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 q uestion of whether the two parameters exhibit the same degree of scale depe ndency. In this study the scale dependency of water permeability (saturated hydraulic conductivity, K-u,) and air permeability (k(a), at a matric wate r potential of -50 cm H2O) was tested at four different sites (three horizo ns at each site), by using two measurement scales (100 cm(3) and 6280 cm(3) ). No clear effect of scale on variability was observed. Air and water perm eability displayed higher variabilities for two structured loamy soils comp ared with two sandy soils. For the more structured soils, the variability b etween measurements was lower for air compared with water permeability. Bot h air and water permeabilities were higher at the large scale compared with the small scale, but this scale-dependent difference was less pronounced i n sandy soils, suggesting a smaller representative elementary volume. For t hree of the four soils, a highly correlated relationship between K-u, and k (a) on both small and large soil samples was observed. For the fourth soil, water retention data revealed that the samples were not sufficiently drain ed at -50 cm H2O to validate a comparison between the two parameters. Predi ctive K-u,(k(a)) relations for the remaining three soils at the two scales compared favorably with a general K-u,(k(a)) relation proposed by Loll et a l. (1999). This study supports the use of a general predictive relation bet ween k(a) near field capacity (at around -50 to -100 cm H2O) and K-w, but c aution should be taken if the soil has a large content of pores that will d rain at or close to a matric water potential of -50 cm H2O.