An examination of the Guelph permeameter for measuring the soil's hydraulic properties

Abstract

The Guelph permeameter method (GPM) is a constant-head well permeameter or shallow well pump-in technique for measuring soil hydraulic properties in the unsaturated zone, i.e., the field-saturated hydraulic conductivity, K fs, and the matric flux potential, φ m. Unfortunately it is not uncommon to obtain negative K fs- and φ m-values with the GPM, especially on heterogeneous soils. The objective of this study was therefore to thoroughly examine the GPM for different soil types over a range of initial soil water content conditions and to evaluate the existence of negative K fs- and φ m-values. Two permeameters of different diameter were constructed to test the GPM. Three sets of 16 wells were prepared at sites of three selected soil types, viz., a Norfolk sandy loam, a Lucedale loam, and a Troup loamy sand, each set having a different well diameter ranging from 36 to 86 mm. Steady state flow in each well was measured at 4 water levels ranging from 50 to 125 mm. For the same wells the measurements were repeated for relatively wet and relatively dry initial soil water content conditions. For yet another site, K fs-values were compared with the saturated hydraulic conductivity values ( K s) obtained by the constant-head method applied to undisturbed soil cores. The in-situ measurements and the undisturbed cores were taken at a depth of 0.34 m at 60 locations on a 50-m × 100-m experimental field. Approximately 40% of the results obtained with the GPM were negative. This was attributed to random heterogeneities and systematic soil textural changes with depth. Fewer negative results were obtained for the coarser soil types. Positive values also seemed to have been affected as indicated by extremely high values of the exponential factor in the hydraulic conductivity-pressure head relation. A regression analysis on the K fs- and K s-values hardly showed any correlation between the results of the GPM and the constant-head method. A co-kriging procedure, using clay content and water content data as covariates, to obtain K s-values for exactly the same locations as where the K fs-values were obtained, gave a slightly higher regression coefficient, but it was still concluded that no significant correlation existed between the two methods.