The exploration of flood plains soils in the State of Rio Grande do Sul (Brazil) has been historically restricted to the binomial 'irrigated rice crop - livestock'. Among several factors that contribute to that situation, the bad drainage conditions are one of the most important. In relation to irrigated rice, this situation promotes late planting and, consequently, productivity reduction. After validation with measured data in a typical flood plain soil of Rio Grande do Sul, the SWAP model was used to simulate scenarios under different drainage conditions. The results were compared with criteria that evaluate soil tillage possibility based on moisture values. This comparison showed a close relationship between subsurface and surface drainage. When surface drainage is satisfactory, soil moisture contents can be compatible with soil tillage, even if subsurface drains are too widely spaced. Otherwise, the increase in number of workable days for mechanization requires very closely-spaced subsurface drains.