Field Application of a Portable Air Permeameter to Characterize Spatial Variability in Air and Water Permeability

The saturated hydraulic conductivity ( K s ) is an essential parameter for modeling water and chemical transport in the vadose zone. Since in situ measurements of K s are complex and time‐consuming, indirect methods that are dependable, fast, and inexpensive with regard to assessing magnitude and spatial variability in K s at the field scale are needed. In situ measurements of air permeability ( k a,in situ ) may fulfill these criteria. In this study, a portable insertion‐type air permeameter was used to measure k a,in situ in the Ap and B horizons at five agricultural field sites in Denmark with soil types ranging from sand to sandy loam. Around 100 k a,in situ measurements were performed within 2 d at each field site. The data showed spatial correlation in k a,in situ at three out of five sites, with correlation distances between 30 and >120 m. On the basis of additional laboratory measurements on large, undisturbed soil samples (6280 cm 3 ), a log‐log linear relationship between air permeability ( k a ) measured at the actual soil‐water content (close to field capacity) and K s was found. The K s – k a relation was in agreement with an earlier predictive relationship based on undisturbed 100‐cm 3 samples from nine other field sites. Using pedotransfer functions for K s based only on soil texture yielded an unrealistic narrow range in predicted K s values whereas pedotransfer functions based on k a,in situ yielded a more realistic prediction range. Measurements of k a,in situ constitute a promising indirect method for assessing spatial variability in K s at the field scale.