Potential Evaporation and the Complementary Relationship

The complementary relationship (CR) provides a framework for estimating land surface evaporation with basic meteorological observations by acknowledging the relationship between actual evaporation, apparent potential evaporation and potential evaporation (E-po). As a key variable in the CR, E-po estimates by conventional models have a long-standing problem in practical applications. That is, the meteorological forcings (i.e., radiation and temperature) employed in conventional E-po models are observed under actual conditions that are generally not saturated. Hence, conventional E-po models do not conform to the original definition of E-po (i.e., the evaporation that would occur with an unlimited water supply). Here, we estimate E-po using the maximum evaporation approach (E-po_max) that does follow the original E-po definition. We find that adopting E-po_max considerably reduces the asymmetry of the CR compared to when the conventional Priestley-Taylor E-po is used. We then employ E-po_max and develop a new physically based, calibration-free CR model, which shows an overall good performance in estimating actual evaporation in 705 catchments at the mean annual scale and 64 flux sites at monthly and mean annual scales (R-2 ranges from 0.73 to 0.75 and root-mean-squared error ranges from 9.8 to 18.8 W m(-2)).Both the 705 catchments and 64 flux sites cover a wide range of climates. More importantly, the use of E-po_max leads to a new physical interpretation of the CR.