EXECUTIVE SUMMARYEvapotranspiration (ET) covers have gained widespread acceptance as a closure feature for waste disposal sites, particularly in arid and semi-arid regions of the southwestern United States. In addition, monolayer ET covers have been selected as the baseline method for closure of waste disposal sites for low-level radioactive (LLW), mixed LLW, and transuranic (TRU) waste disposal sites at the Nevada Test Site (NTS). However, ET covers are also landforms that will be subject to environmental change over time because of processes such as pedogenesis, hydrologic processes, vegetation establishment and change, and biological processes. Often these processes are viewed individually, when, in fact, in most cases they are interrelated or coupled.To better understand the effects of coupled process changes to ET covers, a series of four primary analog sites in Yucca Flat on the NTS, along with measurements and observations from other locations in the Mojave Desert, were selected to evaluate changes in ET covers over time. An important assumption in selecting a temporal sequence of analog sites is that looking back in time provides evidence of how similar features will change in the future. The ET cover constructed for the U3axbl waste cell in the Area 3 Radioactive Waste Management Site of the NTS was used as a baseline for comparison. Field and laboratory studies and literature reviews were conducted to evaluate the possible impacts of change on performance objectives of ET covers elucidated in U.S. Department of Energy, National Nuclear Security Administration Nevada Site Office (NNSA/NSO) Waste Management Program "Integrated Closure and Monitoring Plan" for the NTS. Performance criteria include 1) that the cover provides long-term minimization of migration of liquids through the landfill; 2) that the cover function with minimum maintenance; and 3) that the cover promotes drainage, with minimal erosion or abrasion. Analyses were also carried out to address NNSA/NSO technology needs for ET covers including providing information "on long-term evolution…of these covers as landform features that change because of natural pedogenic, biotic, and climatic processes; and maintaining functional requirements including 'limiting infiltration' and maintaining 'effective high ET ability.'"The analog sites (approximately 30 years, 1,000 to 2,000 years, 7,000 to 12,500 years, and approximately 125,000 years in age) were selected to addresses changes in the early post-institutional control period (the youngest site), the 1,000-year compliance period for disposal of LLW and mixed LLW, and the 10,000-year compliance period for TRU waste sites. Analog sites were selected based on criteria that included: 1) evidence that the sites had been isolated from recent alluvial, colluvial, and erosion processes that would not be expected to be agents on newly constructed ET covers; 2) low surface gradients, eliminating the need to consider processes such as sheet wash that a newly constructed cover would also be designed not to exp...