Effective monitoring of surface barriers intended to isolate and protect waste from the accessible environment requires techniques to assess their performance. Quantifying drainage conditions at any point below the barrier is difficult because field-scale drainage measurements are not well suited for assessing spatial drainage heterogeneity. Measurements of water storage, however, can provide an indirect measure of impending drainage. We measured seasonal water content at a vegetated capillary barrier on the Hanford Site in southeastern Washington to determine effective water-storage monitoring methods. Measurements were made using ground-penetrating radar (GPR) direct ground wave methods operating at 100 MHz. Simultaneous measurements using time-domain reflectometry (TDR) and neutron-scattering probe (NP) were used to determine the sampling depth of the GPR ground wave. The ground wave sampling depth increased only slightly with decreasing moisture content, ranging from 30 to 37.5 cm. The TDR measurements were made using permanently emplaced arrays, eliminating the need for repeated ground disturbance. All three methods showed similar seasonal responses, with the highest water contents observed during the winter months and decreasing through the summer. Unlike TDR and NPs, which are local-scale measurements, GPR exhibited much greater spatial detail across the barrier surface. While measurement uncertainties exist with regard to the sampling depth and dispersive effects, our results indicate that GPR can be an effective and minimally invasive method for providing reasonable near-surface estimates of soil water content changes within an engineered barrier.