Soil Drainage  outflow of water from the soil. More specifically in Bangladesh drainage refers to removal of water from the soil surface, either naturally or by suitable artificial means. Drainage is broadly subdivided into two class namely surface drainage and sub-surface or ground water drainage. Artificial drainage is uncommon in Bangladesh.

In order to describe natural drainage in Bangladesh, the terms, well drained (good); moderately well drained (moderately good); imperfectly or somewhat poorly drained (imperfect/somewhat poor); poorly drained (poor) and very poorly drained (very poor) are used. Associated with these terms are seasonally flooded and intermittently flooded soils. In well drained soils water can move from and/or through the soil reasonably quickly so that the water does accumulate on the surface of the soil for less than few hours at a time and also the soil profile up to a depth of 60-90 cm from the surface does not remain saturated more than two to three days after heavy rainfall. In moderately well drained soils water may remain on the soil surface at a time for a few days following heavy rainfall and the soil mass in the profile up to 90-cm from the surface may remain saturated up to 2 weeks at a time during monsoon. Imperfectly drained soils remain wet for several weeks during the monsoon season and water also can stand for 1 to 2 weeks at a time following heavy rainfall. Poorly drained soils remain wet during monsoon for several weeks and like the imperfectly drained soils water may stand for 1 to 2 weeks at a time following periods of heavy rainfall. Poorly drained soils remain wet for several weeks during the monsoon season and it remains flooded for several weeks but not for the whole year. However, very poorly drained soils remain wet throughout the year.

Well-drained and moderately well drained soils are generally well structured and medium textured. These soils are suitable for upland crops such as wheat, maize, potatoes, peas etc. Rice grows abundantly on soils which are imperfectly, poorly and very poorly drained, less structured and heavy textured. Seasonally flooded soils are generally submerged through rainwater or river water for a few days during the monsoon season, whereas intermittently flooded soils are submerged by rainwater or river water for a period of a few days at a time but several days with intervals during monsoon. Most of the General Soil Types (a group of soils formed in the same way and have a broadly similar appearance) belong to one drainage class only. For example, Brown Hill Soils belong to well drained; Calcareous Brown Floodplain to moderately well drained; Black Terai Soil and Brown Mottled soils belong to imperfectly drained; Shallow Grey Terrace, Deep Gray Terrace and Gray Valley soils to poorly drained; and peat soils belong to very poorly drained class. However, some soil types belong to two drainage classes. Deep Red Brown Terrace Soils belong to moderately well drained and well drained classes; Shallow Red Brown Terrace to moderately well drained and imperfectly drained classes; and Acid Sulphate soils and Acid Basin Clays to poorly drained and very poorly drained classes.

Highland soils, which are subject to surface waterlogging during heavy rainfall or irrigation, will benefit either by open field drains or tile drain. Field drains also benefit dry land rabi crops grown on soils with moderately slow or moderately rapid subsoil permeability. Open drainage in rainy season would be useful only in deep soils. [Aminul Islam]