Most infiltration methods measure the saturated hydraulic conductivity (Ks) of the unsaturated zone at only one depth at or near the surface. The purpose of this study was to extend the falling-head infiltration theory to a multistep falling-head infiltration test (MSFIT) for estimating Ks at various depths in a one-dimensional saturated vertical flow domain. A general analytical solution for Ks is derived for the wetted region of each step in the MSFIT. The Ks values for individual wetted sub-regions are calculated sequentially from the top down based on the fact that the Ks value of the whole wetted region is the depth-weighted harmonic mean of sub-regions for flow perpendicular to the wetted layers. The resolution of the wetted sub-region is determined by the number of steps and the change of the ponded water head. The MSFIT is performed in three independent laboratory two-layer sand columns. The mean value of Ks for the upper coarse sand is 2.13 × 10 -2 cm/sec, and that for the lower fine sand is 3.63 × 1010 -3 cm/sec. Similar Ks results are obtained from 12 core sample measurements. In a field study with the MSFIT, the mean values of Ks estimated from top down are 8.37 × 1010 -4, 2.73 × 1010 -4, and 3.21 × 1010 -5 cm/sec, respectively. The decreasing Ks with depth is similar to the result of small core samples and is consistent with field observations of the soil texture. The proposed MSFIT is simple, flexible, and versatile for obtaining hydraulic conductivity at various depths for one-dimensional saturated vertical flow domains.
All Science Journal Classification (ASJC) codes