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a Jackson School of Geosciences, Bureau of Economic Geology, The University of Texas at Austin, Austin, TX 78758
b U.S. Dep. of Energy, Sandia Natl. Lab., Albuquerque, NM 87185
* Corresponding author (bridget.scanlon{at}beg.utexas.edu)
Received 6 April 2004.
Performance evaluation of evapotranspirative (ET) covers is critical for waste containment. The purpose of this study was to evaluate ET covers at sites in Texas and New Mexico representative of arid and semiarid regions in the southwestern USA using water balance monitoring during 4- and 5-yr periods and water balance simulations using short-term (1–5 yr) and long-term (25 yr) climate forcing. Estimated drainage at the Texas site was related to irrigation while measured drainage at the New Mexico site was restricted to the first 2 yr of the 5-yr monitoring period. Evapotranspirative covers work extremely well in these regions because of the dominance of summer precipitation (62–80%) that corresponds to periods of highest ET. Strong relationships between decreases in soil water storage and vegetation productivity at both sites underscore the importance of vegetation in controlling the water balance in these systems. Simulations of the Texas site indicate that drainage can occur in response to high precipitation near the end of the growing season, but such drainage can be eliminated with a capillary barrier. Inclusion of a capillary barrier increased available water storage by a factor of about 2.5 at both sites. The capillary barrier effect of drainage lysimeters can result in underestimation of drainage and overestimation of water storage relative to covers not underlain by capillary barriers. The data from this study indicate that a 1-m-thick ET cover underlain by a capillary barrier should be adequate to minimize drainage to 
1 mm yr–1 in these arid and semiarid regions. Comprehensive monitoring integrated with modeling is required to assess total system performance to develop a predictive understanding of ET covers.
Abbreviations: AWS, available water storage • CB, capillary barrier • ET, evapotranspirative • GAB, geosynthetic clay layer overlying an asphalt barrier • GCL, geosynthetic clay layer • LAI, leaf area index • PET, potential evapotranspiration • TDR, time domain reflectometry
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