Time Domains for Early‐Time and Steady‐State Pressure Infiltrometer Data

The determination of the field‐saturated hydraulic conductivity, K fs , using a Guelph Pressure Infiltrometer, typically relies on the establishment of steady‐state flow. For media with very low values of K fs , such as the compacted clay liners of landfills, the establishment of steady‐state flow may take months. It then becomes opportune to rely on early‐time observations of infiltration to infer K fs . The time domain in which early‐time data analysis is valid can, for the first time, be delineated by equating the flux from both the early‐time and steady‐state flow expressions. This new delineating boundary value will be indicated by t cr . This will be the critical time at which the early‐time behavior gradually transits into the steady‐state time domain. There is, however, a time zone around t cr in which neither the early‐time analysis nor the steady‐state analysis is appropriate, as flow is neither purely dominated by capillary forces nor gravitational forces. Results of t cr delineations are presented as a function of two field and clay liner parameters: K fs and Δθ, the change in volumetric water content between initial and field‐saturated conditions. Graphical interpretation reveals the existence of the inversely proportional relationships: critical time vs. K fs and critical time vs. applied pressure head. Also evident is the directly proportionate response of the critical time to change in Δθ. We find that the present form of the critical time expression can be reduced to previous formulations concerning characteristic times. We offer these derivations as proof of the validity of this new critical time parameter.