AIr and Water Permeability in Differently Textured Soils at Two Measurement Scales

Air permeability can be used to describe the structure of the soil but may also be used to predict saturated hydraulic conductivity. This raises the question of whether the two parameters exhibit the same degree of scale dependency. In this study the scale dependency of water permeability (saturated hydraulic conductivity, Kw) and air permeability (ka, at a matric water potential of −50 cm H2O) was tested at four different sites (three horizons at each site), by using two measurement scales (100 cm3 and 6280 cm3). No clear effect of scale on variability was observed. Air and water permeability displayed higher variabilities for two structured loamy soils compared with two sandy soils. For the more structured soils, the variability between measurements was lower for air compared with water permeability. Both air and water permeabilities were higher at the large scale compared with the small scale, but this scale-dependent difference was less pronounced in sandy soils, suggesting a smaller representative elementary volume. For three of the four soils, a highly correlated relationship between Kw and ka on both small and large soil samples was observed. For the fourth soil, water retention data revealed that the samples were not sufficiently drained at −50 cm H2O to validate a comparison between the two parameters. Predictive Kw (ka) relations for the remaining three soils at the two scales compared favorably with a general Kw (ka) relation proposed by Loll et al. (1999). This study supports the use of a general predictive relation between ka near field capacity (at around −50 to −100 cm H2O) and Kw, but caution should be taken if the soil has a large content of pores that will drain at or close to a matric water potential of −50 cm H2O.