Characterizing of a capillary barrier evapotranspirative cover under high precipitation conditions

Capillary barrier covers are a type of evapotranspirative (ET) cover in which the water storage capacity of an overlying finer textured soil layer is enhanced by the presence of an underlying coarser textured soil layer. The capillary barrier will fail when the overlying finer textured layer becomes sufficiently saturated, so capillary barriers generally are considered for use in arid or semi-arid areas. As a result, there have been few tests or practical applications of capillary barrier covers in humid regions. In this study, a column test was conducted at a humid site where 2361 mm precipitation occurred during a 541-day monitoring period. Water content, percolation, surface runoff and meteorological data were recorded. The water balance of the column and a breakthrough characterization of the capillary barrier were investigated. Numerical predictions using the Hydrologic Evaluation of Landfill Performance (HELP) model and the VADOSE/W model were performed. Test results showed that surface runoff during the monitoring period was 88 mm, which corresponded to 3.7 % of precipitation. Percolation was 67 mm, which equated to 2.8 % of precipitation. The majority of the precipitation (93.5 %) was evaporated back to the atmosphere. Failure of the capillary barrier occurred only when the critical volumetric water content of the overlying clay layer was exceeded. The breakthrough was caused by consecutive intense rainfall events. VADOSE/W predicted the behavior more accurately than HELP, because VADOSE/W can model the unsaturated flow behavior associated with the capillary barrier covers.