DISCREPANCIES BETWEEN ANALYTICAL SOLUTIONS OF TWO BOREHOLE PERMEAMETERS FOR ESTIMATING FIELD-SATURATED HYDRAULIC CONDUCTIVITY

Abstract

Field-saturated hydraulic conductivity (Kfs) is considered the most important parameter for water flow and chemical transport phenomena in soils. The Richards' and Laplace's solutions of the Guelph Permeameter (GP) and the Glover's solution of the Compact Constant Head Permeameter (CCHP) for calculating Kfs were compared. Steady-state flow rates of water into soil at a single constant head infiltration (H = 20 cm) from a borehole measured with the Guelph permeameter method were used to estimate Kfs values using these solutions. The geometric mean values of Kfs calculated using Richards', Laplace's, and Glover's solutions were 0.112, 0.185, and 0.224 cm h −1 , respectively, for a Duffield silt loam soil. The Glover's and Laplace's solutions, neither of which takes into account the effect of unsaturated capillary flow, produce Kfs values approximately 1.5 to 2 times larger than the Kfs values calculated using Richards' solution. While the Glover's solution gives Kfs values nearly 1.4 times larger than those estimated by the Laplace's solution. The student t-test showed that the mean difference (Md) among Richards', Laplace's, and Glover's solutions were significantly different from zero at p < 0.01. Thus, statistical analyses indicated that the three analytical methods result in dissimilar estimates of Kfs values. Negative Kfs values are often obtained using simultaneous equations of Richards' solution approach in heterogeneous soils and the Richards' solution can only be used at one constant water depth when an value must be estimated or assigned based on the soil texture and structure. Both Glover's and Laplace's solutions can be used in coarse textured soils where the capillarity effect is minimal and initial water content in the soil is near the field capacity level. This indicates that the variability in Kfs estimates depend not only on soil structure, texture, and other soil characteristics, but also on the method of estimation imposed by the borehole analytical solution.