Transient flow from tension infiltrometers: I. The two-parameter equation

Tension disk infiltrometer experiments are generally conducted until appare nt steady state is reached because most of the methods of analysis are base d on Wooding's solution for steady state flux. However, the time necessary to reach steady state may be a penalizing aspect for soils with low permeab ility and the information contained in the transient stages is not utilized . Moreover, these methods assume homogeneous soil and a uniform initial wat er content, which may be unrealistic when a large volume of soil is sampled . In this series, we propose and compare several new methods of analysis th at are based on the transient stage of axisymmetric infiltration. In the fi rst part, we show that a two-parameter equation-one term linear in square r oot of time and one term linear in time-adequately describes the transient now from the disk infiltrometer for both simulated and laboratory tests. Th e technique used for the determination of the two coefficients must meet tw o criteria; it must verify the validity of the two-term equation throughout the duration of the experiment, and it must account for the early-time per turbation that is induced by the sand-contact layer placed between the disk and the soil. We show that the best technique consists in linearizing the data by differentiating cumulative infiltration with respect to the square root of time. Direct nonlinear fitting on cumulative infiltration or infilt ration flux is Likely to lead to unacceptable errors, either because of the undetected invalidity of the two-parameter equation or arising from the in fluence of the contact layer.