Hydraulic conductivity of a silt loam soil as affected by sample length

Soil hydraulic conductivity is often measured with small laboratory samples. Due to sampling artifacts or spatial variability, small-scale laboratory samples may not yield representative conductivity measurements. The objective of this study was to analyze the effect of sample length on the saturated and near-saturated hydraulic conductivity (K). An undisturbed soil core of 9 cm diameter and 25 cm length was extracted from the A horizon of a no-till, silt loam soil, and K was determined at hydraulic heads (matric potential heads) of 0, -4, -8, -12, -16, and -22 cm-H2O under steady-state flow conditions. The core was then dissected incrementally from the bottom in 5 cm segments. At each dissection step, the remaining core and the cutoff core were re-saturated, and K was determined. Measured K varied considerably with core length, with the saturated conductivity being much more variable than the unsaturated conductivities. The use of consecutive 5 cut core segments to determine the effective K resulted in an underestimation of the conductivity compared to direct measurements on longer cores, possibly caused by smearing or compaction of cut surfaces during core dissection. Effective K determined from 5 cm core segments matched well with directly measured K only at hydraulic heads more negative than - 16 cm-H2O. The results of this study highlight that small cores on the order of a few cm may not be representative of a larger soil profile such as the plow layer or crop root zone.