Modelling soil suction changes due to mixed species planting

Abstract

Vegetation-induced changes of soil hydrology (i.e., infiltration rate and soil matric suction) has important implication to the stability of earthen infrastructure. Predicting the plant hydrological effects is challenging when multiple species are present. Most existing models consider only single-species due to lack of reliable and relevant experimental data and difficulty to partition solar radiant energy into individual species in a mixed-species condition. This paper aims to develop an improved hydrological model that can determine the water infiltration rate and suction responses under both single- and mixed-species (one tree and one grass species) condition. The model is verified by double-ring infiltration tests and field monitoring conducted in bare, grass-only, tree-only and mixed tree-grass plots. The model was subsequently used for parametric analyses. The mixed species plot had the lowest infiltration rate and preserved the greatest suctions because the tree-grass competition for water created high soil moisture deficit. Parametric study identifies a threshold ratio of tree and grass leaf area index of 2.0, beyond which the effects of grass root-water uptake on water competition with the adjacent tree were negligible. Induced suction in the mixed tree-grass plot was close to that in a tree-only plot. Contribution of tree root-water uptake to induced suction in a mixed species plot did not increase further when the leaf area index ratio exceeds 5.0.