On a treatment of the spatial distribution of pressure head in a steep soil layer during a big storm from the viewpoint of its long-term evolution process

A difficulty of predicting runoff characteristics from catchment properties such as geology. topography, soil and vegetation is caused by their heterogeneous spatial distributions. The regularities are derived from a nested structure of developments with different time scales: topography is evolved due to orogenic movement and erosion, soil development is repeated through landslide occurrences, and plants have a life cycle. Revealing the nested structure must be a key for evaluating an effect of catchment properties on the runoff prediction. Our study project 'Prediction of catchment runoff changes based on elucidating a nested structure consisting of the developments of topography, soil and vegetation' challenges a unique simulation for a long-term soil evolution process supported by vegetation roots and an efficient drainage system like natural pipes between the periods of landslide occurrences. In this presentation, I will discuss how we can generally treat the spatial distribution of pressure head in a soil layer on hillslope for a big storm triggering the landslide initiation. Most people may answer this will be dependent on a given storm hyetograph only because of an unsteady process, but here we are considering the meaning of its distribution in a long historical evolution of the soil layer on a steep hillslope in an active tectonic region such as Japan and the West Coast. A quasi-steady state created within a soil layer in response to a given hydraulic condition can give us important information for developing a simulation model for the evolution process of a soil layer.