Daylight Simulation Modeling Topography

I guess I don't understand why you can't include the trees and topography in the simulation?

As Jill stated, why can;t you? You can definitely include topography into your simulation and it should be to determine actual levels. How its modeled depends on your program. Use simple geometries for items in the near vicinity. I have modeled a mountain, (what we call mountains in PA) for a project that was located on the north slope of the mountain and for one located in the valley in the Pittsburgh. Morning and evening low sun angles were non- existent because of the terrain. Vegetation can be tricky,especially if its deciduous trees, and what values of transparency do you use based on tree type and depth of the woodline. Vegetation is a great element to use to help with direct solar issues. I use simple planes for trees and make my best judgement on transparency of the canopy. You can sometimes get this information about the canopy density from arborist books to help determine the transparency.