There are tons of ways to measure evaporation, and most of these approaches are some sort of interconnected. And it is always confusing to term evaporation in different contexts.
Before you know more about Penman, you need to understand what is turbulent flux <q'w'>(angle quotation means average, same as bar) and how people parameterize them.
First, regard
turbulence as the
deviation of mean in temporal statistical sense, so as the transfer of momentum, heat, and moisture. Second, we are using a zero-order closure method
similarity theory to parameterize the mean qualities needed.(see turbulence closure problem) Similarity theory can be interpreted as parameterizing the mean qualities, but in fact it doesn't mean using any simulation or calculation to retrieve those values. It is based on the similarity between the connected dimensionless groups, which is profound and powerful. Here, one thing that confusing me before was the K theory. K theory is an one-order closure method, by that it means the second order flux term is parameterized by one order variables, such as the mean profiles.
By applying Taylor Hypothesis, one can replace the temporal fluctuations in flux by the
spatial changes of mean qualities. (mean: averaging over time)
Bulk transfer model is based on this simplified idea so that the product of the flux <q'w'> can be rewritten as the product of bulk difference in wind speed and humidity.
(to be continued)