Changes in pore size distribution and hydraulic properties of forest soil resulting from structural development

SummaryTo examine how rainwater dynamics on a forested hillslope change as a result of forest biological activities altering the development of pore structure, we analyzed the pore size distributions and hydraulic properties of forest soils under disturbed and undisturbed conditions. Undisturbed soil samples were collected in nine forests on Mt. Rokkou and Sakurajima Island, Japan, and disturbed soil samples were prepared using the same samples. We measured water retention curves, saturated hydraulic conductivities, and particle size distributions. Results showed that while pore size distributions and saturated hydraulic conductivities of disturbed soils were mainly influenced by particle size distributions, undisturbed forest soils had relatively similar pore size distributions and saturated hydraulic conductivities after secondary soil pores were formed as a result of forest biological activities. The undisturbed forest soils contained more large pores and fewer intermediate-size pores than the disturbed soils. We also found that undisturbed forest soils had a higher water content than the disturbed soils. As secondary soil pores developed, the median pore radius and the width of the pore size distribution increased, and the soil water diffusivity tended to decrease. The saturated hydraulic conductivity was proportional to the square of the median pore radius for all samples. Numerical simulations conducted assuming the mean hydraulic properties for disturbed and undisturbed soils revealed that the development of soil structure in forest soils has the effect of decreasing storm runoff and increasing base flow discharge.