ESTIMATING SOIL HYDRAULIC PROPERTIES USING TENSION INFILTROMETERS WITH VARYING DISK DIAMETERS

Tension infiltrometers have become a popular instrument for field determination of soil hydraulic properties, such as hydraulic conductivity (KS) of saturated soil and the parameter (α) used in exponential expressions of the hydraulic conductivity function. However, results different from other independent field or laboratory measurements are often obtained using the steady-state approximate solutions. This is likely caused by the variable sizes of the infiltrometer disk used for the infiltration measurement and/or the limitations of steady-state solutions for small disk dimensions. To determine the effect of disk sizes on parameter estimation, we measured infiltration in two soils (Arlington sandy loam and Sparta sand) with tension infiltrometers of several disk diameters (5.5-34.5 cm). For each disk size, the infiltration was repeated at multiple supply potentials and continued until steady-state so that replicated parameter estimates were obtained for each disk size. Results indicate that estimated values of KS and α appeared to vary with the size of the infiltrometer disk used. Variations in estimated KS and α values for different disk sizes or for different potential increments for the same disk were greater than the potential overestimation with the steady-state solution when compared with an improved solution for small disk sizes. Discrepancies between tension infiltrometer and other methods in practice are probably caused by variability within each method, such as soil heterogeneity or simplifying the hydraulic conductivity function to the exponential expression, rather than by limitations in the steady-state solution for small tension infiltrometer disk sizes.