Fundamental studies on the pipe drainage in large sized paddy fields.

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Recently in large sized paddy fields, the control of subsurface drainage due to pipe drainage is considered to be important. The matter of pipe drainage in paddy fields with layred soils, especially the drainage effects of the back filled trench, were investigated by means of numerical analysis due to the finite element method. The following results were obtained.(1) We proposed the method of numerical analysis which can obtain not only the equipotential lines but also the stream lines in the steady-state Darcy flow throughout layred soils.(2) By the numerical analysis, we found that hydraulic conductivity of both the back filled trench and the layr, which lie over the trench, largely affected the flow patterns and the discharge from drain pipe. For the numerical analysis, constant depth of ponding water at ground surface is upper boundary condition. As the lower boundary, we defined an impermeable layr, which is deeper than 1.0m.(3) After the drainage of surface water, the draw down of the ground water level and the decrease of both the soil water contents and the pipe drain discharge were numerically analyzed.(4) By the field observations, it was verified that in each of the plowed layr, the hard pan and the original sub layr the relationship between bearing capacity of soils and the soil water contents were linear, respectively.