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

近年来，在大面积稻田中，由于管道排水而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管道排水问题，特别是回填沟的排水效果。获得了以下结果。(1)本文提出了一种数值分析方法，它不仅可以得到层状土中稳态达西流的等势线，而且可以得到流线。(2)通过数值分析发现，回填沟及其上覆土层的导水系数对排水管的流态和流量有很大的影响。在数值分析中，上边界条件为地表积水深度恒定。作为下边界，我们定义了一个不透水层，这是深1.0米。(3)通过数值模拟，分析了地表水抽干后，地下水位下降、土壤含水量和排水管流量的减少情况。(4)通过现场观测，证实了翻耕层、硬土层和原始下卧层的土壤承载力与土壤含水量均呈线性关系。

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.