The Relationship between Perc Time and Field-saturated Hydraulic Conductivity for Cylindrical Test Holes

Correlations between Perc Time (PT) and field-saturated hydraulic conductivity (Kfs) are often used in development of on-site water recycling and treatment facilities that operate by infiltration into unsaturated soil. The accuracy and utility of these correlations is suspect, however, because they differ greatly from each other, and they derive from empirical regressions or simplified analyses that incompletely describe the factors affecting PT and Kfs. An accurate and physically based analytical expression relating PT to Kfs for cylindrical test holes was used to compare and assess six PT versus Kfs correlations. Some of the correlations appear to apply only for specific test conditions, while others are of unknown origin, or seem inconsistent with established soil water flow theory. It was consequently recommended that the proposed analytical expression relating PT to Kfs be used in place of the existing PT versus Kfs correlations, as it provides PTKfs results that are functionally linked, founded on rigorous infiltration theory, and based on known and well-defined borehole test conditions.