When running a simulation for a SWMM network, InfoWorks ICM checks whether a rainfall event contains rain gauge boundary information.
If no boundary information is found in the rainfall event, rainfall profiles from the SWMM rain gage associated with a subcatchment will be applied to the area defined by the subcatchment's boundary and no spatial variation will occur.
However, if boundary information is found in the event, rainfall profiles from the SWMM rain gage will be applied to the area defined by the rain gauge boundary. This allows rainfall to spatially vary within and across subcatchments.
In order to apply spatially varying rainfall from a rainfall event to a SWMM network:
- The SWMM subcatchment's X and Y coordinates must lie within the defined rain gauge boundary.
- The SWMM subcatchment's Rain gage ID field must be set to the applicable SWMM rain gage.Note that a rain gage must be specified for a subcatchment although its setting will be ignored when spatially varying rainfall from an event is applied.
- The SWMM rain gage's Rainfall profile is linked to in the appropriate profile in the rainfall event that is associated with the rain gauge boundary information.
Boundary information overrides the profile number entered in the Rainfall profile field of the SWMM rain gage as follows:
- If the Rainfall profile field is zero or blank the subcatchment 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 subcatchment whose Use area-averaged rain option is unchecked, InfoWorks ICM assigns the rainfall profile whose rain gauge region contains the subcatchment. It decides which region contains the subcatchment as follows:
- If the subcatchment has x and y coordinates, InfoWorks ICM uses these to calculate which region contains the subcatchment. If the subcatchment was imported and values updated using the #G flag, the X and Y values represent the centroid of the subcatchment calculated from the boundary data.
- If the subcatchment has no coordinates, InfoWorks ICM uses the coordinates of its Drains to node or subcatchment to calculate the region.
- If the Drains to object 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 gage regions, using only the rain gage 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 property to calculate the rainfall for the subcatchments as the area-average of the rainfall from all rain gage regions that overlap the subcatchment. Any areal reduction/calibration factors set for the rain gage regions are incorporated in the calculation of the area-averaged rainfall. If the subcatchment falls wholly or partly outside any rain gage 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
Instead, the values used are those that would apply if using only the rainfall polygon that covers the greatest area of the subcatchment.
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 gage region (if any) the centroid of the element is located and applies the rainfall profile accordingly.