Abstract
Following the preceding report on an ill-drained paddy field, we give an account of the features of underdrainage in well-drained paddy fields. It is demonstrated that, in the reclaimed land from the sea in Kojima Bay, paddy fields with the combined mole-pipe drainage or the pipe drainage are dealt with in connection with mechanisms on the methods of underdrainage.
We explained in full the conditions of soil and successively investigated the amount of discharge from underdrainage, level of groundwater table, soil moisture and bearing capacity.
So we knew that the amount of discharge from pipe drains was about 1. 5 1/sec/ha when water moved downward through the subsoil to a depth of 5 cm below the surface, and on the contrary the amount of discharge from combined mole-pipe drains was 39 1/sec/ha and its downward speed reached to 40 cm for 4 hours. These effects are distinguished from the effects of those results in pipe draining. There are some differences in the condition of cracks between both experimental fields, resulting in a great difference in the coefficient of permeability. It is indicated that we have to investigate the mechanisms of the combined mole-pipe drainage system because soil cracks due to the different systems of underdrainage show themselves in subsoil in different ways.
