In 2016, the Centre for Ecology and Hydrology (CEH) and Wallingford HydroSolutions (WHS) released another version of the revitalised rainfall-runoff model (ReFH2) which allows users to generate flood peak flows and hydrographs from given rainfall events for both catchment and development sites. For a full description of the approaches, the reader is referred to their document which can be freely accessed at:-
There are 2 components to the ReFH 2 model. The first is a new runoff volume and routing model, ReFH2, which was added to InfoWorks ICM in version 6.5. The second component is the FEH 2013 generated design rainfall which was added in version 7.5. This blog post will go on to describe the implementation of both components within InfoWorks ICM/ICMLive and how users can use these to generate rainfall events and runoff within these software packages.
The first point to note, is that the equations used to generate FEH2013 rainfall and ReFH2 runoff are proprietary and have not been published publicly. Therefore, unlike the previous FEH and ReFH approaches, it has not been possible to code these equations directly into the InfoWorks ICM/ICMLive simulation engines. The user therefore requires a copy of the ReFH2 software with a licence obtained from Wallingford HydroSolutions (WHS). The ReFH2 software must be available when the FEH2013 rainfall is generated and the relevant calculations are undertaken for determining the parameters for the subcatchments ReFH2 runoff. However, once the rainfall and runoff parameters have been determined, InfoWorks ICM/ICMLive simulations do not need access to the ReFH2 software and runs can be distributed onto other machines/servers as is desired.
Like all rainfall the FEH2013 generated rainfall is created by using a Rainfall Event Editor. In the list of available rainfall models the appropriate FEH2013 Rainfall generator should be selected.
Once selected the following dialog is provided. The user can then import the XML data from the FEH2013 web service at https://fehweb.ceh.ac.uk. At the time of the original implementation in ICM, the descriptors were only available for catchments. Shortly afterwards a new point format has been developed which is available from the website. In order to use the newer point descriptors you will need to use version 9.0 of InfoWorks ICM or later. The method of bringing in the two types of XML file into InfoWorks ICM is identical.
Once, the FEH2013 parameters are imported, the user can choose the relevant return periods and durations. Multiple return periods and durations can be specified using a comma to separate the chosen return periods/durations. The user can also specify whether to use the ReFH2 derived Alpha coefficient, areal reduction factor and seasonal correction factor values or use user-specified values. If the ReFH or ReFH2 runoff models are being used the initial soil moisture parameter, Cini and the initial baseflow, BF0 can also be specified. When you click OK the settings and parameters are sent to the ReFH2 software and the rainfall profiles generated.
Once the rainfall profiles have been generated, these can be viewed as a profile and in graph view as would occur with other generated rainfall. Note that the rainfall displayed in the rainfall graph and the profile is the gross rainfall prior to the application of the seasonal correction factor and the areal reduction factor. It is only during the simulation that the seasonal correction factor and areal reduction factor are applied.
The FEH 2013 rainfall can then be used with existing runoff models such as the fixed, Wallingford, NewUK or any other runoff model that you may have applied FEH and ReFH rainfall to in the past. There is however a new ReFH2 runoff model which will be described in the next section.
ReFH2 Runoff Model
ReFH2 is a runoff model which calculates the volume of runoff and the routing of that runoff volume. For a full description of the model the reader is recommended to review the ReFH2 technical description mentioned previously. The ReFH2 runoff model can be used in conjunction with the aforementioned FEH2013 generated rainfall or with rainfall generated from another source. Within InfoWorks ICM, the ReFH2 runoff model is set within the runoff surface properties as per the other runoff volume and routing models. Within the Runoff Surface, both the runoff volume and runoff routing models should be set to ‘ReFH’. This allows the user to specify either the original ReFH runoff model or the newer ReFH2 runoff model.
The subcatchment should then use a land use which has a ReFH runoff surface applied to it. The subcatchment property sheet also has the field ‘ReFH/ReFH2 Parameters’ where the appropriate catchment descriptors are added.
It is within the ReFH/ReFH2 parameters that the user makes the choice between the ReFH and ReFH2 runoff models. The catchment descriptors can be imported from the rxml format ReFH2 project files or from the xml files available from the FEH web service.
Once imported the parameter values should be updated. Once the parameter values have been set, the user can commit the network and use the network as part of a simulation.
With regards results, when using the ReFH2 runoff model (or indeed the ReFH runoff model) the user can graph runoff, as per any runoff model, the soil moisture content, C, which is tracked throughout the simulation and the baseflow. This allows the user the opportunity to assess the correct application of Cini and BF0.
Closing the Water Balance
Appendix 1 of the ReFH 2.2 Technical Guidance Document describes a method to close the water balance (i.e. conserve volume) for the ReFH runoff model. Previously it was possible, particularly for long duration events to have the situation where the baseflow volume, and therefore the total ReFH runoff, was greater than the rainfall volume in some cases where BR was greater than 1. An approach to close the water balance is described in the ReFH2.2 technical guidance document and this was implemented within InfoWorks ICM 7.5. The following is output from one test subcatchment showing the results within InfoWorks 7.0 and InfoWorks 7.5 showing the effect of the new routine to ensure volume balance (in this case the BR parameter was manually adjusted to demonstrate the issue).