Surface barriers reduce water percolation through contaminated vadose zones, but confirming flow reduction is challenging. We propose a hydraulic-conductivity factor as a conservative indicator of the reduction in soil-water flow beneath surface barriers. The factor can be formulated using measured soil-water contents or pressures without knowledge of saturated hydraulic conductivities or hydraulic gradients. Factor determination does not require direct measurements of water flux, hence it is cost effective. Pressure- and water-content-based formulas were demonstrated using data from a drainage experiment and the former was further demonstrated for an interim barrier over buried tanks, one of which leaked radioactive waste at the Hanford Site. Three years after barrier emplacement, hydraulic conductivity decreased at 1-, 2- and 5-m measurement depths. Numerical simulations explored the relations among flux and conductivity factors. Drainage rates changed slowly at depth, with years to decades for substantial flux reduction at the underlying water table after surface barrier emplacement.