Fig. 8. Independent test of new models of air permeability as a function of air-filled porosity, k_a(), for Hjørring subsurface soils (12 samples from 4–5-m depth and 12 samples from 6–7-m depth): (a) log-transformed air permeability at matric potential of –100 cm H₂O, log(k_a,100), as a function of air-filled porosity, ₁₀₀ [estimated and regression curves for k_a,100 (Eq. [12] with equivalent pore diameter for gas flow = 150 µm and Eq. [13]) are given]; (b) scatterplot comparison of predicted and measured log(k_a). Calculated RMSE values by Eq. [2] using log(k_a) data are given.