2016 Volume 37 Issue 4 Pages 559-578
In order to evaluate the effects of slope gradient on unsaturated rainwater infiltration, this study conducted numerical experiments by solving the two-dimensional Richards’ equation for saturated-unsaturated soil water flow. We assumed slopes with a horizontal length of 10 m, a vertical soil thickness of 2 m, and slope gradients of 12.5° through 42.5°. The initial condition was attained by continuously supplying rainwater at a constant intensity corresponding to an annual effective rainfall. Then, rainfall intensity was increased to a level corresponding to a heavy storm event, and the dynamics of infiltrated rainwater were analyzed. The results indicated that, as the slope gradient increases, water infiltration becomes more diffusive in regions close to the soil surface. That is, in steeper slopes, water content increased more rapidly and required a longer time to reach a constant value than in gentler slopes. The diffusive infiltration phenomena in the steep slope are attributable to the horizontal water flux qx, which contributed to rapid and delayed increases in water content immediately below and above the wetting front, respectively. These phenomena were observed in most parts of the steep slope, except for the downstream and upstream edges. At the downstream edge, the steep slope produced slower wetting front movements than the gentle slope. At the upstream edge, wetting front movements were faster in the steep slope than in the gentle slope. It is suggested that in the steep slopes the diffusive infiltration phenomena tend to contribute to more rapid increases in pore water pressures at the bottom of the soil profile.
