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

近年来，在大型水田中，由于管道排水引起的地下排水的控制被认为是重要的。采用有限元数值分析方法，研究了层状土壤稻田的管道排水问题，特别是回填沟槽的排水效果。得到了以下结果：(1)提出了一种数值分析方法，不仅可以得到层状土壤中的等势线，而且可以得到层状土壤中的流线。(2)通过数值分析，我们发现沟道上方的回填沟槽和沟层的渗透系数对流态和排水管的流量都有很大的影响。在数值分析中，地表积水深度恒定为上边界条件。(3)对地表水排水后地下水位的下降、土壤含水率和管涌流量的降低进行了数值分析。(4)通过野外观测，验证了在耕层、硬土层和原土层中，土壤的承载能力分别与土壤含水率呈线性关系。

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.