Abstract
Even if it is a common sense to install some coarse materials as the draining layer underlying the planting layer, we have a little of information on the role of the coarse materials in drainage process. This study evaluated it numerically by using the unsaturated Darcian soil water simulator with alternating two kinds of boundary conditions. The physical properties of the volcanic ash soil and the 5-10 mm crushed crumbs were applied to calculation. At steady state, the thickness of the upper layer, planting layer, has a large effect on the ponding depth of soil surface, but that of the lower layer, coarse materials, has a little of effect on it. Under the rainfall intensity 50 mm/hr, the ponding depths for the upper layers of 100 and 200 mm result in 396-453 mm and 851-905 mm, respectively. At 100 mm planting layer, the critical rainfall intensities for ponding occurrence have the values between 9.0 and 14.3 mm/hr, and they were increased with the thickness of the lower draining layer.
