Characterization of Macropore Flow Mechanisms in Soil by Means of a Split Macropore Column

Abstract

Naturally occurring macropores such as root channels and earthworm holes have been shown to affect water flow and solute transport processes in soils. To further our understanding of the specific processes that are involved in macropore flow, we have developed a Split Macropore Column (SMC) which consists of a semi‐cylindrical soil column with a semi‐cylindrical macropore at the main axial of its planer surface. The SMC allows one to control a number of important physical parameters such as the size, density, and continuity of the macropores for miscible displacement studies, as well as an opportunity to observe visually the macropore's impact on flow processes at the critical matrix‐macropore interface. After rigorous testing, we found that this system is a useful instrument to study macropore flow mechanisms. Using these SMCs, we performed a series of miscible displacement experiments to study the effects of soil type, flux rate, macropore size, and macropore to matrix size ratio. Our initial results show three zones of flow through the column: (i) film flow at the matrix–macropore interface, (ii) in the matrix directly adjacent to the macropore, and (iii) in the remaining matrix. The initial data suggest that macropore flow is influenced more by the matrix–macropore size ratio than by the macropore size.