Vegetation (Grass Trees)+ Building Density more than 1

Kristi · October 17, 2023, 2:49pm

Hello,
I am running the UWG with different inputs as shown,

etc.
As far as I know, the engine considers the rule that the tree+grass cover + building density should be less or equal to one. I obtained the error just once when this addition was bigger than one and it didn’t run the simulation, but now even though it can be 1.5, 1.6, or even bigger I don’t obtain the error and the simulation runs and gives me the temperature values. Is there any change in the simulation code and is it logical and correct that it still runs?
Moroever, in order to understand the vegetation influence being it tree or grass, I kept the other parameters constant and increased the grass or tree from 0.1-0.9 ratio. The engine run gave me results and I tried to compare the results with each other. The difference is so small from 0.1 to 0.9 (grass or tree cover).
I would really aprecciate your help in the matter, I want to know if the engine runs correctly or after the addition is bigger than 1 it just assumes as 1?
Thank you so much,

I also want to add that the model is a real urban model and the data for vegetation grass and tress are same as in the real-life model.

chris · October 18, 2023, 8:43pm

Hi @Kristi ,

Yes, that is the case and you can see this here in the source code:

github.com

ladybug-tools/dragonfly-uwg/blob/master/dragonfly_uwg/properties/model.py#L391


      
          
          def _autocalcualted_tree_coverage(self, ground_area):
              """Autocalculate the tree coverage from the model context shades."""
              veg_shds = []
              for shd in self.host.context_shades:
                  if shd.properties.uwg.is_vegetation:
                      veg_shds.extend(shd.geometry)
              tree_area = self.compute_horizontal_area(veg_shds)
              if ground_area <= 0:
                  return 0
              return tree_area / ground_area if tree_area / ground_area <= 1 else 1
          
          def _weighted_property(self, attribute, weights):
              weight_val = sum([getattr(bldg.properties.uwg, attribute) * w
                                for bldg, w in zip(self.host._buildings, weights)])
              return round(weight_val, 5)
          
          @staticmethod
          def _autocalculated_traffic(story_count):
              """Autocalculate the traffic wattage from the average story count."""
              if story_count <= 3:

The only way that you would be able to get a coverage fraction more than 1 is if you start putting trees over the roofs of the buildings. In this case, we just set it to 1 assuming that you’ve already covered the streets in trees if you are starting to cover the roofs.

You should search this forum for related posts and you’ll see that this question has been asked several times. While I think it’s true that people overestimate the urban heat island benefit that they get from vegetation (especially grass), the UWG also does not do a great job accounting for the heat dissipated through evapotraspiration at the moment given that it just takes a “latent fraction” and does not do a whole moisture balance calculation. This will hopefully change in the future.