蓮華池集水區不飽和土壤的水力特性

Abstract

本研究探討森林集水區不飽和土壤的水力特性。研究區域位於蓮華池四號集水區，於坡面選定六條樣線，每一條樣線依照山脊、山腹、山谷分別選定三個位置，每一位置分別對深度0 cm及深度20 cm，使用張力滲透計進行現地滲透試驗，供分析不同壓力水頭條件下的水力傳導度。同時，採取不擾動土壤試體分析物理性質，供對照現地滲透試驗結果，說明大孔隙率對水分滲透流動的影響。並且，使用水力傳導度以及不同壓力水頭條件下導水孔隙率的計算結果，比較蓮華池四號與五號集水區的水力特性。 蓮華池四號集水區在不同深度的土壤孔隙率有顯著差別，土壤深度0 cm 的總孔隙率比深度20 cm多了10％左右，其中壓力水頭在＞−10 cm範圍之孔隙率的差異，為造成在不同深度總孔隙率差異的主因。由於水分在大孔隙與部分的中孔隙中流動相當快速，水力傳導度受到壓力水頭變化影響，由飽和轉為不飽和之水力傳導度轉變點在壓力水頭−10 cm左右。經由迴歸分析，飽和水力傳導度與大孔隙率、碳含量、乾總體密度等具有良好之相關性（r2＞0.70）。 使用壓力鍋測定法測得壓力水頭＞−3 cm的土壤大孔隙率在深度0 cm與深度20 cm分別為8.82％與5.55％，然而由使用Waduwawatte et al. (2004a)的方法（WSX方法）計算得壓力水頭−0.6 ~ −3 cm的導水大孔隙率只佔0.014％與0.001％。但水分流動經過此部份導水大孔隙的比例達53.97％及42.44％。 其次，由四號集水區與五號集水區的資料分析比較，不同土壤深度的平均飽和水力傳導度有顯著差異，五號集水區比四號集水區多了一次方。在壓力水頭−0.6 ~ −3 cm 條件下，使用WSX方法計算的導水大孔隙率就深度0 cm 的土壤而言，五號集水區為四號集水區的8.6倍，深度20 cm 則為42倍，顯示天然闊葉林淺層土壤的導水孔隙率高於人工杉木林，而且，飽和水力傳導度也較高。結果顯示不同林相型態會改變土壤的水力傳導特性，且會改變該地區之地表逕流量與土壤保水能力。

The studies investigate about hydraulic properties of unsaturated soil in forest watershed. Tension infiltrometer was used to measure infiltration rate, that to calculate hydraulic conductivities under varied water pressure head condition. Study area was at Lienhwachi watershed No. 4. Six locations were respectively selected from ridge, hillslope and valley. Each location comprised soil depths of 0 cm and 20 cm for field infiltration test. At the same time, undisturbed soil samples near to infiltration test location were excavated to analyze their physical properties, that were corresponded to the results of the infiltration test to understand the influence of the macropores on hydraulic conductivity. Further, hydraulic conductivity and calculated water-conducting porosity of watershed No. 4 were compared with watershed No. 5. According to the analyzed data, soil porosity of watershed No. 4 are significantly discernible in different depths. The total porosity in depth of 0 cm is about 10% more than that in depth of 20 cm. In different depths, total porosity discrimination result from macroporosity and a portion of mesoporosity that corresponding to water pressure head > −10 cm. Since water flows rapidly in macropore, macroporosity and a portion of mesoporosity influence saturated hydraulic conductivity in different depths. From saturated to unsaturated, the break point of hydraulic conductivity is at water pressure head about −10 cm. According to correlation analysis, saturated hydraulic conductivity were significantly correlated to macroporosity, carbon content, and dry bulk density (r2 > 0.70). Macroporosity are about 8.82% and 5.55% in soil depths of 0 cm and 20 cm respectively measured by pressure-plate apparatus at water pressure head more than −3 cm. However, water-conducting macroporosity calculated by Waduwawatte et al. (2004) approach at water pressure head between −0.6 and −3 cm are about 0.0014% and 0.001% in depth of 0 cm and of 20 cm. In addition, 53.97% and 42.44% of water flows pass through the water conducting macropores in depth of 0 cm and 20 cm respectively. Besides, average saturated hydraulic conductivity of different depths in watershed No. 4 are significantly discernible with watershed No. 5. Average saturated hydraulic conductivity of watershed No. 5 is one order more than that of watershed No. 4. At water pressure head > −3 cm, water-conducting macroporosity of soil depth of 0 cm of watershed No. 5 was 8.6 times with watershed No. 4 and soil depth of 20 cm was 42 times, that show water-conducting macroporosity of shallow soil depths of natural forest is greater than plantation. From the results, it clearly show that change forest type could alter unsaturated hydraulic properties of surface soil and consequently alter the amount of surface runoff and soil water storage.