蓮華池林地土壤水力傳導度不同測定方法之比較

Abstract

本研究探討蓮華池林地土壤的水力傳導度特性。研究區域位於蓮華池四號集水區(四號)及五號集水區(五號)，於坡面選定四條樣線，每一條樣線在山脊、山腹處分別選取一點，使用雙環入滲計對深度0 cm及20 cm進行現地滲透試驗。同時，以直徑20 cm、長度40 cm的採土圓筒採取大型不擾動土樣，於實驗室進行定水頭與變水頭滲透試驗，以比較現地及室內的測定方式其飽和水力傳導度是否有差異。另外採取不擾動土壤試體分析物理性質，供對照現地與室內的試驗結果，說明土壤總體密度、大孔隙等對飽和水力傳導度的影響。 雙環入滲計的試驗結果，五號的平均飽和水力傳導度大於四號一次方左右。五號的土壤大孔隙率顯著高於四號，而且，五號的土壤為粘質壤土，其砂粒含量較高，四號的土壤為坋質粘土，粘粒含量高。上述因素是造成五號的飽和水力傳導度較四號為高的原因。經迴歸分析，飽和水力傳導度與大孔隙率、乾總體密度等具有良好的相關。其次，比較雙環入滲計與室內定水頭與變水頭滲透試驗結果，雙環入滲計試驗所得之飽和水力傳導度大於定水頭滲透試驗結果，原因為定水頭試驗時從試體下方供水，受到深度30~40 cm的土壤較0~20 cm緻密的影響，造成試體的飽和水力傳導度偏低；而變水頭試驗所得之飽和水力傳導度大於雙環入滲計的試驗結果，原因為變水頭滲透試驗時壓力水頭較高，以及在滲透性良好的土壤可能會有高估飽和水力傳導度的情形。另外，雙環入滲計的結果與前人之Guelph滲透計、張力滲透計的結果比較，顯示雙環入滲計與Guelph滲透計所得飽和水力傳導度相近，而張力滲透計所得之飽和水力傳導度小於雙環入滲計及Guelph滲透計。 本研究另以室內人工模擬降雨，進行大型不擾動土樣之不飽和滲透試驗，依Brooks and Corey之不飽和水力傳導度與壓力水頭的函數分析結果，五號的孔隙分佈參數(η)值較四號小，五號的總體密度低，土壤疏鬆且含有較多的大孔隙，顯示五號土壤孔隙組成的不均質性比四號明顯。而四號的限界毛管水頭(Ψcr)值大於五號，顯示四號的土壤質地偏細且細孔隙含量偏高，是造成飽和水力傳導度較低的原因。

The study investigate about the saturated hydraulic properties of soil in forest watershed. This study area was at Lienhuachih watershed No. 4 and No. 5, where we choose four transects respectively selected from ridge and hillslope. Each location comprised soil depths of 0 cm and 20 cm, where Double-ring infiltrometer was used to measure field infiltration rate, and calculate the hydraulic conductivities in each point. At the same time, use the 20 cm of diameter and 40 cm of length cylinder to dig the large-scale undisturb soil samples, in order to compare the difference measurement types (Constant-Head Permeability Test and Falling-Head Permeability Test ) whether the conclusions are differ or not. Besides, collect small undisturbed soil to do physical analysis so as to compared the field and lab infiltration results and illustrate the influence of soil texture, large macroporosity, coarse porosity to the penetrate rate. By the way, we compared the difference properties in Guelph infiltrometer, tension infiltrometer and double-ring infiltrometer. The results of double ring infiltration test, the average saturated hydraulic conductivity in watershed No.5 are large than in No.4. The soil texture of No.5 is clay loam, and there is more sand; the texture of No.4 is silt clay, there is more clay. The clay water retention capacity is better than sand, so that the saturated hydraulic conductivity is watershed No.5 more large than No.4. In comparison with double ring infiltration, constant head permeability test and falling head permeability, the saturated hydraulic conductivity of double ring infiltration is all large than constant head permeability. The reason that constant head permeability’s water flow is through down to above, the soil at 30 cm is closely knit than 0~20 cm so that the saturated hydraulic conductivity is low. The saturated hydraulic conductivity of falling head permeability test is all large than double ring infiltration. The reason that falling head permeability test’s pressure head is large than double ring’s, and the falling head permeability test in good permeability soil may overestimate. Besides, we compare the hydraulic conductivity of Guelph infiltrometer, tension infiltrometer and double-ring infiltrometer. The saturated hydraulic conductivity of tension infiltrometer is lower than Guelph infiltrometer and double-ring infiltrometer, and the results of Guelph infiltrometer is closely by double-ring infiltrometer. The lab simulated rainfall conclusion, use the Brooks and Corey method to get the unsaturated hydraulic conductivity properties with water pressure head. The soil pore size distribution parameter (η) of No. 5 watershed is more large than No. 4, show that the soil porosity structure in watershed No.5 was more heterogeneous. There are more abundant soil aggressive structure and loose properties. The critical capillary head of No.4 is large than No.5, appeared that soil texture in No.4 is silt clay and micropores content is relatively high, that’s the reason why there has the smller saturated hydraulic conductivity.