On the thermodynamic foundations of the complementary relationship of evaporation

摘要

The simultaneous thermodynamic pathways (i.e., isenthalps) of the air at the measurement height and at the vegetated land surface under isobaric and adiabatic wetting/drying cycles of the environment make it possible to define the actual evaporation rate with the help of three (one measured and two derived) vapor pressure (and corresponding temperature) terms. From the first-order approximation about the constancy of the relative average speed which the two isenthalps are travelled at during drying out of the environment, a nondimensional, linear form of the complementary relationship (CR) of evaporation naturally emerges, but now expressed by vapor pressures (and temperatures, respectively). Without an artificially low Priestley-Taylor parameter value this linear CR would overestimate the evaporation rates because the surface warms faster than the constant relative speed assumption permits. With the appropriate estimation of the wet-surface temperature and employment of realistic boundary conditions, the latter leading to a nonlinear CR, land evaporation rates can be estimated fairly accurately with minimal input variables (air temperature, humidity, wind speed and net surface radiation) and without any information of land surface properties. Not only actual but three potential evaporation rates can also be defined by linking the temperature/vapor pressure coordinates of the air and the surface isenthalps, thus reproducing certain existing formulations of the CR as well as re-creating an existing hybrid (containing, both non-dimensional vapor pressure and evaporation terms) version of it.