In-situ Visualization of Flow Mechanisms in Macroporous Soils using 4D
X-Ray Computed Tomography
Abstract
Transfer of mass between macropores and the soil matrix is an important
control on flow and solute transport in the vadose zone. Few empirical
techniques are available to explicitly investigate how the fast flows in
macropores interact with the slower flows in the matrix to allow the
flow system to evolve over time. In this study, time-lapse X-ray
Computed Tomography (CT) scans are used to obtain quantitative 4D (i.e.,
transient three-dimensional) images of infiltration in two soil columns:
one homogenous, non-macroporous and one containing a network of
desiccation cracks. Water was applied to the top of each column at
increasing rates over the flow period. High resolution (80 micron) CT
images of the columns were collected throughout the infiltration
experiments at 7-minute intervals. These images were processed to obtain
time-varying maps of water content that provide insights to the
evolution of the flow patterns and mechanisms of interaction between the
macropore and matrix domains. Flow in the non-macroporous column was
observed to be nearly uniform, whereas flow behavior in the macroporous
column was dependent on the influent water flux. At low infiltration
rates, film flow occurred in the macropores with comparatively little
imbibition from macropore to matrix. At high infiltration rates, the
macropores filled with water and imbibition to the matrix increased.
Results demonstrate that wetting of the soil is a complex process
reflecting contributions from downward infiltration through
macropore-matrix networks and lateral wetting from the macropores.