Wet Surface Evaporation Insight from Temperature-Vapor Pressure Diagrams

Abstract:

Potential evaporation from wet surfaces plays a role in many methods to estimate actual, drying-surface evaporation. The behavior of Penman’s (1948) evaporation equation combining the surface energy budget with mass and energy transfer equations can be illustrated on temperature – vapor pressure (T-e) graphs. Key to this depiction are two straight and parallel lines on the graph representing constant enthalpy of the air at measurement height and at the surface skin, with the spacing between the lines representing a combination of aerodynamic resistance variables and available energy. The equations on which Penman based his formula can easily be solved numerically for T_0w, the temperature the surface would have if was saturated, keeping all other variables constant, without invoking Penman’s famous assumption. Wet surface evaporation is proportional to the vapor pressure difference between the measurement height and the surface skin. Equilibrium evaporation, based on the slope of the vapor pressure curve at T₀w is also easily represented in the graph. The difference between the correct wet surface evaporation rate and Penman’s approximation are immediately visible on the graphs. Different wet surface evaporation rates are compared using data from a tropical savanna in Australia. Visualization on a T-e diagram provides insight into evaporation processes.