Generalized complementary functions, which describe the relationship between the ratio of actual evaporation over the Penman potential evaporation (E/E_Pen) and the proportion of the radiation term in E_Pen (E_rad/E_Pen) have not been widely used at annual time scales. In this study, the generalized nonlinear advection-aridity function (GNAA) and sigmoid generalized complementary function (SGCF) were evaluated for annual evaporation estimation with calibrated parameters in the Loess Plateau of China. For all 15 catchments, the lowest values of annual E_rad/E_Pen were found to be much larger than zero, and the annual E/E_Pen increased approximately linearly with annual E_rad/E_Pen. This complementary curve at an annual timescale differs from those at daily timescales, which require different parameterizations of the GNAA and SGCF. For the GNAA, parameter c, which is often set to zero for daily time scales, must be well calibrated. For the SGCF, the upper and lower limits of E_rad/E_Pen must be constrained. After calibration, both the SGCF and GNAA performed well at estimating annual evaporation. In addition, the calibrated Priestley-Taylor coefficient from the SGCF was found to be closer to the widely accepted value (1.26) than that determined from the GNAA.