The saturated hydraulic conductivity (Ks) is an essential parameter for modeling water and chemical transport in the vadose zone. Since in situ measurements of Ks are complex and time-consuming, indirect methods that are dependable, fast, and inexpensive with regard to assessing magnitude and spatial variability in Ks at the field scale are needed. In situ measurements of air permeability (ka,in situ) may fulfill these criteria. In this study, a portable insertion-type air permeameter was used to measure ka,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 ka,in situ measurements were performed within 2 d at each field site. The data showed spatial correlation in ka,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 cm3), a log-log linear relationship between air permeability (ka) measured at the actual soil-water content (close to field capacity) and Ks was found. The Kska relation was in agreement with an earlier predictive relationship based on undisturbed 100-cm3 samples from nine other field sites. Using pedotransfer functions for Ks based only on soil texture yielded an unrealistic narrow range in predicted Ks values whereas pedotransfer functions based on ka,in situ yielded a more realistic prediction range. Measurements of ka,in situ constitute a promising indirect method for assessing spatial variability in Ks at the field scale.

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