ET cover systems use one or more vegetated soil layers to retain water until it is either transpired through vegetation or evaporated from the soil surface. These cover systems rely on the water storage capacity of the soil layer, rather than low hydraulic conductivity materials, to minimize percolation. ET cover system designs are based on using the hydrological processes (water balance components) at a site, which include the water storage capacity of the soil, precipitation, surface runoff, evapotranspiration, and infiltration. The greater the storage capacity and evapotranspirative properties, the lower the potential for percolation through the cover system. ET cover system designs tend to emphasize the following:
• Fine-grained soils, such as silts and clayey silts, that have a relatively high water storage capacity
• Native vegetation to increase evapotranspiration
• Locally available soils to streamline construction and provide for cost savings