Evaluation of two generalized complementary functions for annual evaporation estimation on the Loess plateau, China

Abstract Generalized complementary functions, which describe the relationship between the ratio of actual evaporation over the Penman potential evaporation (E/EPen) and the proportion of the radiation term in EPen (Erad/EPen) 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 Erad/EPen were found to be much larger than zero, and the annual E/EPen increased approximately linearly with annual Erad/EPen. This complementary evaporation relationship at an annual timescale differs from those at daily timescales, and requires different parameterizations of the GNAA and SGCF. For the GNAA, parameter c, which is often set to zero for daily time scales, need to be well calibrated with available data. For the SGCF, the upper and lower limits of Erad/EPen 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.