Greetings,

TLDR : I am trying to consider the process of evaporation within the stigmas of apple flowers so that I can build a heat budget model out of it. How should I proceed?

I am trying to build a physical model that predicts stigma temperature (reproductive organ of a flower) based off atmospheric variables. To start off, I read through most of the 4th edition of the book by Monteith and Unsworth and am building my model based off a pre-existing leaf budget model. In most leaf budget models, they will assume a plate surface but in my case, I am computing a sphere instead. Also, they consider evapotranspiration and that there is leaf boundary conductance as well as stomatal + cuticular resistance. Stigmas from apple flowers do not transpire nor do they have stomatal and cuticular because they don't need to regulate their temperature through evaporation. They still diffuse some level of evaporation however because they are saturated in water. When a stigma dies, it still remains rigid in place unlike a leaf that withers. However, I am unsure about how I should consider this evaporation and hence why I am asking for guidance.

Here is what I’ve attempted so far :

Following the methods proposed by Monteith and Unsworth, I compute the latent heat exchange by only considering the stigma boundary conductance over a small spherical object (0.5 - 1mm of diameter). This leads to erroneous and excessive values in latent heat exchange and results in overall stigmas that are 10-15 degrees cooler than air temperature (while they should be 2-3 degrees higher during the day). It's almost like the models considers that the lack of stomatal + cuticular resistance to be a way for the water in the stigma to easily diffuse throughout the air (because of considerable evaporation).

My idea is that what i'm doing right now is basically considering the stigma to be a spherical object of pure water, while this is clearly not the case; there are biological barriers that prevent all the water from the stigma to evaporate into the air. However, I have no idea what those barriers may be and even less of an idea about how to program it. I feel like I am missing an important step in my reflection, but I can't pinpoint it precisely. Would anybody be able to help me out?

For additional precision:

1) during the day, stigma temperature seems to be generally higher than that of the air. It is the reverse during the night. In cloudy skies, during both night and day the stigma temperature is somewhat equal to air temperature.

2) completely excluding both evaporation and transpiration lead to an overall better prediction, but the results are even better if I just consider the stigma to be like a small spherical shaped leaf with stomatal and cuticular. However, in all cases the RMSE of the predictions are not better than the RMSE of the air temperature (which signifies that its meaningless for me to build a model if I can just use air temperature and have better results).

Sorry for the long post, thank you!