When running a simulation, InfoWorks ICM checks whether a rainfall event contains rain gauge boundary information. If boundary information is found, InfoWorks ICM automatically uses the linked rainfall profile for all subcatchments or buildings that fall within the rain gauge boundary. Therefore it is not necessary to edit the network each time a different rainfall event is used in order to enter the correct rainfall profile for each subcatchment or building.
Boundary information overrides the profile number entered in the Rainfall profile field of the Subcatchment or Building in the following way:
- If the Rainfall profile field is zero or blank the subcatchment or building is assumed to have zero rainfall and no change is made. This behaviour is the same as with normal rainfall.
- If the Rainfall profile field is non-zero, InfoWorks ICM ignores the current contents of the field during simulation and uses the rain gauge boundary profile instead.
Evaporation profiles are applied to rain gauge regions as follows:
- If the evaporation data consists of one profile only, the evaporation rate on that profile is applied to all rain gauge regions.
- If the evaporation data consists of more than one profile, the evaporation profile corresponding with the rainfall profile associated with the rain gauge region is applied.
By default, for a particular subcatchment (subcatchment's Use area-averaged rain box unchecked) or a building, InfoWorks ICM assigns the rainfall profile whose rain gauge region contains the subcatchment or building. It decides which region contains the subcatchment or building in the following way:
- If the subcatchment or building has x and y coordinates, InfoWorks ICM uses these to calculate which region contains the subcatchment or building. If the subcatchment or building was imported and values updated using the #G flag, the x and y values represent the centroid of the subcatchment or building calculated from the boundary data.
- If the subcatchment or building has no coordinates, InfoWorks ICM uses the coordinates of its drainage node to calculate the region.
- If the drainage node has no coordinates, InfoWorks ICM assumes coordinates of 0,0.
If the x, y point falls exactly on more than one boundary, or falls within overlapping regions, the region with the lower profile number is used.
If rain gauge boundary data is associated with a rainfall event, a InfoWorks ICM simulation will fail under the following circumstances:
- If the rainfall event contains profiles for which rain-gauge boundaries have not been defined.
- If any subcatchments receiving rainfall cannot be assigned to a rain gauge boundary as described above.
Area-averaged rainfall
For very large subcatchments, where the subcatchment overlaps multiple rain gauge regions, using only the rain gauge region that contains the centroid can mean that the simulated subcatchment rainfall is not representative of the rainfall over the subcatchment as a whole.
Check the subcatchment's Use area-averaged rainfall box to calculate the rainfall for the subcatchments as the area-average of the rainfall from all rain gauge regions that overlap the subcatchment. Any areal reduction/calibration factors set for the rain gauge regions are incorporated in the calculation of the area-averaged rainfall. If the subcatchment falls wholly or partly outside any rain gauge region, the simulation will fail with an error during pre-processing. If the subcatchment does not have a boundary, the option is ignored.
The following variables are not area-averaged:
- Antecedent rainfall
- Local evaporation
- Evaporation from multiple evaporation profiles
- Temperature from multiple temperature profiles
- Runoff initial conditions
Instead, the values used are those that would apply if using only the rainfall polygon that covers the greatest area of the subcatchment.
Note that the recommended way to set runoff initial conditions for a subcatchment is via the Subcatchment or Surface tabs on the Rainfall Event Editor.
Applying spatially varying rainfall to 2D mesh elements
Spatially varying rainfall and evaporation can also be applied to 2D mesh elements when carrying out 2D Simulations.
For rainfall to be applied to a 2D mesh element, the Apply rainfall etc directly to mesh elements option of the 2D Zone in which the mesh is generated must be selected.
For each mesh element, InfoWorks ICM checks in which rain gauge region (if any) the centroid of the element is located and applies the rainfall profile accordingly.