Bypass flow in soil

Abstract

Bypass flow in soil is a frequently observed phenomenon that describes rapid and nonuniform downward water movement along highly conductive pathways through the unsaturated soil. Such preferential flow is caused by a number of different mechanisms that originate primarily from local heterogeneities. In structured soils, vertically continuous earthworm burrows and root channels, or cracks and fissures of an inter-aggregate pore network mainly serve as preferential flow paths. The interrelations between flow and transport processes and soil structural properties under varying initial and boundary conditions are complex and process-based models need to be improved. Current approaches for a quantitative description of preferential flow mostly assume that the structured soil consists of separate but interacting porous domains. The dual-permeability models assume two mobile flow domains. These models mainly differ in the description of flow in the preferential flow domain (i.e., either Richards’ equation assuming capillarity or kinematic wave approach for gravity flow) and with respect to the mass transfer formulation (i.e., from pressure head or saturation-based first-order type formulations to more complex nonlinear formulations or numerical solutions of the local flow equation). Many challenges remain, in particular, how to capture dynamic soil structure effects and to improve quantitative descriptions of bypass flow. A better understanding of the susceptibility of soils to preferential flow is of importance when studying problems related with unintended contaminations of ground and surface waters. At the same time, the quantitative analyses of water and element balances of ecosystems are methodologically problematic as long as preferential leaching rates cannot adequately be measured or calculated.