IRRIGATION REQUIREMENT OF TRANSPLANTED MONSOON RICE IN BANGLADESH

The main rice crop of Bangladesh is grown in the monsoon season as a t ransplanted rainfed crop referred to as T. Aman. Yields are often low because of late-season water stress associated with rainfall variabili ty and late planting. To realise the yield potential of modern varieti es, water control, including supplemental irrigation, needs to be impr oved. Using a daily water-balance simulation technique on a cropping s ystem of rainfed pre-monsoon rice (Aus) followed by irrigated T. Aman, this study estimates for representative stations in 24 districts the probable planting dates and supplemental irrigation required to avoid significant water deficit during the reproductive phase of the T. Aman crop. The results are presented as isolines on a country map, and it is concluded that supplemental irrigation is required throughout the c ountry for optimal performance. The probable irrigation requirement du ring the reproductive period increases from 0 mm in the Northeast to 2 25 mm in the Northwest, neglecting seepage and distribution losses. So me implications for a small-scale water harvesting irrigation method a re discussed. Thus, neglecting distribution and application losses, a 2 m deep ditch of an area equal to 5% of the rice area is required to satisfy an intermediate, probable reproductive field irrigation requir ement of 100 mm for infinitly large areas. The pond/land area ratio in creases to 15% for smaller areas for a pyramidal-shaped pond because o f side slope requirements for pond stability. Economic sustainability requires that the area ratio should be less than 9%, limiting the irri gation requirement to less than 180 mm and the pond depth to more than 1.1 m. In the Western part, the method is therefore likely to be unec onomical for shallow ponds on small areas of non-clay soil unless addi tional benefits are derived from the pond.