A computer program for the calculation of both the surface and the groundwater runoffs from a watershed in a mountainous area is newly coded. This program is mainly based on the Finite Element calculation of the three dimensional groundwater flow in a high permeable layer developing beneath the slope surface in a watershed. The amount of the surface flow come out at each saturated nodal point located on the surface can be evaluated by taking the difference between the rain volume given in the point and the calculated volume of rain which infiltrates into the layer from the point. As the runoff phenomena can be physically simulated by the use of this program, the hydrogeological condition in a watershed may be back-analytically estimated if the runoff data is given.
The hydrogeologic condition in the Inuyama test field is successfully estimated from the analysis of the runoff data which corresponds to a short durated but intense rainfall. It is found that the permeable layer in this field should have the nature that the permeability under the unsaturated state rapidly increases if the moisture content in the layer exceeds a critical value.