傳統入滲試驗方法主要量測地表單一深度下之均質土壤飽和水力傳導係數(Ks)，本研究提出之連續落水頭入滲試驗(MSFIT)，以一維飽和垂直水流之落水頭入滲理論為基礎，延伸推導連續落水頭入滲模式解析解，可用於估測不同深度下之飽和水力傳導係數。一維飽和垂直水流之水力傳導係數可總和表示為各分層之調和平均形式，藉由調合平均定義即可由上至下計算各分層之飽和水力傳導係數值。連續落水頭入滲模式可藉由入滲率與潮濕鋒運移深度倒數之線性迴歸式推估Ks，落水頭試驗次數與地表積蓄水位變化量將影響Ks分布解析度。為驗證連續落水頭入滲試驗之線性迴歸模式，本研究進行三組獨立之雙層土壤分布之砂箱試驗，結果顯示MSFIT線性迴歸模式估測之水力傳導係數值與12個採樣分析所得之值相近，且模式估測之分層水力傳導係數結果亦與砂箱土壤分布設計呈現出相同之遞減趨勢；MSFIT線性迴歸模式亦應用於現地試驗中，模式估測結果與現地觀測之土壤質地及採樣分析結果相近。除了利用觀測地表積蓄水位資料之MSFIT線性迴歸模式外，本研究亦提出以含水量觀測資料為主之MSFIT等量潮濕鋒模式，其模式依據觀測特定深度下之潮濕鋒運移時間關係來估測Ks。數值模式結果指出由初始含水量變化至飽和含水量之時間過程，中間時間點之位置可有效代表等量潮濕鋒到達時間。MSFIT等量潮濕鋒模式亦利用雙層土壤分布之砂箱試驗進行驗證，試驗結果顯示MSFIT等量潮濕鋒模式估測所得之水力傳導係數值與傳統定水頭試驗及MSFIT線性迴歸模式估測值皆相近。本研究提出之連續落水頭入滲模式不僅操作簡單，亦可有效推估獲得不同深度之一維飽和垂直水流之力傳導係數值。

Most infiltration methods measure the saturated hydraulic conductivity (Ks) of the unsaturated zone at only one depth at or near the surface. The purpose of this dissertation is to extend the falling-head infiltration theory to a multi-step falling-head infiltration test (MSFIT) for estimating Ks at various depths in a one-dimensional saturated vertical flow domain and to validate its results using laboratory and field data. A general analytical solution for Ks from the linear regression of infiltration rate and the reciprocal of the depth of the wetting front is derived for the wetted region of each step in the MSFIT. The Ks values for individual wetted sub-regions are calculated sequentially from the top down based on the fact that the Ks value of the whole wetted region is the depth-weighted harmonic mean of sub-regions for flow perpendicular to the wetted layers. The resolution of the wetted sub-region is determined by the number of steps and the change of the ponded water head. The MSFIT is performed in three independent laboratory two-layer sand columns for model verification. The estimated hydraulic conductivity shows a consistent decrease in three independent laboratory sand column tests and is in agreement with the value obtained from twelve core samples. The MSFIT was applied to a field site. The field results compare well to those of small core samples and field observation of the soil texture. An alternative using effective arrival time for estimating Ks at various depths was proposed. Numerical experiment shows that the effective arrival time of the wetting front can be well approximated by the average of the start and finish times of the change in water content. The MSFIT using effective arrival time approach was applied to a sandbox experiment in the laboratory. The experimental results are consistent with the hydraulic conductivity obtained using the constant-head ponded infiltration method and the measurements using linear regression approach. The proposed MSFIT is simple, flexible, and versatile for obtaining hydraulic conductivity at various depths for one-dimensional saturated vertical flow domains.

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