Abstract
Rapid underdrainage is necessary for the cultivation of upland crops in fields converted from clayey paddy fields in the Hokuriku Area. Therefore, in order to accelerate subsurface drainage shallow underdrain pipes were constructed at about 0.4m below the soil surface at intervals half that of ordinary underdrain pipes installed at about 0.7m deep. In this paper, the characteristics of the system's underdrain discharge during heavy rainfall over a period of 5 years were investigated. The following results were obtained. In the early years of the converted field, the total underdrain discharge was almost shallow drain discharge in the plot having an ordinary drain combined with a shallow drain system and the peak underdrain discharge was greater than that in the plot having only an ordinary underdrain. Surface stored water and excess water in the plow layer soil were drained quickly and effectively by shallow underdrainage. Following this, ordinary underdrain discharge increased year by year. After 5 years, the peak underdrain discharge of the plot having the underdrain system was nearly equal to that ofthe plot that developed macropores such as shrinkage cracks. Ordinary underdrainage and shallow underdrainage play an important role in eliminating excess water in the plow layer soil and lowering the groundwater table.
