Abstract
Since the amount of water requirement for puddling irrigation is large, investigating the factors that determine the water requirement is important. In this study we observed the process of water ponding during irrigation for puddling in a paddy field with peat subsoil of relatively high permeability, and analyzing the components of the water requirements. Before the irrigation, soil water contents and air volume ratios in soil profile were measured and infiltration tests using cylindrical infiltrometers were conducted. As a result, the surface water ponding spreaded with a constant velocity, indicating that the vertical drainage loss should not be significant and the water requirement mainly depend on the soil water deficit for saturation. This character of the field is not caused by the permeability of hardpan or shallow subsoil, but is brought about by a clay layer exists below 1m deep. The result implies that the puddling water requirement in this type of lowland paddy field is independent on the size of the field lot.
