Approximate Solutions for the Soil Water Movement during Drainage under a High Groundwater Table Condition

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  • 地下水位が浅い土層内で生じる排水時の土壌水分変化を表す近似解
  • チカ スイイ ガ アサイ ドソウナイ デ ショウジル ハイスイジ ノ ドジョウ

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Innovative approximate solutions regarding an unsaturated water flow equation for internal drainage under a high groundwater table condition have been derived. This was obtained based on approximate separation of variables, i.e., depth z and time t. For soil whose unsaturated hydraulic conductivity was described as an exponential function of the matric potential, soil water flux f was approximated by the product of two functions, i.e., the drainage rate to groundwater (a function of t) and the distribution function (depending on z only). When groundwater level is fairly high, the distribution function is nearly equal to that derived from the linear differential equation, whereas the linear function of z is applicable for low groundwater level. Drainage rate r (t) could be determined by the “water balance equation” of the soil layer. With some limitations, numerical results obtained by the approximate solutions agreed well with the finite-difference solutions, includinig those for soil whose water characteristics are of the Brooks and Corey type. These solutions provided better understanding of various relations, for exaniple, between the drainage rate and water flux at an arbitrary depth.

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