Abstract:
Although Aus and Aman rice cover 89% of the total cropped ar'ea and yield 83.7% of the total rice output, these two crops are still grown under 'providence' methods ~ that is, by impounding rainfall inside the cropped area and depending entirely upon rainfall as a soil moisture source. The high uncertainty involved regarding availability of rainfall during transplanting and during the crop growing periods of these two predominantly rainfed crops has often meant that investments made in these crops were lost, although the mean annual rainfall in the country varies from 1400 mm in Rajshahi to over 5000 mm in Sylhet and 80% of this rainfall occurs during theAus and Arnan crop growing periods. andanaly'sing the risks associated with rainfed Aus and Arnan cul tivatidns. Exper,imental plots were taken at the farm of Bangladesh Rice Research Institute, Joydebpur. There were in all five treatments ranging from totally rainfed'plots to 25 mm to,SO mm standing water on the plots. The water balance method was applied for determining the crop water requirement with provisions for measuring rainfall, evaporation and seepage and percolation. From the study, it was observed that with supplementary irrigation increase in yield varying from 8% to 71% over rainfed condition occured during the study period for different treatments. The amount of supplementary irrigation varied from 69 mm in Aus season to 573 mrn in Aman season for the treatment
of 25 to 50 mm standing water on the plots. For the saturated plot treatment a 57% higher yield over rainfed condition was observed during the Aman season and required 14.4 mrn of water from irrigation. From the analysis of rainfall data it was observed that rainfall during the crop growing period has to be supplemented by irrigation to meet the crop water requirement and to ensure a dependable yield during the Aus and Aman seasons. From probability analysis of rainfall it was found that the Aus crop requires supplementary irrigation in one out of four years and in case of Aman crop the supplementary irrigation is required in three out of four years. The amount of supplementary irrigation required varies from 0 to 67 mm for Aus season and 67 mm to 309mm for Amah season with rainfall probabilities varying from 50 to 75 percent and 25 to 75 percent for Aus and Aman seasons respectively.
