Numerical calculation of secondary discharge peak from a small watershed using a physically based watershed scale infiltration simulation

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.

Keywords:

• Numerical simulation
• Rainfall-runoff process
• Richards' equation
• Secondary discharge peak

Acknowledgments

We thank Mr. Shigeru Kuwabara, Mr. Makoto Kuwabara, all the staff of the Field Museum Kusaki, and those of Oya, the Field Science Center, of the Tokyo University of Agriculture and Technology, for obtaining field data and for assistance with our research activities.