Spatially Varying Rainfall
Rainfall varies in space as well as in time, and the two effects interact.
Small rain cells typically produce short-duration storms. Small rain cells have a short life, or move rapidly across the catchment. As these cells are of the order of a kilometre across, there is a significant spatial variation in rainfall intensity.
Large rainfall cells tend to produce longer duration storms. Large weather systems tend to produce large rainfall cells. There is therefore less spatial variation of intensity.
It is usual to measure rainfall at a point, using a rain gauge. To make use of this rainfall data we need to allow for the spatial variation. The effect of this is to smooth out the variation of intensity. When the rainfall is at its most intense at the rain gauge it is unlikely to be of equal intensity at all other points in the catchment. The average rainfall over the catchment will therefore be less than the measured value. Conversely, if there is a brief period of no rainfall at the rain gauge it is unlikely that all rain has stopped across the catchment. The average rainfall is therefore greater than the rainfall measured at that point.
By default InfoWorks ICM represents the effect using a simple moving average smoothing. The degree of smoothing depends on the area of the catchment being considered.
To turn off the application of rainfall smoothing when running a simulation, uncheck the Apply rainfall smoothing check box in the Schedule Hydraulic Run View.
The equation applied by InfoWorks ICM (based on the equation in the Wallingford Procedure) is:
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where: pt is the rainfall intensity at time t pt-1 is the rainfall intensity at time t-1 p't is the smoothed rainfall at time t m is the smoothing factor given by:
where: AT is the catchment area (km2), calculated as the sum of the Total Area values of all subcatchments using the rainfall profile dt is the data timestep (seconds) |
Rain Gauge Boundaries
InfoWorks ICM supports spatially varying rainfall by allowing Rain Gauge Boundaries to be stored with each profile in a Rainfall Event.
When using observed rainfall, there may be data for a number of different rain gauges placed in different locations. The region covered by a particular rain gauge can be defined in the GeoPlan View, ensuring that the rainfall profile from the correct rain gauge is applied to subcatchments and 2D Mesh elements falling within the rain gauge boundaries.
When a simulation is run using a rainfall event containing rain gauge boundaries:
- Existing rainfall profiles associated with subcatchments are automatically replaced with the correct profile based on subcatchment location
- Rainfall profiles are automatically applied to 2D mesh elements based on mesh element location (provided the Apply rainfall option is selected for the 2D Zone.)
By default, the rain gauge region that contains the centroid of the subcatchment will be used to provide rainfall data for the entire subcatchment. For very large subcatchments, where the subcatchment overlaps multiple rainfall polygons, this can mean that the simulated subcatchment rainfall is not representative of the rainfall over the subcatchment as a whole. Check the subcatchment Use area-averaged rainfall field to use data from all rain gauge regions that the subcatchment overlaps.
See Using Spatial Rainfall in Simulations for full details.
Detailed instructions on using rain gauge boundaries can be found in the Rain Gauge Boundaries topic.
