MEASURING HYDRAULIC CONDUCTIVITY IN A CRACKING CLAY SOIL USING THE GUELPH PERMEAMETER

Abstract

Knowing the variation of field-saturated hydraulic conductivity (Kfs) with space, time and antecedent soilmoisture conditions can be important for optimum management of cracking clay soils. The Guelph Permeameter (GP)method is a potentially valuable technique for measuring Kfs and its variation in cracking soils, but concerns existregarding (1) impacts of smearing and compaction in the GP well on the Kfs measurement, (2) accurate detection of whensteady flow from the GP is attained, and (3) time required for the GP to reach steady flow (equilibration time). Theseconcerns were investigated for a series of GP measurements in a cracking clay soil (Vertic Xerochrept) in Sicily by testinga plucking implement for removal of smearing and compaction, and by comparing three approaches for detecting steadyflow and equilibration time. The mean Kfs values (11.4-22.1 mm h1) obtained from the GP measurements were 1-3 ordersof magnitude larger than what might be expected for a clayey texture, indicating that ponded infiltration from the GPwells occurred primarily through highly permeable soil shrinkage cracks, rather than through the low permeability clayeymatrix. Wells that were treated with the plucking implement produced a non-significant (P < 0.05) factor of 1.94 increasein mean Kfs relative to untreated wells, suggesting that this method for removing smearing and compaction functionedprimarily by reopening partially smeared-over shrinkage cracks or other macropores. Relatively short mean equilibrationtimes were obtained for the GP measurements (11-42 min), suggesting that the GP method can measure the soilsantecedent Kfs before wetting from the well causes enough soil swelling to produce a declining permeability. Thetraditional four equal readings (FR) procedure for estimating GP equilibration time substantially underestimated themore recent visual estimation (VE) and the new cumulative drop (CD) procedures. This underestimation by the FRprocedure resulted in overestimates of Kfs relative to the VE and CD procedures for Kfs values less than about 4 mm h1.It was concluded that the GP method can be a viable technique for measuring Kfs in cracking clay soil.