Modelling effects of root growth and decay on soil water retention and permeability

Plant roots can change the soil water retention curve (SWRC) and saturated permeability (k(sat)) of vegetated soils. However, there is no model that could capture both the effects of root growth and root decay on these soil hydraulic properties simultaneously. This note proposes a new void ratio function that can model the decrease and increase in soil void ratio due to root occupancy (upon growth) and root shrinkage (upon decay), respectively, in an unsaturated vegetated coarse-grained soil. The function requires two root parameters; namely, root volume ratio and root decay ratio, both of which can be readily measured through root excavation and image-based analysis. The new function is incorporated into a void ratio-dependent SWRC model for predicting the SWRC of vegetated soils. Similarly, the same function can be combined with the Kozeny-Carman equation for predicting k(sat). The model prediction is then compared with a set of new field test data and an existing laboratory dataset for a silty sand vegetated with plant species under the family Schefflera. Good agreements are obtained between the measurements and predictions.