Evapotranspiration (ET) covers are a type of cap placed over contaminated material, such as soil, landfill waste, or mine tailings, to prevent water from reaching it and to isolate it from people and wildlife. They differ from other types of caps in the way they prevent water from seeping into the waste. (See Community Guide to Capping.) ET covers store water from rainfall and snowmelt until: 1) drier or warmer weather evaporates the water, or 2) plant roots take up the water and release it to the air as water vapor through the leaves and stems. This process is called “transpiration.”