I'm trying to understand transpiration estimation methods. The FAO recommends a standard Penman-Monteith methodology for modeling evapotranspiration based on meteorological and vegetation parameters (ie. wind speed, leaf area index). Many eco-hydrology models make use of the Penman-Monteith equations, some with minor modifications.
I understand the Penman-Monteith equations, and modification thereof, are simplifications of complex processes. It would be impractical to measure actual transpiration in all conditions where that information is useful, so we model it instead. That said, field observations in the development or support of models is valuable for verifying knowledge gained through models.
My question is: what field studies have developed or supported Penman-Monteith equations (or modified versions of it)? In a sense there is indirect verification of the equations through field measurement verification of results from models that implement the Penman-Monteith equations. Are there any direct field measurements of (evapo-)transpiration that support (or even helped develop) the Penman-Monteith equations, to demonstrate the equations' validity in real world conditions?