ReFH2 Method (ReFH2 Rainfall Runoff Method)
    • 21 Sep 2022
    • 16 Minutes to read
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    ReFH2 Method (ReFH2 Rainfall Runoff Method)

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    Article Summary

    Required Data Entry

    On opening a ReFH2 boundary unit within the Flood Modeller interface, you are presented with four tabs: Catchment, Rainfall, Options and Results. The first two tabs specify the input data for ReFH2; the Options tab allows for some model adjustment and the Results tab displays the values for the generated hydrograph and its intermediate constituent components (you can only view the Results tab if you have a licensed installation of the ReFH2 software).

    Catchment tab

    The Catchment tab required data entry detailed as follows:



    Default value



    Baseflow index catchment descriptor

    (Imported from catchment file)




    Catchment descriptor - DDF model parameter c.




    Contributing catchment area (km2)


    >0; warning if area < 0.038 or area > 9868


    Country of applicability (England, Wales, Scotland or Northern Ireland)




    Catchment descriptor - DDF model parameter d1.




    Catchment descriptor - DDF model parameter d2.




    Catchment descriptor - DDF model parameter d3.




    Mean drainage path length catchment descriptor (km)




    Mean drainage path slope catchment descriptor (m/km)




    Catchment descriptor - DDF model parameter e.




    Easting coordinate of catchment outflow (not used in calculations)




    Catchment descriptor - DDF model parameter f.



    Node Label

    Node label identifier


    12 chars max


    Northing coordinate of catchment outflow (not used in calculations)



    Use plotscale equations

    Tick if using the ReFH2 plot scale equations.


    Value in dat file: plottrue (ticked) or plotfalse (case insensitive)


    Catchment descriptor - proportion of time catchment soil moisture deficit (SMD) was below 6mm during the period 1961-1990. (range 0.0 - 1.0)


    0 ≤ PROPWET≤ 1


    Standard annual average rainfall (mm) - catchment descriptor.




    Urbanisation flag. Set to URBANISED if using the urban model, else use the rural model


    ‘urbanised’ or ‘rural’ (case insensitive)


    Extent of urban/suburban land cover (range 0.0 - 1.0) - catchment descriptor.


    0 ≤ URBEXT ≤ 1

    Allow event parameter override

    Option to allow settings in the 1D simulation (ief) file to scale or adjust ReFH2 parameters. (valid from v5.1)


    True or False


    Baseflow index catchment descriptor, updated with 2019 values (required for ReFH v2.3+). (valid from v5.1)


    >0. Must be present if ReFH Version ≥2.3 AND xmlflag=xmlfalse

    ReFH2 version

    ReFH engine version to use. (valid from v5.1)

    2.2 (if blank in existing unit)

    2.3 (for newly created units)

    2.2 or 2.3

    altbar, aspbar, aspvar, farl, fpext, fpdbar, fploc, ldp, rmed_1h, rmed_1d, rmed_2d, saar4170, sprhost, urbconc2000, urbloc2000, urbext1990, urbconc1990, urbloc1990, c_1km, d1_1km ,d2_1km ,d3_1km ,e_1km ,f_1km …

    “Unused” catchment parameters are still read in from catchment descriptor file. These data are needed to form a valid intermediate xml file for the ReFH2 software; the latter six are the point rainfall DDF model parameters.

    Rainfall tab

    The Rainfall tab requires data entry detailed as follows:



    Default value


    Time step (∆t)

    Time interval used in unit hydrograph and rainfall profile (hrs).



    Duration (D)

    Storm duration (hrs). Needs to be rounded to the nearest odd integer multiple of ∆t.


    (2n+1)∆t, where n is an integer

    Flood Return Period (T)

    Return period (years)




    Season flag - 'SUMMER' or 'WINTER'


    'SUMMER' or 'WINTER' (case insensitive)

    Design rainfall method (DDFMETHOD)

    DDF rainfall model to use (1999 or 2013). If using the 2013 model, then a pre-existing xml catchment descriptor input file, containing DDF 2013 model parameters (from the FEH Web Service), must be specified



    1999 or 2013. If 2013, then xml file must exist

    Note: The following variables are only relevant if ReFHVer ≥ 2.3



    Default value



    Rainfall uplift factor, e.g. to represent climate change effects


    Should be ≥1 Though it does accept values <1

    (0 = ignored)

    Use DDF 2013 rainfall from descriptor file

    Allows rainfall data in catchment descriptor xml file to be utilised.

    Location of catchment file specified on Options tab.


    XMLTrue, XMLFalse or XmlDDFOnly


    Note this option works in conjunction with descriptor file checkbox on Options tab.



    Import DDF 2013 rainfall

    Enables rainfall data from a catchment descriptor xml file to be imported into the table. This can be either for editing to create a custom rainfall dataset or just for reviewing the data.


    Tabular data not used but still saved to network file if option to use descriptor file is ticked.


    Number of ensuing storm durations in DDF2013 table


    Must be >0 if nDDF2013T >0


    Number of ensuing return periods in DDF2013 table


    ≤24, must be >0 if mDDF2013D >0

    T1 …. Tn

    Return periods in DDF2013 table


    >0; Ti+1 > Ti


    Storm durations for each row in DDF2013 table


    >0; Dj+1 > Dj


    DDF2013 rainfall values for storm duration i, return period j


    >0; Pi+1,j≥ Pi,j; Pi,j+1≥ Pi,j

    The rainfall data table is only editable if the tickbox options to use data from an xml descriptor file (on the rainfall and options tabs) are both unticked. In addition, tabular rainfall data can only be used with the FEH 2013 design rainfall option. The table allows for individual cells to be edited. The right-click menu on the table also provides options to cut, copy or paste individual or blocks of cells. Pasting can be to other parts of the table or to 3rd party software, e.g. MS Excel, for further editing (you can copy data back form Excel into the table as well).

    V2.3 options tab

    The data settings on the v2.3 options tab are only applied if the ReFH2 engine version on the Catchment tab is set to v2.3. Furthermore, in v2.3, all settings on this tab are optional and only need values specifying if they are required for your particular analysis. The options provided on this tab are as follows:



    Default value


    Seasonal correction factor

    usescf (check box flag)

    scf (value)

    A seasonal correction factor (SCF) was introduced within the ReFH method to support the use of summer and winter design inputs. The SCF converts the DDF estimate of design rainfall depth based on annual maximum rainfall into an estimate of seasonal design rainfall through simple multiplication. The SCF is a correction factor depending on location, season, duration and selected return period. A detailed description of the development of the SCF is given by Kjeldsen et al. (2006) with a shorter summary provided in the FEH Supplementary Report No1 (Kjeldsen, 2007).



    ‘scfTRUE’ or ‘scfFALSE’


    scf=0 if usescf=’scfFALSE’

    Areal reduction factor

    usearf (check box flag)

    arf (value)

    ARF = The ratio of rainfall depth over an area to the rainfall depth of the same duration and return period at a representative point in the area.

    The estimates of design rainfall calculated using the DDF model are point values as the model is based on data from individual gauges. To allow the estimation of catchment average design rainfall, areal reduction factor (ARF) are adopted from the existing FSR/FEH method.



    ‘arfTRUE’ or ‘arfFALSE’


    arf=0 if usearf=’arfFALSE’

    Urban sewer capacity (m3/s)

    usesc (check box flag)

    sc (value)

    Urban sewer capacity (m3/s). See ReFH2 inline help for more detail.

    Used in conjunction with urban exported drained area – see below.



    ‘scTRUE’ or ‘scFALSE’


    sc=0 if usesc=’scFALSE’

    Urban exporting drained area (km2)

    useeda (check box flag)

    eda (value)

    ReFH2 allows the user to specify both a sewer flow capacity (m³/s) and the exported drained area (km²). This area cannot be greater than the urban area. Drainage may either be exported into adjacent catchments (this can commonly occur in combined sewer systems) or the drainage system may be discharge to the watercourse downstream of the point of estimation. The setting of exported drained area requires a detailed knowledge of the drainage systems.

    See ReFH2 inline help for more detail.



    ‘edaTRUE’ or ‘edaFALSE’


    Mapped urbanised area (km2)

    useurbanarea (check box flag)

    urbanarea (value)

    Mapped urbanised area within the catchment (km2). See ReFH2 inline help for more detail.



    ‘urbanTRUE’ or ‘urbanFALSE’

    0 ≤ urbanarea ≤ catchment area

    Urban impervious runoff factor

    Useirf (check box flag)

    irf (value)

    The nett rainfall from the impervious surface is assumed to be a fraction of the rainfall incident upon the surface.

    See ReFH2 inline help for more detail

    irf=0 ⟹ useirf=’irfFALSE’



    ‘irfTRUE’ or ‘irfFALSE’


    Urban imperviousness factor

    Useif (check box flag)

    If (value)

    The fraction of the urban area that is impervious

    See ReFH2 inline help for more detail

    if=0 ⟹ useif=’irfFALSE’



    ‘ifTRUE’ or ‘ifFALSE’

    0≤ if≤1

    Tp scaling factor (rural to urban)

    usetpscaling (check box flag)

    tpscaling (value)

    Factor applied to rural Tp to derive urban time-to-peak.

    See ReFH2 inline help for more detail.

    tpscaling=0 ⟹ use tpscaling =’tpscFALSE’



    ‘tpscTRUE’ or ‘tpscFALSE’

    0≤ tpscaling ≤1

    Urban depression storage (mm)

    useds (check box flag)

    ds (value)

    Rainfall in excess of this storage will contribute to the pervious runoff within the urban area through percolation through cracks or edge run off and percolation.

    See ReFH2 inline help for more detail



    ‘dsTRUE’ or ‘dsFALSE’


    Note: These settings will still all be written to your network file (dat file) even if they are unticked to not be used. In this case they will have a “False” flag against them, which will instruct the ReFH2 software to ignore them.

    Options tab

    The Options tab required data entry detailed as follows:



    Default value


    Simulation Type (bfonly)

    Flow values to use during simulation, can be one of: base flow only ('BFONLY'), peak flow only ('PFONLY') or full hydrograph (blank).

    For example, if 'BFONLY' is selected then the baseflow contribution is used for the boundary flow value for the whole simulation.



    Boundary Type (hymode)

    Boundary mode flag: 'HYDROGRAPH' denotes a generated hydrograph (i.e. the unit behaves as a QTBDY); 'HYETOGRAPH' indicates that it behaves as a REBDY-type unit, applying the Rainfall profile to a Rainfall-Only boundary (the latter must be used in conjunction with lateral inflow unit)




    Minimum Flow (minflow)

    Minimum flow (m3/s). Forces the resultant hydrograph to be not less than the specified flow. This may be required for program stability



    Generate Report

    Pdf output report flag. Set to true if a pdf output report is required; false otherwise


    reptrue (ticked) or repfalse (case insensitive)

    Report output folder

    Folder location of the pdf report. Must exist if Generate Report ticked (set to true).


    255 chars max; Folder must exist if used

    Hydrograph Scaling (SCALING)

    Hydrograph scaling method ('FULL' or 'RUNOFF'). 'FULL' scales the whole hydrograph; 'RUNOFF' scales only the quick runoff component of the hydrograph.

    Used in conjunction with SCFLAG and scfact.




    Scaling factor value (scfact)

    If SCFLAG='PEAK' then all hydrograph ordinates are scaled (by a constant value) to achieve a peak flow of scfact.

    If SCFLAG='SCALE' then all hydrograph ordinates are multiplied by scfact.

    Used in conjunction with SCFLAG and SCALING.



    Scaling method (SCFLAG)

    Hydrograph scaling option ('PEAK' or 'SCALE'). 'PEAK' scales the hydrograph to fit a peak; 'SCALE' scales the hydrograph by a given factor, scfact.

    Used in conjunction with SCALING and scfact.




    Time Delay (tdelay)

    Optional delay time (hrs), e.g. if tdelay=2hrs then the hydrograph will start 2hrs after the start time of the simulation.




    Descriptor filename (xmlfile)

    Filename of xml file (if used; can be blank if not).


    Must exist if xmlflag=xmltrue or if using DDF 2013 model

    (255 chars max)

    Read descriptor (xmlflag)

    Ticked (set to xmltrue) if using a pre-existing catchment descriptor xml file; false (unticked) otherwise


    xmltrue (ticked) or xmlfalse (case insensitive)

    Catchment descriptor type

    Determines which set of descriptors to utilise from specified catchment descriptor file (xml file).


    Catchment or Point

    The following variables have changed behaviour/constraints for Flood Modeller v5.1 (incorporating ReFH2.3):




    4.2 or 5.1

    Flood Return Period

    ≤1,000 for ReFHVer=2.2

    ≤100,000 for ReFHVer≥2.3



    ‘DEFAULT’ is only applicable if ReFHVer≥2.3


    • ‘Xmltrue’ – reads catchment descriptors AND DDF parameters from Xmlfile
    • ‘Xmlfalse’ – reads catchment descriptors AND DDF parameters from .dat file
    • ‘XmlDDFOnly’ – reads catchment descriptors from .dat file; reads DDF parameters from Xmlfile

    NB If ‘Xmlfalse’ AND m=n=0, AND DDFMODEL=2013 then the computational engine attempts to read DDF parameters from xmlfile (and issues a fatal warning if unable to do so).

    Note: the following defaults are implied (i.e. the ReFH2 engine overrides whatever is set in the unit data, under the given circumstances)

    • Plotscale is forced to be true if using point descriptors
    • Rainmodel is forced to be 2013 if using point descriptors
    • Rainmodel is forced to be 2013 if the ReFH2 engine version used is v2.3 (or later)
    • If catchment area is specified explicitly within the unit data (CAREA) and a pre-defined descriptor file is being used, then this area is only used if the descriptor file is a point descriptor file; otherwise area is read from the catchment descriptor file

    Generating Results

    Prior to viewing the Results tab or running a simulation, the minimum input requirements are:

    • The Catchment Descriptors (Catchment Tab)
    • Storm Duration
    • Data Interval
    • Return Period (Rainfall Tab)

    Although a recommended timestep range and storm duration are provided on the Results Tab, an estimate of these must still be provided initially.

    Catchment descriptors may be imported from an xml file obtained from the FEH Web Service (note; this must be purchased), a csv file produced by the FEH CD-ROM (using the Import Catchment Descriptors button), or may be input manually. Note that only those descriptors used in the calculations are imported, in addition to the Easting and Northing grid reference, which are provided for information only (although may be used by post processing tools, e.g. TabularCSV, to extract coordinates).

    The season (Rainfall Tab) has a default storm season setting of 'WINTER', but this can be changed to ‘SUMMER’. An appropriate selection can be influenced by the degree of urbanisation in the catchment (for URBEXT > 0.125 a summer storm is recommended).

    By clicking on the Results tab, the user is effectively launching a boundary mode simulation of Flood Modeller of a model containing the single ReFH2 unit. This produces the output hydrograph and its constituents in tabulated form, and many of the intermediate calculated parameters. Recommended values for design storm duration and timestep are given, which can be subsequently checked against those originally entered. N.B. if subsequently adjusting the storm duration or time interval, these should be rounded so that the duration divided by the interval is an odd integer. The hydrographs and hyetographs may also be viewed graphically via the Plot button. The hydrograph (.zzh) and summary data (.zzb) output files are also produced and are able to be viewed via the appropriate button on the tab. Occasionally, the simulation may fail, e.g. due to bad input data or an inappropriate licence, and like any Flood Modeller simulation, will generate a run error and produce diagnostic output in such instances. The user is advised to check this output (N.B. The error dialogue box may appear partially hidden behind the "Please Wait..." dialogue box).

    If the ReFH2 executable fails to run (E1797), there will be an accompanying message in the diagnostics (*.zzd) file of the format: “Error running ReFH2 executable; return code: n ”, where n is usually the return code specified in the ReFH2 documentation. In the first instance, the user is referred to this. However, there are other codes, not directly returned from the ReFH2 executable. If this is the case, usually these are error codes determined by the Windows system, e.g. refer to for details. Specifically, the following two may be encountered:

    • n=258 – ReFH2 executable timed out. By default, 30s is allowed for the ReFH2 executable to run and process the data. This should be much longer than ReFH2 software takes to run, and is introduced as an additional safety measure to Flood Modeller from hanging should an unexpected interruption occur in the ReFH2 software.
    • n=9999 – [Non-Windows system code]. Unspecified return code. The ReFH2 executable process could not be launched for an undetermined reason.

    Theory and Guidance

    The ReFH2 boundary unit utilises the ReFH2 (Revitalised Flood Hydrograph 2) software to derive an inflow hydrograph for a catchment or sub-catchment. The ReFH2 method can be applied to both rural and urbanised catchments and can utilise either the FEH 1999 or the FEH 2013 rainfall models. ReFH2 is an updated version of the original ReFH method and was released by Wallingford HydroSolutions ( in 2015.

    The hydrograph produced by the method is utilised in Flood Modeller as a boundary condition, equivalent to a Flow Time Boundary. It can therefore form inputs to a hydrodynamic or routing model. This inflow can be applied at the upstream end of a reach or may be distributed over a reach in conjunction with the Lateral Inflow unit. When run in boundary mode, the resultant hydrograph may be viewed using the Graphical or Tabular postprocessors; furthermore, summary hydrograph (.zzh) and data input (.zzb) files may be viewed prior to running a full simulation; this preview functionality is also available within the Flood Modeller interface.

    Alternatively, the rainfall component of the ReFH2 Boundary can be used as a direct rainfall boundary (equivalent to an REBDY), when used in conjunction with a Lateral Inflow unit, by selecting the Hyetograph Boundary type option.

    The ReFH2 Boundary is a rainfall-runoff model using procedures developed by CEH to update the original ReFH Method and the FSR/FEH Rainfall Runoff Method. The key changes from ReFH1 are:

    • Revision of equations for estimating model parameters (Tp, CMAX, BL, BR) from catchment descriptors.
    • Improved representation of urban processes (implementation of Kjeldsen 2013 urbanisation)
    • The catchment descriptor (regression) equations used to derive Tp, CMAX, BL and BR at ungauged locations have been re-estimated and updated to include URBEXT2000 (ReFH1 used URBEXT1990).
    • Special consideration to Scottish data:
      • Use of more Scottish gauged data (overlapping rain and flow data) for model calibration
      • Development of Scotland specific equations for the estimation of initial model conditions
    • Improved handling of “alpha” (a) parameter:
      • Calibration of the method up to and beyond the 200-year design event (ReFH “alpha” parameter only calibrated to T=150 years)
      • Alpha performance related to SAAR and return period (in ReFH alpha is related only to return period)
      • Option to make estimates of initial model conditions (CINI) independent of pooling-group bias, i.e. alpha not invoked (applied by default for Scotland)
    • Applicable on permeable catchment, ReFH is not recommended for use if BFIHOST > 0.65
    • ReFH2 can utilise FEH99 and FEH13 depth duration and frequency rainfall models
    • Scaling of parameters for use on small catchments (plot scale). Plot scale is assumed to be <0.5km2, i.e. below the limit of extracting catchment descriptors from the FEH Web Service.

    Enhancements within the ReFH2.3 model (as per the website), and its incorporation within Flood Modeller include:

    • The option to apply climate change factors to design rainfall.
    • The option to specify several parameters directly within Flood Modeller, including ARF and SCF
    • Closing the water balance within the ReFH rural model.
    • Closing the water balance within the ReFH urban model, including the introduction of depression storage.
    • Incorporation of the new BFIHOST19 descriptor from the FEH Web Service.
    • Modelling of FEH13 rainfall is no longer limited to a maximum of 1:1000 year event.
    • ReFH2.3 includes an option to use the legacy ReFH2.2 model, allowing you to recreate results created in older version of the software or revisit existing projects.

    More information on the theory adopted within the ReFH2 method can be found on the Wallingford Hydrosolutions website ( ), e.g. the ReFH2 Technical Guidance Document , or in the user guide that accompanies the ReFH2 software. It is strongly recommended that users of the ReFH2 Boundary read the Technical Guidance Document , produced by Wallingford Hydrosolutions. You should also be conversant with the methods described in the Flood Estimation Handbook and must be aware that design flow hydrographs produced by the ReFH2 method can still be highly inaccurate (as the model parameters are based solely on catchment descriptors).

    Users should note that you cannot use observed rainfall data with the ReFH2 Method (you would need to revert to ReFH1 to incorporate observed rainfall into your boundary calculations).

    When using the ReFH2 Method you should also note that separate methods have been derived specifically for Scottish catchments. These are invoked by selecting Scotland for the country setting. Currently equations are the same for all other UK countries (so selecting any other country will produce the same output).

    Datafile Format

    The following describes the format of a ReFH2 unit within a 1D river network file (.dat file). You can view this file in a text editor, e.g. Notepad.

    Line 1 - Keyword 'ReFH2BDY' (case insensitive)

    Line 2 - Label

    Line 3 - Minrev, ReFHVersion

    Line 4 - z, easting, northing, ngr

    Line 5 - T, D, ∆t, SEASON, URBANISED, PLOTSCALE, override, DescType, CCUplift

    Line 6 - Xmlflag

    Line 7 - Xmlfile

    Line 8 - Reportflag

    Line 9 - Reportfolder

    Line 10 - CAREA, COUNTRY, SAAR, URBEXT2000

    Line 11 - DDFMETHOD, c, d1, d2, d3, e, f


    Line 13 - tdelay, bfonly, SCFLAG, scfact, hymode, SCALING, minflow

    Line 14 - altbar, aspbar, aspvar, farl, fpext, fpdbar, fploc, ldp

    Line 15 - rmed_1h, rmed_1d, rmed_2d, saar4170, sprhost, urbconc2000, urbloc2000

    Line 16 - urbext1990, urbconc1990, urbloc1990

    Line 17 - C_1km, d1_1km ,d2_1km ,d3_1km ,e_1km ,f_1km

    The following lines are only valid for Minrev ≥ 5.1

    Line 18 - usescf, usearf, usesc, useeda, useurbanarea, useirf, useif, usetpscaling, useds

    Line 19 - scf, arf, sc, eda, urbanarea, irf, if, tpscaling, ds

    Line 20 - mDDF2013D (referred to below as m), nDDF2013T (referred to below as n)

    The following lines are only valid if m and n > 0

    Line 21 - <blank>, T1, T2, …, Tn

    Line 22 - D1, P1,1, P1,2, … P1,n

    Line 21+i - Di, Pi,1, Pi,2, … Pi,n

    Line 21+m - Dm, Pm,1, Pm,2, … Pm,n

    Some of these parameters are entered as default values automatically by the Flood Modeller interface. These are defined as follows (in case you wish to manually edit values in a 1D river network (.dat) file):



    Default value



    Minimum supported Flood Modeller revision number


    4.2 or 5.1(fixed)


    National Grid reference; optional


    String (≤20 chars)


    Elevation (m AD). Not used in ReFH2 calculations.



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