Spatial distribution of soil structure in a suburban forest catchment and its effect on spatio-temporal soil moisture and runoff fluctuations

Abstract

Intensive hydrological observations and model calculations reveal that heterogeneous distributions of soil thickness and soil physical characteristics greatly influence long-term spatio-temporal fluctuations of soil moisture and runoff in a suburban forest, which has rich (thickly wooded) forest but undeveloped forest soil. This paper presents a simple, physical-contour-based model that can describe topography and spatially heterogeneous distributions, and uses the model to clarify how spatial heterogeneous variability influences long-term rainfall runoff processes. The 1.5-ha study catchment features deciduous-evergreen secondary forest and is located near an urban area about 30 km southeast of Nagoya City, in central Japan. This model can simulate saturated/unsaturated throughflow, excess flow, and soil moisture, and has not been calibrated. No input parameters have been fitted and all input parameters are based on published and measured data. Rainfall runoff processes are affected by structural distributions of colluvial/residual soil and spatial heterogeneous distributions of soil thickness. The heterogeneous soil thickness distribution contributes, in particular, to headwater conservation, which moderates storm flow and inhibits drought water flow. The magnitude of headwater conservation and the alignment of individual hillslope elements help to control runoff. The mechanism is attributable to the greater degree of headwater conservation in a suburban forest compared with a homogeneous natural forest. The model also clarifies and predicts water circulation on a larger scale than has been possible to date.