6 METHODS FOR DETERMINING SORPTIVITY AND HYDRAULIC CONDUCTIVITY WITH DISC PERMEAMETERS

The disc permeameter has become a popular apparatus for measuring in s itu the sorptivity, S, and hydraulic conductivity, K, of the soil at s ome prescribed potential. A number of different methods have been prop osed for calculating S and K using the flow rate, Q(t), from the disc. Measurements of Q(t) on a Kokotau silt loam soil were made using disc s with radii of 50 and 102 mm. Measurements were made at potentials of -20, -40, and -100 mm with the 102-mm disc and at potentials of -20 a nd -40 mm with the 50 mm disc. S and K were calculated using six diffe rent methods. Three of the methods use Wooding's equation of flow from a disc: Q(infinity)/pi r(2) = Delta K[1 + 4 lambda(c)/pi r], where Q( infinity) is the steady-state flow rate; r is the radius of the disc s ource; Delta K = K(Psi(o),) - K(Psi(n)) (where Psi(o) and Psi(n) are, respectively, the potential of the source and the initial potential of the soil); and lambda(c) is the macroscopic capillary length scale. T hese three methods and a method based on a linear diffusion model gave similar values of S and/or K. The method of Youngs, which uses the ea rly- to medium-time infiltration data to calculate K, gave very low,an d inconsistent values of K. Calculations of K were also made from valu es of S at two different potentials. Here S as S-o was first calculate d by using the early-time values of Q(t(1/2)). These values of K were similar to those calculated using the methods based on Wooding's equat ion. This suggests that measuring S at two or more different potential s, using the disc permeameter, would provide a very rapid method for c haracterizing the hydraulic properties of soils, provided the soil is fine textured and S can be measured either from the early-time behavio r or using the method of Warrick.