Hydraulic properties of unsaturated soil in Lienhwachi Watershed

• Main Authors: Chin-Yuan Hung, 洪志遠
• Other Authors: 陳明杰
• Format: Others
• Language: zh-TW
• Published: 2007
• Online Access: http://ndltd.ncl.edu.tw/handle/64937264922934273759

碩士 === 臺灣大學 === 森林環境暨資源學研究所 === 95 === 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.