Perhaps this will help.  Cowardin et Al used 2 meters depth to divide wetlands with plants and what he called deep water habitats, but I am not sure that is what you want.  If the water in lake can develop a thermocline, then that may be the defining depth that controls surface water temperature and evaporation rate. 

Dear William,

Thanks for your timely answer. Your answers and the two papers were much helpful.

I agree with Changliang Shao

Normally in deep water lake you will observe thermal stratification. In shallow water lake it will not be observed.

Ashfaque Ahmed, thanks, you are quite right. It is one of the important views.