Enhancement of seepage and lateral preferential flow by biopores on hillslopes

Natural soils are generally populated with a wide variety of macropores formed from physical processes and/or biological activity. These macropores can have a large influence on the lateral flow of water in hillslope soils even when those macropores are not continuous or connected directly to ponded water. The concept of self-organization of flow paths described by SIDLE et al. (2001) is analyzed through numerical simulation of variably-saturated flow in a large cylinder of soil containing a population of disconnected macropores. It is demonstrated that there is a threshold water pressure at which the macropores will become active, and above this threshold the then active network of macropores significantly increases the effective conductance of the soil volume. In the case examined here the increase exceeded 40%. The analysis presented provides a context for the explanation of soil pipe formation by the process of seepage erosion. An analogy is drawn between percolation theory in porous media and the concept of self-organization of flow pathways at the hillslope scale.