the year and maximum water demand occurs in March. First assessment of availability and demand of water resources in the critical month of March was made by the Master Plan Organisation (now Water Resources Planning Organisation) and presented in the National Water Plan prepared in 1986. In 1991 MPO updated the National Water Plan together with the demand and supply of water.
Gross water demand is based on the irrigation requirement, salinity control in the estuaries, riverine fisheries, inland navigation, fisheries and salinity control, and domestic and industrial uses. There are about 7.56 Mha (million hectare) of cultivable land of which about 6.9 Mha of agricultural land can be brought under irrigation at full development by the year 2018. The total water requirement has been estimated at 24,370 Mm3 (million cubic metre) which includes 14,209 Mm3 for agriculture, 9,910 Mm3 for navigation and 170 Mm3 for domestic and industrial use. The total water supply in this month is 23,490 Mm3 which comprises 5,360 Mm3 of groundwater, 6,390 Mm3 from regional rivers, beels and haors and 11,740 Mm3 from main rivers. Agricultural water requirement is 58.6% of the total, navigation, salinity control and fisheries demand 40.7% and domestic and industrial need accounts for only 0.7% of total demand. Out of these entire requirement, 77.2% is expected to be provided by surface water and the balance 22.8% is expected to come from groundwater. Water balance, shown in the Table, was estimated by the MPO in 1991 for the month of March for a 1983 benchmark year followed by 1990 and the likely position in the years 2005 and 2018. The year 2018 was contemplated as the projected earlier possible date for full development of the Ganges and Brahmaputra barrage projects.
