ESTIMATING SOIL HYDRAULIC-PROPERTIES USING TENSION INFILTROMETERS WITH VARYING DISK DIAMETERS

Tension infiltrometers have become a popular instrument for field dete rmination of soil hydraulic properties, such as hydraulic conductivity (K-S) of saturated soil and the parameter (alpha) 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 l ikely caused by the variable sizes of the infiltrometer disk used for the infiltration measurement and/or the limitations of steady-state so lutions for small disk dimensions. To determine the effect of disk siz es on parameter estimation, we measured infiltration in two soils (Arl ington sandy loam and Sparta sand) with tension infiltrometers of seve ral 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 d isk size. Results indicate that estimated values of K-S and alpha appe ared to vary with the size of the infiltrometer disk used. Variations in estimated K-S and alpha values for different disk sizes or for diff erent potential increments for the same disk were greater than the pot ential overestimation with the steady-state solution when compared,vit h an improved solution for small disk sizes. Discrepancies between ten sion infiltrometer and other methods in practice are probably caused b y variability within each method, such as soil heterogeneity or simpli fying the hydraulic conductivity function to the exponential expressio n, rather than by limitations in the steady-state solution for small t ension infiltrometer disk sizes.