Comparison of formulating apparent potential evaporation with pan measurements and Penman methods

Abstract

The apparent potential evaporation Epa represents the atmospheric evaporation demand which is essential in estimating the actual evaporation. In practice, Epa is usually formulated by different functions based on Penman method or by pan measurements. However, it remains unclear which formulation is preferred for different ecosystems. In this study, we adopted two formulations of Epa by the Penman methods (EpMO based on the Monin–Obukhov similarity theory and Epu2 based on the Rome wind function) and another two formulations by the pan evaporation measurements (ED20 observed by the D20 pan and EE601 observed by the E601B pan) at multi eddy covariance (EC) stations including different ecosystem types. The four formulations were compared in estimating actual evaporation (E) with the aid of a generalized complementary function. Our results show that: 1) For short vegetations (i.e., grass and crop), ED20 is a competitive candidate because this in-situ measurement can accurately reflect the dynamics of the underlying surface through observation, and it is widely accessible and easy to deploy; 2) For high vegetations like forests, EpMO is the best candidate because this theoretical formulation incorporates both evaporation demands from the high canopy and ground surface, which cannot be reasonably captured by the other three candidates; 3) When the canopy information (e.g., displacement height and roughness lengths) is not available, Epu2 can be regarded as a useful alternative formulation of Epa with moderate accuracy in evaporation estimation.