Assessment of liquid (soil water) flux under unsaturated flow conditions is an important factor in permitting and design of evapotranspirative final cover systems (ET covers) for landfills. Unsaturated flow flux assessments are employed to demonstrate regulatory compliance for permitting of ET covers, to develop construction specifications for approved ET covers, and to establish performance criteria for monitoring of constructed ET covers. Ideally, unsaturated flow flux assessments for ET covers should be based upon site-specific data and should be made using boundary conditions and soil profiles as close as possible to prototype conditions. In practice unsaturated flow flux assessments for ET cover performance have typically been based upon either direct flux measurements from lysimeter test sections or unsaturated flow modeling. The unsaturated flow model may often be calibrated using either percolation and run-off data from lysimeter test pads or internal soil moisture content data from the as-built cover. Lysimeter test pad data are adversely impacted by unrealistic lysimeter boundary conditions, difficulties in accurately measuring small values of percolation, and the inability of unsaturated flow models to accurately predict runoff, while flux values predicted using models calibrated solely upon internal soil moisture content data are of questionable reliability. Therefore, choice of the appropriate approach with which to demonstrate regulatory compliance is often a question of "the lesser of two evils." However, consideration of the limitations of these two approaches suggests that for municipal solid waste landfills in arid and semi-arid climates the use of internal soil moisture measurements from a prototype cover in situ may be preferable for both regulatory compliance evaluation and design of ET covers due to a more accurate simulation of prototype conditions as well as other practical engineering considerations. Copyright ASCE 2006.