Effect of Pore Radius Distribution of Forest Soils on Vertical Water Movement in Soil Profile.

Abstract

In order to clarify the effect of soil pore radius distribution g(r) on the water movement in soil profile, one-dimensional vertical flow equation was solved by using the model for soil hydraulic properties which was derived by applying lognormal distribution laws to g(r). Results showed that the recession hydrograph from a soil profile becomes gradual as the median of g (r) increases. For soil with a relatively large median of g (r), the recession hydrograph becomes steep as the width of g (r) increases. When the median of g(r) is relatively small, the hydrograph has more gradual decreases in recession flow as the width of g(r) increases. This critical median value increases as the length of soil profile (L) becomes small. By comparing pore radius distributions of various soils, it was shown that the undisturbed forest soils yield greater amount of recession flow than the disturbed loamy soils. 70 percent of the disturbed sandy soils yield greater amount of recession flow than the forest soils with L of 100cm. When L is 50cm, hydrographs of half of the sandy soils have greater decreases in recession flow than those of the forest soils. Hydrographs from the crumb-structure forest soils have more gradual decreases in recession flow than those from the granular-structure forest soils. Recession hydrographs from the massive forest soils are steeper than the those from the crumb and granular-structure soils. As a result, it was indicated that well-developed forest soil is effective as the surface soil to increase baseflow from hillslope.