Runoff Water all the water flowing from a drainage area. In other words that part of the precipitation which runs off the land surface into streams, in contrast to those parts which either soak into the ground or evaporate. Runoff is a serious problem in Bangladesh and produces natural hazards for agriculture and habitation. Firstly, runoff does not allow development of mature soils on old rock or landscapes of weathered materials, as these materials are always washed away with water. Secondly, it causes flood annually in the monsoon season. Water levels sometimes rise rapidly on basin lands due to runoff water. Lower Purnabhaba Floodplain (Rajshahi); Lower Atrai Basin (low lying areas between barind tract and Ganges River Floodplain); Sylhet Basin; in basins within the Northern and Eastern Piedmont Plains; in broad valleys within the madhupur tract and in some basins in the Brahmaputra and the Ganges River floodplains are flooded as a result of local runoff over the surrounding paddy fields.
deforestation and erosion in hill areas in adjoining parts of India as well as within Bangladesh have also led increasing severe runoff and flood bringing down appreciable amounts of sediments on to some floodplain land of Sylhet basin. In tista floodplain (Rangpur and adjoining regions) except close to active river channels, flooding is caused predominantly by runoff from local rain and raised groundwater table.
Due to runoff aided by the two mighty rivers, the ganges and the brahmaputra, traversing thousands of kilometers through the vast catchment areas in India and Tibet, cause drainage congestion, inundation and lastly flood in the monsoon season in Bangladesh. The enormity of the problem may be anticipated from the fact that the combined maximum discharge of 50 million cusec of water carrying a sediment load of about 24,000 million tons annually from outside the country. Thirdly, runoff gave rise to acidity in deep red brown soils of Madhupur, Barind and Akhaura tracts. The only benefit of runoff is the creation of new land in the river estuaries especially in the Meghna estuary.
A high rate of runoff will be produced by a crusted and/or compacted soil, which has a low infiltration rate. Agricultural land having proper tillage operations could increase infiltration rate and surface drainage capacity, reducing runoff. [Md Khurshid Alam and M Aminul Islam]