Prediction of Macropore Water and Air Flow at the Field Scale: Empirical Models v/s Lattice Boltzmann Simulations.

Preferential flow through soil macropores governs the transport of pollutants and also affects infiltration and runoff generation. However, macropore flow currently cannot be reliably predicted at the field scale because of inherently large spatial variability. The aim of this study was to perform field scale characterization of preferential flow and investigate the performance of predictive empirical models and X-ray Computed Tomography (CT) coupled lattice Boltzmann simulations. For this purpose, 65 cylindrical soil columns (6 cm diameter and 3.5 cm height) were sampled from the top soil (5 to 8.5 cm depth) in a 15×15 m grid from an agricultural loamy field located in Silstrup, Denmark. All soil columns were scanned with an industrial CT scanner (129 µm resolution) and later used for measurement of saturated water permeability and air permeability at -30 and -100 cm matric potentials. Distribution maps and semivariograms for both water and air permeabilities reflected no significant spatial correlation. Empirical predictive models showed poor performance as they were not able to realistically capture macropore flow. This is because both water and air permeabilities were poorly correlated with soil texture, bulk density, etc. and strongly correlated with X-ray CT derived mean pore diameter. Air-filled porosity at -30 cm matric potential compared well with X-ray CT derived porosity at 129 µm resolution. The X-ray CT derived soil macropore structure provided the boundaries for lattice Boltzmann simulations, in which fluid-solid boundary was treated as a non-slip boundary and solved by means of the bounce-back approach. The simulated permeability was compared with measured saturated water permeability and air permeability at -30 and -100 cm matric potential, and the results showed good agreement. While computationally intensive, the results of this study were very encouraging and may pave the way for establishment of a digital soil physics laboratory in future.