| Summary: | 碩士 === 國立臺灣大學 === 森林環境暨資源學研究所 === 103 === The study applied four saturated-unsaturated hydraulic conductivity models, which were Gardner exponential model (GE), Boorks and Corey model (BC), Gardner rational power model (GP) and van Genuchten-Mualem model (VGM), to the simulation of hydraulic conductivities under varied water pressure head condition. Three mathematical methods including solving the simultaneous equations, regression analysis, and numerical simulation were adopted to decide the parameters in the four models. Study area was at Lienhuachih watershed No. 4 and No. 5. Four locations were selected along ridge, respectively from the two watersheds, and each location comprised soil surface and the depth of 20 cm for field infiltration test by tension infiltrometer. As the soil became desiccated after the infiltration test, undisturbed soil samples of the infiltration test location were excavated for analyzing their physical properties, that to understand how the soil physical properties affect the measured hydraulic conductivity. Besides, the saturated hydraulic conductivities at the locations which had been measured by the double-ring infiltrometer by Yeng-Bang Tsai (2013) were used in the study to modify the hydraulic conductivities that simulated by the models under near-saturated condition.
According to the analyzed data, soil physical properties of watershed No. 4 were more homogeneous than that of watershed No. 5, and the discrepancy of different depths were also smaller than that of watershed No. 5. The data of field infiltration test showed that the dispersion of hydraulic conductivities of the soil surface and the depth of 20 cm of watershed No. 4 were both smaller than that of watershed No. 5. Therefore, at the results of establishing saturated-unsaturated hydraulic conductivity models, the parameters including soil text/structure parameter and the saturated hydraulic conductivity were more similar between the locations of watershed No. 4 than that of watershed No. 5. In field infiltration test, the condition of higher water pressure head that usually has more gravitational force but less of capillary force would make water only flow through parts of large soil pores instead of filling up whole pores that probably lead to underestimate the hydraulic conductivity. In the condition of more rainfall to the soil surface before the field infiltration test, it revealed that the soil pore could be effectively filled and therefore enhance the hydraulic conductivity.
The mathematical method of numerical simulation could get the lowest index of error, RMSE, which meant the method was the best way to estimate the parameters in four models. Besides, the RMSE results of GP model and VGM model were both lower 10-6, better than the results of GE model and BC model. According to GP model and VGM model, calculated values of the soil text/structure parameter were between 12 m-1 to 36 m-1, which meant that the soil of research site at Lienhuachih was well-structured, coming up to the analyzed results of the soil physical properties.
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