Macropores are known to be important pathways for the rapid transport of water, solutes and colloids in soil.
Nevertheless, we still know very little about how the topology and geometry of macropore networks govern
water flow configurations and velocities in natural soil. In this study, we aimed at gaining more insight into
macropore flow by using X-ray tomography to quantify air–water distributions in the macropore networks of
undisturbed topsoil and subsoil columns of a clay soil at varying steady-state flow rates. We observed that while
large fractions of the macropore network remained air-filled, the air phase only became entrapped when the
irrigation rate was very close to the saturated hydraulic conductivity of the soil. The data enabled us to
parameterize a kinematic wave model for water flow following the approach proposed in Jarvis et al. (2017a).
Follow-up experiments would be required to evaluate whether these kinematic wave parameters derived by Xray
imaging match with those obtained from outflow measurements. We found that quantitative X-ray imaging of
macropore flow through soils still remains a challenging task. We recommend that future experiments are
conducted on smaller soil samples to improve image resolution and minimize experimental time spans as well as
X-ray image noise and illumination bias. Such experiments could also include 3-D tracer imaging to identify the
imaged macropore networks transporting most of the water (i.e. the backbone) at varying steady irrigation rates.