How to modify evaporation factor from trees to forests
Default values for the evaporation factor of trees are set to a value consistent with the placement of individual trees. This means that if an individual tree is loaded in as a singular polygon, their evaporation factor is set properly. However, an entire forest may be loaded as a single polygon. Where a polygon representing an individual tree may have about 1m² of polygon per tree, a forest may have closer to 30m² or 40m² per tree. Consequently, applying this factor to a larger surface area results in an overestimation of total evaporation. To correct this, you can take the following steps.
Editor → Current Situation (Ribbon tab) → Buildings (Ribbon bar) → The Building to edit (Left panel)
How to change the evaporation factor for forests:
- Determine the forest (or larger group of trees) for which the evaporation factor should be changed.
- Determine the ratio of surface area to amount of trees, for example by dividing the area of the forest by the amount of trees in the forest.
- Select the Buildings which represents the forest in the Editor.
- Go to the "Details" tab.
- Find the "Water transpiration" Function Value.
- Divide the "Water transpiration" value by the ratio determined earlier to get a new, lower evaporation factor.
- Enter that new factor as the "Water transpiration" Function Value for this forest.
Notes
- To do this en-masse for Buildings with a tree-line function, consider modifying the Function Values of the relevant Functions instead. Note that this may mean you will want to perform the opposite operations on individual trees where appropriate.
- If you have a GeoJSON file with data on tree density, you can combine that file with a GeoJSON file containing exported Buildings from your Project. You can use that information to calculate a new evaporation factor for all tree-like Functions, and then import that data into your Project again as update for your existing Buildings.