The Calculation of ReFH Initial Baseflow in InfoWorks ICM and changes for version 7.5

InfoWorks_ICM_medium  v7.5

In InfoWorks ICM version 7.5 there will be a few changes to the way that the Revitalised Flood Estimation Handbook (ReFH, http://evidence.environment-agency.gov.uk/FCERM/Libraries/FCERM_Project_Documents/FD1913_3259_TRP_pdf.sflb.ashx),  initial baseflow (BF0) is calculated. This blog is released prior to the release of 7.5 so that users are aware of the shortcomings of the current approach in which InfoWorks ICM calculates the ReFH initial baseflow.

Currently where ReFH rainfall is applied to multiple subcatchments, all subcatchments will have an initial baseflow calculated based on the area of the catchment used to generate the rainfall (ie, the profile catchment area added in the rainfall generator) unless a value was specified directly on the ‘ReFH Subcatchment tab’. Therefore for subcatchments using the ReFH runoff volume model, the same initial baseflow will be provided regardless of the subcatchment area. Clearly, this can have a significant impact upon the initial baseflow in the subcatchments and is incorrect bahaviour. The correct behaviour would be for the initial baseflow to be scaled by the subcatchment area. This only affects the use of the global initial baseflow and does not affect the values specified in the ‘ReFH Subcatchments’ tab of the rainfall event which should be used in preference to the values on the ‘Globals’ tab.

In 7.5, the ReFH rainfall generator has undergone some changes related to the calculation of initial baseflow for those subcatchments which use the ReFH/ReFH2 runoff model and the ReFH rainfall. These changes are summarised in the following sections.

User (Catchment) and User (areal) Options for Initial Baseflow

The ReFH rainfall generator now has a new user (areal) option for BF0. This allows the initial baseflow BF0 to be specified per unit subcatchment area (m3/s/ha), which is consistent with ReFH recommendation.

There is also a new “User (catchment)” option. Using this, the BF0 value is specified as a flow (m3/s). Within the simulation, the initial baseflow in a subcatchment is calculated by multiplying this BF0 value by the ratio of the subcatchment area (in the network) to the catchment area (in the rainfall profile). In other words, the specified BF0 value is assumed to apply to the catchment area, and is treated as being proportional to area.

‘User’ values from models created in earlier versions of ICM are now treated as ‘User (Catchment) values and are scaled by area ration when used in the simulation, as described above.

Figure 1: The new User (Catchment) and User (areal) fields for the ReFH Rainfall generator.

Figure 1: The new User (Catchment) and User (areal) fields for the ReFH Rainfall generator.

Best practice, as it was prior to version 7.5, is to use the ‘ReFH Subcatchment’ tab on the rainfall event grid view to directly input values for each subcatchment. The value specified here will be used in preference to the setting on the event generator window.

Figure 2: The ReFH Subcatchment tab in which soil moisture content and initial baseflow can be specified for each subcatchment.

Figure 2: The ReFH Subcatchment tab in which soil moisture content and initial baseflow can be specified for each subcatchment.

Calculated Values of BF0 are scaled by area

In versions of ICM before 7.5, any network subcatchment using a ReFH or ReFH2 rainfall profile would use the calculated BF0 value directly, regardless of whether the subcatchment area (in the network) was the same as the rainfall profile catchment area (in the rainfall event).

In version 7.5, when “ReFH” or “ReFH2” is selected for the calculation of BF0, the value calculated now applies to the area specified for the catchment on the rain event generator. It is then scaled linearly to the subcatchment area when a simulation is carried out.

To mark the change, the field is now labelled “ReFH(2) catchment BF0” rather than “ReFH(2) BF0”.

Event profile properties initial baseflow (BF0) scaled by area ratio

Right-clicking on a profile in the grid view and selecting “Profile Properties” brings up a dialog which now includes the catchment area, taken from the value on the event generator grid, which itself generally comes from the FEH web service.

In versions of ICM before 7.5, all subcatchments using a particular profile would have the same initial baseflow, regardless of subcatchment area.

In the upcoming version 7.5, to calculate the initial baseflow in a subcatchment during a simulation, the BF0 value shown on the above described dialog is now scaled by the ratio of the subcatchment area to the catchment area. Thus the initial baseflow is now proportional to subcatchment area.

The label for this field has been changed from “Local ReFH baseflow/BF0” to “Catchment ReFH baseflow BF0”, to emphasise that the value is applicable to the catchment.

Figure 3: The new Catchment ReFH Baseflow BF0 (m3/s) parameter. Also note that the catchment area now appears in the profile properties.

Figure 3: The new Catchment ReFH Baseflow BF0 (m3/s) parameter. Also note that the catchment area now appears in the profile properties.

Global Initial Baseflow (BF0) Per Unit Area

In the upcoming version 7.5, on the “Globals” tab of the rainfall grid editor, there is a new “ReFH initial baseflow per unit area” field. If specified, this is used in preference to the “ReFH catchment initial baseflow” field, which is a flow. This allows the global initial baseflow to be proportional to subcatchment area.

 

Figure 4: The Catchment Runoff Data Globals Tab with the new choices of adding the ReFH initial baseflow as a catchment flow (to be pro-rated depending on subcatchment area) or as a baseflow per unit area.

Figure 4: The Catchment Runoff Data Globals Tab with the new choices of adding the ReFH initial baseflow as a catchment flow (to be pro-rated depending on subcatchment area) or as a baseflow per unit area.

Recent software fix to the application of ReFH initial baseflow and initial soil moisture content

Within a ReFH rainfall event, if the values of initial baseflow (BF0) and initial soil moisture (Cini) were set to ReFH, then the software should’ve calculated these parameters using the ReFH method. However, this was not the case in version 6.5 when using the ReFH runoff model. In this case both parameters were set to zero. This can be checked by opening the rainfall event grid editor (right click on the event), right clicking in the rainfall grid and selecting ‘profile properties’. The local ReFH soil moisture and the local ReFH baseflow are both zero. This bug was introduced in version 6.5 and was fixed in version 7.0.1. The software bug only affected the ReFH runoff model and not the ReFH 2 runoff model. Note that since version 7.0 it is now possible to plot the Cini value for each ReFH subcatchment as part of the subcatchment results.

Current Software Bug in the calculation of initial baseflow for ReFH models.

Prior to version 7.5 there was also a software bug which affected the calculation of initial baseflow for ReFH models. The user-specified initial baseflow was only calculated for those rainfall profiles which had a zero area catchment. If the rainfall profile within the rainfall generator had an area applied, as would be expected, then the ReFH calculated value would be used, even if a user-defined value has been specified. This has been subsequently addressed in 7.5. The specification of a user initial baseflow via the ‘ReFH Subcatchment’ tab is not affected by this bug.

Summary

Changes have been made to the way that InfoWorks ICM calculates the Initial Baseflow value for subcatchments using the ReFH runoff model. Previously, the initial baseflow value was assigned to all subcatchments regardless of area. In version 7.5 the software will now scale the initial baseflow based on the ratio of subcatchment area to rainfall profile area. The user also has a choice of whether to specify a user-defined initial baseflow in terms of flow or as flow per unit catchment area. These changes will only affect those subcatchments which use the ReFH or ReFH2 runoff models, it will not impact on any other runoff model. Despite the changes, best practice to specify user defined values for the initial soil moisture content and initial baseflow would be via the ‘ReFH subcatchment’ tab.

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    About Duncan Kitts

    Duncan Kitts is a Senior Support Engineer with Innovyze in the United Kingdom, specializing in River modelling and 2D modelling. He has over 9 years experience of modeling the key hydraulic processes involved in both fluvial systems and urban drainage environments. Duncan is responsible for providing support of both infoworks ICM and Infoworks RS.
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