Numerical calculation of secondary discharge peak from a small watershed using a physically based watershed scale infiltration simulation |
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Authors: | Katsushige Shiraki Yoshiki Shinomiya Rieko Urakawa Hiroto Toda Kikuo Haibara |
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Institution: | (1) Graduate School of Tokyo University of Agriculture and Technology, Saiwai-cho 3-5-8, Fuchu-shi, Tokyo 183-8509, Japan;(2) Shikoku Research Center, Forestry and Forest Products Research Institute, Kochi, Japan |
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Abstract: | Numerical infiltration simulations were performed to reproduce secondary discharge peaks in a mountainous forest watershed
(watershed area, 1.89 ha; average topsoil depth, 2.61 m; and bedrock geology, Mesozoic–Paleozoic) using a simplified physically
based three-dimensional saturated and unsaturated water-flow model based on Richards’ equation. We were able to calculate
the quick discharge during rain and a secondary discharge peak at the watershed simultaneously, using observed topographical
information, the topsoil depth distribution, and soil hydraulic characteristics, and by dividing the watershed by 2.5 m horizontally
and ten cells vertically. Although the calculated hydrograph did not agree entirely with the observed hydrograph, we conclude
that the characteristics of the observed hydrograph were explained with better accuracy using the smaller soil porosity patterns
than using the observed patterns. We verified that the simulation method based on Richards’ equation was effective to analyze
the rainfall-runoff processes toward the intended watershed. Computational comparisons clarified that lower soil porosity
quickens the timing of secondary discharge peaks and increases their volume. Additional examinations, such as the distribution
of soil hydraulic characteristics and the actual condition of Hortonian overland flow, are necessary to simulate rainfall-runoff
processes precisely at the intended watershed. |
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Keywords: | Numerical simulation Rainfall-runoff process Richards’ equation Secondary discharge peak |
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