Hydraulic conductivity function data from the technical literature for different soil types, defined using different experimental approaches, are used to assess the uncertainty involved in empirical prediction models. Empirical predictions were found to lead to an error in hydraulic conductivity of 1 to 4 orders of magnitude, with the greatest discrepancies at low moisture contents. This finding emphasizes the importance of soil-specific characterization of hydraulic properties for use in design, using either flow pump permeameter testing or column infiltration testing. An infiltration example is used to highlight the impact of uncertainty in predicted hydraulic conductivity functions on engineering analyses. Uncertainty in the hydraulic conductivity function was found to have the greatest impact on suction profiles observed during wetting under infiltration rates close to the saturated hydraulic conductivity.