Generic Rainfall-Runoff Boundary

Introduction

The generic event rainfall-runoff boundary (GERRBDY) provides a range of different hydrological model components to be integrated within the same hydrological boundary unit.

The rainfall models implemented to date focus on the globally applicable SCS (US Soil Conservation Service) model, and an observed rainfall model. In the future, the interface will also support point precipitation data available from NOAA.gov. The loss models implemented include the SCS curve number and Green-Ampt models. A transformation model can be selected from SCS unit hydrograph or Clark unit hydrograph, and multiple methods are available for ascertaining time of concentration/lag. Simple constant and recession baseflow models can be defined.

The main idea behind the GERRBDY unit is to enter details for your entire catchment, split into defined sub-basins if desired. Rainfall models, loss models, transformation models and baseflow models can be defined and then applied to your catchment in the model summary table. The advantage of entering multiple sub-basins is the user’s ability to then apply a rainfall profile, loss through infiltration, time of concentration (or time of lag) and routing method independently for each sub-basin. This could be considered particularly advantageous if a defined storm profile only affects part of the catchment, or if the land type differs and so loss through infiltration will want to be defined seperately, for example.

Note that it is not necessary to split your catchment into multiple sub-basins, but this will mean a single rainfall profile, loss model, routing method and baseflow must be applied to the entire sub-basin.

Another key feature of the new Generic Rainfall Runoff unit is the ability to add DCIA (directly connected impervious areas) to the sub-basins. Any rain received here will be transferred directly to the outlet without attenuation or delay, regardless of any lag defined for the sub-basin.

Catchment Details Tab

On this tab, enter the details of your catchment, split into sub-basins as you deem fit.

Reasons for defining an area of your catchment as a different sub-basin are as follows:

1. To apply a different rainfall profile to an area. This could be the same profile at a later time (by adding an appropriate rainfall delay)
2. To apply a different loss model to an area, for example, to represent diffferent land types. Note that curve number loss via a composite CN (to account for different land types in a single sub-basin) is available.
3. To apply a different transformation model to an area (hydrograph method and/or time of concentration/lag)
   Note

   All or part of a sub-basin can be defined as DCIA (Directly Connected Impervious Area). Any rain falling on DCIA will all be converted to flow (i.e. without the application of any loss model or transformation model assigned to that sub-basin) but additional lag applied ot the sub-basin will also apply to the contribution from the DCIA to the total flow summation.
4. To apply additional lag to a sub-basin, i.e. to delay the contribution from this sub-basin to the total flow summation
5. To apply a different baseflow model. Note that the baseflow model will "start" after any additional lag applied to the sub-basin (but prior to any rainfall delay). The baseflow value at this "starting time" is then repeated for all times prior to this for the purpose of the total flow summation (this is so the contribution to the total flow from this sub-basin doesn't "jump in" following any additional lag applied to the sub-basin). This means careful attention needs to be paid to the initial baseflow value if applying a recession model to a sub-basin with additional lag.

See the Results Tab for further information on the calculations within the unit, both on a sub-basin level, and for the total flow summation.

More information on how to use the catchment details tab can be found here: How to setup a Generic Rainfall Runoff Boundary

Theory and Guidance

Multiple sub-basins can be added here. At least one sub-basin must be specified. The advantage of entering multiple sub-basins is the user’s ability to then apply a rainfall profile, loss through infiltration, time of concentration (or time of lag) and routing method independently for each sub-basin. This could be considered particularly advantageous if a defined storm profile only affects part of the catchment, or if the land type differs and so loss through infiltration will want to be defined seperately, for example.

Note that it is not necessary to split your catchment into multiple sub-basins, but this will mean a single rainfall profile, loss model, routing method and baseflow must be applied to the entire sub-basin.

Another key feature of the new Generic Rainfall Runoff unit is the ability to add DCIA (directly connected impervious areas) to the sub-basins. Any rain received here will be transferred directly to the outlet without attenuation or delay.

Key features of the 'Catchment Details' tab:

• Click ‘Add’ to add a row to the table to define a new sub-basin.
• Users can enter their own name for the sub-basin by typing directly into the table. Default name is:
  subbasin_m, where m is sequential numbering.
• Area must be specified in the ‘Area’ column. This must be strictly positive. This can be edited by typing directly into the table.
• DCIA proportion can be optionally added to each sub-basin. Default value is 0. This can be edited by typing directly into the table.
• Highlight a row (or rows) and click ‘Remove’ to delete sub-basin(s).
• The ‘Total Area’ field is automatically filled and not editable by the user.

The user enters the outlet location (not defined for 2D application) and catchment area manually. Note that the location data is optional as the position of the boundary node on your Flood Modeller map view is defined in the gxy file associated to your 1D network (i.e. where you position the node on the map either initially or by using the move nodes feature);
Default (and US customary) units are given as:

• Catchment area: km²(sq mi)
• DCIA proportion: km²/km²(sq mi/sq mi) i.e. dimensionless

Data file format

Catchment details create the following data file boundary identifiers. Please note that the following lines are written automatically to the .dat file from the user interface when you save your GERRBDY unit - the format is only given here as a reference.

Rainfall Profiles Tab

On this tab, enter the details of rainfall profiles to be applied to your defined sub-basins.

A variety of options are given for users to enter their rainfall profile(s). Click a link below to jump to the relevant section within this help file.

1. SCS rainfall profiles
2. Observed rainfall models. These could be user-entered series or from data from a library.

Key features of the 'Rainfall Profiles' tab are:

• The user must give details of at least one rainfall profile;
• A second profile can be added by clicking ‘Add New Profile’;
• The user can enter their own name for the rainfall profile(s). Default name is
  rainfallprofile_m, where m is sequential numbering;
• A ‘Delete Profile’ button is provided. This cannot be used unless you have multiple profiles defined (i.e. you cannot delete all rainfall profiles at once);
• User can select “Start at unit time zero” or (second radio button) “delay by” option to delay the start time of the rainfall. “delay by” option has second field (manual entry) for non-negative real entry, default is 0. NB Current value for the units entered here shall be hours only. Minutes may be an option for the future.
  

More information on how to use the rainfall profiles tab can be found here: How to setup a Generic Rainfall Runoff Boundary

SCS design storms

Theory and Guidance

Select the method 'SCS Storm' to choose from the standard profiles (i.e. Type I, Type Ia, Type II, and Type III). Later, in the 'Model Summary' tab, you will have the option to apply this storm profile (or any other profiles defined) to any of your defined sub-basins.
Key features of the SCS rainfall are:

• Generic rainfall model parameters are presented in a column on the left hand side of the tab;
• The SCS rainfall duration defaults to 24 hours; however, shorter durations may be modelled (in which case, the corresponding maximal rainfall sub-interval of this duration within the standard 24-hour profile is selected);
• The rainfall depth is to be entered by the user manually;
• The areal reduction factor should default to 1;
• The data interval defaults to 0.1 hours for the SCS rainfall model, however this may be manually changed to a multiplier of rainfall duration;
• The user can specify a 'storm extent' using an ESRI polygon shapefile. However, it should be noted that entering a file here is optional. The entry here is for reference only. When this boundary unit is utilized in a 2D model you are required to specify a storm extent shape file, however this is done in the 2D simulation interface and the file reference is saved as part of your 2D model simulation file (xml file);
• The storm profile is to be selected by the user from a list of permitted options (i.e., Type I;Type Ia; Type II; and Type III); the selected profile is shown in an associated chart as both cumulative % rainfall and % rainfall by data interval;
• A ‘Delete Profile’ button is provided. This cannot be used unless you have multiple profiles defined (i.e. you cannot delete all rainfall profiles at once)

Default metric (and US customary) units are given as:

• Storm duration: hours
• Rainfall depth: mm (inches)
• Data interval: hours

User specified storms: manual entry and from storm libraries

Choose from observed rainfall models or entry from a storm library as specified below:

Observed rainfall model

Theory and Guidance

Select the method 'User Specified' to define a storm profile of your choice. Later, in the 'Model Summary' tab, you will have the option to apply this storm profile (or any other profiles defined) to any of your defined sub-basins.
Key features of the observed rainfall model are:

• User gives the rainfall profile a name in field provided. This automatically updates on the tab label once they click anywhere outside of the ‘Profile name’ field.
• User enters ‘Storm Duration’ – default value is 4 hours. Entry must be strictly positive.
• User enters ‘Rainfall Depth’ – default value is 0. Editable. Entry must be strictly positive.
• ‘Areal Reduction Factor’ is at default value of 1 but is editable by the user.
• User enters ‘Data Interval’ – default value is 0.5. Editable. Entry must be strictly positive.
• ‘Time’ column of table is automatically filled with 0.5, 1, 1.5, ... ,4 (based on default data interval and storm duration). These times automatically update as a user enters a different time interval and/or storm duration. The ‘Time’ column of the table is not editable directly by the user.
• User enters rainfall depths directly into the ‘Rainfall’ column of the table.
• A plot of the profile is shown in the ‘Plot’ tab for reference.
• ‘as proportion of rainfall depth’ checkbox allows the user to specify depths in the table as proportions. If this box is checked, the sum of the entries in the ‘Rainfall’ column of the table should equal 1.
• A ‘Delete Profile’ button is provided. This cannot be used unless you have multiple profiles defined (i.e. you cannot delete all rainfall profiles at once)

Default (and US customary) units are given as:

• Storm Duration: hours (hours)
• Rainfall Depth: mm (inches)
• Data Interval: hours (hours)

Library [FRQSIM] storms

Theory and Guidance

Select the method 'Library Storm' to choose from a number of defined FRQSIM library storm profiles. Details are also provided to edit the library data and thus add in storms of your choice. Later, in the 'Model Summary' tab, you will have the option to apply this storm profile (or any other profiles defined) to any of your defined sub-basins.

Key features of the library rainfall model are:

• User gives the rainfall profile a name in field provided. This automatically updates on the tab label once they click anywhere outside of the ‘Profile name’ field;
• ‘Storm Duration’ and ‘Rainfall Depth’ are automatically filled from data in library profile selected;
• ‘Areal Reduction Factor’ is at default value of 1 but is editable by the user;
• Table is automatically filled with time and depth data from library profile selected;
• Storm profiles can be edited (i.e. you can add your own storm to the library), details below;
• Plot of profile is shown in ‘Plot’ tab for reference. The selected profile is shown as both cumulative % rainfall and % rainfall by data interval;
• A ‘Delete Profile’ button is provided. This cannot be used unless you have multiple profiles defined (i.e. you cannot delete all rainfall profiles at once).

To edit the library:

• The file containing the available storm profile libraries is called "storm_profile_libraries.csv" and should be located in your application data folder:
  ...\Users\username\AppData\Roaming\Flood Modeller
• You can view this file, edit profiles or even add new profiles or new libraries of multiple profiles. However, take care to maintain the format for all profiles (i.e. as used for the supplied profiles). In addition you can add comment rows to the files, e.g. to describe particular profiles. Comment rows must start with "//"
• An example of this format is shown below:

  [My Design Profiles]

  //Comment: These profiles have been used in Project A

  Profile1 (4 hrs; 20.86 mm; 0.5 hrs),4,20.86,0.5,0,0.58,1.22,2.55,12.33,2.29,1.17,0.56,0.16,

  Profile2 (4 hrs; 10.5 mm; 1 hrs),4,10.5,1,0,0.5,2.5,5,2.5,

• You can edit the storm profile in a text editor or in a spreadsheet application (such as MS Excel). A recommended methodology for adding new profiles is to copy an existing row, paste this as a new row and then edit values in this new row. This is the best method for ensuring the format is not corrupted.
• If you have a problem editing the profile data then Flood Modeller can regenerate the original version of the file. To do this just delete or rename your edited copy of the file (in the above data folder). Flood Modeller will then recreate the default version of the storm profiles file in your data folder the next time you open a generic rainfall boundary unit properties window.

Default (and US customary) units are given as:

• Storm Duration: hours (hours)
• Rainfall Depth: mm (inches)
• Data Interval: hours (hours)

Rainfall Profiles - Data file format

Rainfall profiles create the following data file boundary identifiers. Please note that the following lines are written automatically to the .dat file from the user interface when you save your GERRBDY unit - the format is only given here as a reference.

Datafile format - start of rainfall

Where CD is the last line number of the catchment details data

The Master time interval MASTERT will be calculated from the interface after the rainfall profiles have been entered. This will be the greatest common divisor of all the rainfall time intervals and time delays entered. Note a time delay can be given by the user per defined rainfall profile, and (later in the model summary) two additional time delays can be given per defined sub-basin. All of these time delays, together will all rainfall time intervals, must be multiples of MASTERT.

Data file format - SCS storm

The SCS rainfall profiles create the following data file boundary identifiers. Please note that the following lines are written automatically to the .dat file from the user interface when you save your GERRBDY unit - the format is only given here as a reference.

where, for profile N , PRP is the last line of data for the previous rainfall profile, profile N-1 ; for profile 1, PRP is the last line of the multiple rainfall profiles data (CD+3)

CD is the last line number of the catchment details data.

Data file format - user entered storm

The user entered rainfall profiles create the following data file boundary identifiers. Please note that the following lines are written automatically to the .dat file from the user interface when you save your GERRBDY unit - the format is only given here as a reference.

where, for profile N , PRP is the last line of data for the previous rainfall profile, profile N-1 ; for profile 1, PRP is the last line of the multiple rainfall profiles data (CD+3).

CD is the last line number of the catchment details data.

Data file format - storm profile from library

The library entered rainfall profiles create the following data file boundary identifiers. Please note that the following lines are written automatically to the .dat file from the user interface when you save your GERRBDY unit - the format is only given here as a reference.

where, for profile N , PRP is the last line of data for the previous rainfall profile, profile N-1 ; for profile 1, PRP is the last line of the multiple rainfall profiles data (CD+3)

Multiple Rainfall Profiles data file format

Please find below an example of multiple rainfall profiles written to a data file. Please note that the .dat file is written automatically from the user interface when you save your GERRBDY unit - the format is only given here as a reference.

The Master time interval MASTERT will be calculated from the interface after the rainfall profiles have been entered. This will be the greatest common divisor of all the rainfall time intervals and time delays entered. Note a time delay can be given by the user per defined rainfall profile, and (later in the model summary) two additional time delays can be given per defined sub-basin. All of these time delays, together will all rainfall time intervals, must be multiples of MASTERT.
We note that (for the example given above) MASTERT is 0.25. This is the greatest common divisor of:

• data intervals for defined profiles. In this case our first profile has a data interval of 1 hour, second profile has a data interval of half an hour.
  and
• Time delays for defined profiles. In this case the first profile has a start time of zero but the second has a 15 minute delay.

Loss profiles Tab

On this tab, enter the details of loss profiles to be applied to your defined sub-basins.

A variety of options are given for users to enter their loss model profile(s). Click a link below to jump to the relevant section within this help file.

1. SCS curve number (CN) method. Options are provided for user entered, published or event data.
2. Green & Ampt (G&A) method. Options are provided for user entered or published data.

Key features of the 'Loss Profiles' tab are:

• The user has to provide details of at least one loss model, although the option of 'none' can then be chosen later in the 'Model Summary' tab. In this case, the gross rainfall received (as detailed in the rainfall profiles section) will be used directly to create the flow hydrograph;
• The user can add a second loss model by clicking ‘Add New Profile’;
• The user can enter their own name for the loss profile(s). Default name is:
  lossprofile_m, where m is sequential numbering;
• A 'Delete Profile' button is provided. This cannot be used unless you have multiple profiles defined (i.e. you cannot delete all loss profiles at once);
• Although not expected to be a common occurrence, the user has the option to enter unrelated, independent models to apply to sub-basins (for example using the CN method for one sub-basin and the G&A method for another) i.e. selection of an initial loss method does not restrict the selection of further loss models.

More information on how to use the loss profiles tab can be found here: How to setup a Generic Rainfall Runoff Boundary

SCS Curve Number method

As mentioned above, this option covers three eventualities – user entered data, published data and event data. Initially we discuss the details for the user entered option:

SCS curve number method - user entered

Theory and Guidance

Select the method 'SCS - User Specified' to manually enter a curve number to be applied. Later, in the 'Model Summary' tab, you will have the option to apply this loss model (or any other loss profiles defined) to any of your defined sub-basins.
Key features of the SCS loss model are:

1. User gives the loss model profile a name in field provided. This automatically updates on the tab label once they click anywhere outside of the ‘Profile name’ field.
2. Select ‘CN – user specified’ from drop-down box in ‘Method’ field.
3. User enters a singlecurve number to be applied to the entire sub-basin. This field is left blank by default.
4. Initial abstraction ratio is set at 0.2 s by default.

Note: the curve number must lie between 1 and 100. The software gives a warning if your curve number is less than 40 or greater than 98 (not recommended in this case, see TR-55 for further details)
Default metric (and US customary) units are given as:

• Curve number: dimensionless

SCS curve number method - entry by Published data

Theory and Guidance

Select the method 'SCS - Published data' to open a table for entering curve number data. Any rows from tables 2.2a, 2.2b, 2.2c or 2.2d in TR-55 can be added: select a land type, cover description, soil group to obtain the CN. Items in the table can also be adjusted (e.g. impervious area proportion, antecedent moisture condition), and the curve number will automatically update based on the adjusted value(s). Later, in the 'Model Summary' tab, you will have the option to apply this loss model (or any other loss profiles defined) to any of your defined sub-basins.

Key features of the SCS loss model are:

• User gives the loss model profile a name in field provided. This automatically updates on the tab label once they click anywhere outside of the ‘Profile name’ field;
• Select ‘CN from published data’ from drop-down box in ‘Method’ field;
• User selects land type, cover description and soil group, and clicks ‘Add’ to add this data in a row of the table. This automatically fills the land type, cover description and soil group columns. The CN column will be filled with the relevant curve number. None of these four columns are editable in the table - delete the row if you make a mistake at this point;
• Once a row is entered into the table, the AMC column will be automatically filled with “2” (by default). This is editable for most rows – other options (available via the drop-down box) are “1 (dry)” and “3 (wet)”. Adjust this value and the CN will update automatically based on the following relationship [Hawkins, et al. 1985]
  where CN , CN and CN are the curve numbers for ARC 1, 2 and 3, respectively.
  Please note that the AMC shall be 2 and not editable for the following land types:

• The user manually enters an area proportion in the ‘Area’ column. No default value is given. Note this is the area of the sub-basin not defined as DCIAoccupied by the chosen land-type, i.e. sub-basins are split into land and DCIA, the land is then split again by type;
• A row or rows can be highlighted in the table and then clicking the ‘Remove’ button will delete these rows;
• The composite curve number (once calculated) is shown in the ‘Composite Curve Number’ field. This is not editable by the user;
• Initial abstraction ratio is set at 0.2 s by default.

SCS loss model - estimate from event data

Theory and Guidance

Select the method 'SCS - event data' to provide rainfall and runoff data for events. The maximum retention and composite CN will be calculated automatically. Later, in the 'Model Summary' tab, you will have the option to apply this loss profile (or any other profiles defined) to any of your defined sub-basins.
Key features of the SCS- estimate from event data method are:

1. User gives the loss model profile a name in field provided. This automatically updates on the tab label once they click anywhere outside of the ‘Profile name’ field.
2. Select ‘CN from event data’ from drop-down box in ‘Method’ field.
3. Click ‘Add’ to add an event to the table.
4. Edit the ‘Rainfall’ and ‘Runoff’ columns by typing directly into the table.
5. Initial abstraction ratio is set at 0.2 s by default.
6. A 'Remove' button is provided to remove events. Highlight a row and click 'Remove' to delete the data.

Note: The initial abstraction entered is used for calculation in the loss model. the rainfall and runoff data entered into the table calculates the curve number CN assuming the relation Ia = 0.2s. Adjusting the initial abstraction will affect the initial abstraction used in the loss model procedure but not for the estimation of the curve number.

Default (and US customary) units are given as:

• Rainfall: mm (inches)
• Runoff: mm (inches)

Loss models: Green and Ampt infiltration

For the Green and Ampt loss model, the user can select between 'user specified' and 'Published data'. The only difference between these methods are the number of parameters that are editable in the Green-Ampt table. We therefore detail both of these methods in this section.

Theory and Guidance

Green and Ampt (1911) presented an approach to determine loss via infiltration based on fundamental physics, together with results that matched empirical observations. Select the method 'Green-Ampt' to utilise this methodology in your model. Later, in the 'Model Summary' tab, you will have the option to apply this loss model (or any other loss profiles defined) to any of your defined sub-basins.
Key features of the Green-Ampt loss model are:

• User gives the loss model profile a name in field provided. This automatically updates on the tab label once they click anywhere outside of the ‘Profile name’ field.
• Select ‘G&A – user specified’ from drop-down box in ‘Method’ field.
• Initially, click the 'Add' button and select a land type from the drop-down box. This will automatically populate the effective saturated water content, saturated hydraulic conductivity and wetting front suction fields with published data for the selected soil texture. For the 'user specified' option, these fields are all editable, whereas for the 'published data' option they are not. Published data values are given here . Please note that these values are in the units specified, and changing your units in Flood Modeller will not adjust these values.
• The ‘Area’ column must contain the proportion of the total sub-basin after any DCIA has been removed , as in the CN case. See above for further details. The values in the ‘Area’ column must sum to 1.
• The ‘Remove’ button allows the user to highlight row(s) for deletion.

Default metric (and US customary) units are given as:

• Initial water content: fraction e.g., cm3/cm3 (inches3/inches3)
• Effective saturation water content: fraction e.g., cm3/cm3 (inches3/inches3)
• Saturated hydraulic conductivity: cm/h (in/hr)
• Wetting front suction head: cm (inches)
• Area: km2 (square miles)

Loss Profiles - Data file format

Loss profiles create the following data file boundary identifiers. Please note that the following lines are written automatically to the .dat file from the user interface when you save your GERRBDY unit - the format is only given here as a reference.

Datafile format - start of loss profiles

where FRP is the last line data of the final rainfall profile

Data file format - SCS CN user specified

'SCS CN - user specified' loss profiles create the following data file boundary identifiers. Please note that the following lines are written automatically to the .dat file from the user interface when you save your GERRBDY unit - the format is only given here as a reference.

where, for profile N , PLP is the last line of data for the previous loss model profile, profile N-1 ; for profile 1, PLP is the last line of the multiple loss models profile data (FRP+3)

Data file format - SCS CN published data

'SCS CN - published data' loss profiles create the following data file boundary identifiers. Please note that the following lines are written automatically to the .dat file from the user interface when you save your GERRBDY unit - the format is only given here as a reference.

where, for profile N , PLP is the last line of data for the previous loss model profile, profile N-1 ; for profile 1, PLP is the last line of the multiple loss models profile data (FRP+3)

Data file format - SCS CN event data

'SCS CN - event data' loss profiles create the following data file boundary identifiers. Please note that the following lines are written automatically to the .dat file from the user interface when you save your GERRBDY unit - the format is only given here as a reference.

where, for profile N , PLP is the last line of data for the previous loss model profile, profile N-1 ; for profile 1, PLP is the last line of the multiple loss models profile data (FRP+3)

Data file format - Green Ampt

Green Ampt (user specified or published data) loss profiles create the following data file boundary identifiers. Please note that the following lines are written automatically to the .dat file from the user interface when you save your GERRBDY unit - the format is only given here as a reference.

where, for profile N , PLP is the last line of data for the previous loss model profile, profile N-1 ; for profile 1, PLP is the last line of the multiple loss models profile data (FRP+3)

Multiple Loss Profiles data file format

Please find below an example of multiple loss profiles written to a data file. Please note that the .dat file is written automatically from the user interface when you save your GERRBDY unit - the format is only given here as a reference.

Transformation Profiles Tab

On this tab, enter the details of loss profiles to be applied to your defined sub-basins.

The transformation model details are two-part: a method is required for calculating the travel time (time of concentration or time of lag) together with a method for calculating the hydrograph. The following options are provided for the hydrograph methods (click a link to jump to the relevant section of this help file):

1. The SCS unit hydrograph
2. The Clark unit hydrograph

For travel time, the user can specify the time of concentration or the time of lag. Further, the relation between the time of concentration and time of lag is clearly stated and adjustable by the user should they require this. The following options are provided for the timing methods (click a link to jump to the relevant section of this help file):

1. Time of concentration - user-specified
2. Time of concentration - NRCS method
3. Time of concentration - TR-55 method
4. Time of concentration - Kerby
5. Time of concentration - Kerpich
6. Time of concentration - Kerby/Kerpich combined method
7. Time of lag - user specified
8. Time of lag - Snyder

Key features of the 'Transformation profiles' tab are:

• The user must provide at least one transformation model.
• As with the rainfall and loss profiles, multiple transformation model profiles can be specified. The user can add a second transformation model by clicking ‘Add New Profile’.
• The user can enter their own name for the transformation profile(s). Default name is:
  transformation profile_m, where m is sequential numbering.

Although not expected to be a common occurrence, the user has the option to enter unrelated, independent models to apply to sub-basins (for example using the SCS unit hydrograph method for one sub-basin and the Clark hydrograph for another) i.e. selection of an initial transformation method does not restrict the selection of further transformation models.

More information on how to use the transformation profiles tab can be found here: How to setup a Generic Rainfall Runoff Boundary

The SCS Unit Hydrograph Method

Theory and Guidance

Select the transformation method 'SCS' to manually enter dimensionless unit hydrograph peak rate factor (DUHPRF) and define this transformation model for application to sub-basins. Later, in the 'Model Summary' tab, you will have the option to apply this transformation model (or any other transformation profiles defined) to any of your defined sub-basins.
Key features of the SCS transformation model are:

• User gives the transformation profile a name in field provided. This automatically updates on the tab label once they click anywhere outside of the ‘Profile name’ field;
• Select ‘SCS’ from drop-down box in ‘Method’ field;
• DUH Peak rate factor default value is 484 but this is editable by the user. This should not be set lower than 100 of greater than 600 (see NRCS: Technical Release 55 for further details);
• Timing details must be provided by the user – these are discussed further in the next sections.

The DUH peak rate factor defines the shape of the dimensionless unit hydrograph (DUH) A default value of 484 is adopted (the minimum permissible value is 100; the maximum permissible value is 600). A look-up table to guide the user in selecting an appropriate value for the DUH peak rate factor is provided here.
Default (and US customary) units are given as:

• DUHPRF (US customary only)

The Clark Unit Hydrograph Method

Theory and Guidance

Select the transformation method 'Clark' to manually enter a storage coefficient and define this transformation model for application to sub-basins. Later, in the 'Model Summary' tab, you will have the option to apply this transformation model (or any other transformation profiles defined) to any of your defined sub-basins.
Key features of the Clark transformation model are:

• User gives the transformation profile a name in field provided. This automatically updates on the tab label once they click anywhere outside of the ‘Profile name’ field.
• Select ‘Clark’ from drop-down box in ‘Method’ field.
• User must select a time/area method from the drop-down box provided. ‘Default’ method is currently the only method available. This is based on the default relationship:
  
  where A is the cumulative watershed area contributing at time t ; A is the total watershed area; and t is the time of concentration.
• User must enter a storage coefficient in the field provided. No default value. Must be positive.
• Timing details must be provided by the user – these are discussed further in the next sections.

Default (and US customary) units are given as:

• Storage Coefficient: hours (hours)

Timing Method

For either SCS or Clark transformation methods, a time of concentration and/or time of lag must be specified. Choose from the following methods (click a link to jump to the relevant section of this help file):

1. Time of concentration - user specified
2. Time of concentration - NRCS method
3. Time of concentration - TR-55 method
4. Time of concentration - Kerby
5. Time of concentration - Kerpich
6. Time of concentration - Kerby/Kerpich combined method
7. Time of lag - user-specified
8. Time of lag - Snyder

We discuss the options for specifying a time of concentration initially.

Time of concentration - user specified

Theory and Guidance

Select the method 'TOC - user specified' to manually enter a time of concentration. Later, in the 'Model Summary' tab, you will have the option to apply this transformation profile (or any other profiles defined) to any of your defined sub-basins.

• User selects radio button of ‘Specify time of concentration’
• Select ‘User Specified’ option.
• This disables the ‘Time of Lag’ field and enables the ‘Time of concentration’ field at the bottom of the page.
• The time of concentration can now be entered directly into the field provided.
• On clicking outside of the ‘Time of concentration’ field, the ‘Time of Lag’ field should be automatically updated. This should contain the value
  TOC/TOL multiplier * Time of concentration
• The TOC/TOL multiplier is also editable by the user. If this is edited, clicking outside of the field will automatically update the ‘Time of Lag’ field.
• The ‘Time of Lag’ field is not editable by the user.

Default (and US customary) units are given as:

• Time of concentration: hours (hours)
• Time of Lag: hours (hours)

Time of concentration - NRCS (watershed lag)

Theory and Guidance

Select the method 'TOC - NRCS' to enter parameters to calculate the time of concentration by the NRCS (watershed lag) methodology. Later, in the 'Model Summary' tab, you will have the option to apply this transformation profile (or any other profiles defined) to any of your defined sub-basins.
Key features of the NRCS time of concentration method are:

• User selects radio button of ‘Specify time of concentration’
• Select ‘NRCS (Watershed Lag)’ option.
• This disables the ‘Time of Lag’ field and the ‘Time of concentration’ field at the bottom of the page. Neither are editable by the user.
• User enters parameters for Flow length and Slope. No default values given.
• User can adjust the Retardance factor. Default value of 60.
• Once these parameters have been given, the ‘Time of concentration’ field should be automatically updated with the calculated time of concentration.
• The ‘Time of Lag’ field should be automatically updated. This should contain the value
  TOC/TOL multiplier * Time of concentration
• The TOC/TOL multiplier is editable by the user. If this is edited, clicking outside of the field should automatically update the ‘Time of Lag’ field.

Default (and US customary) units are given as:

• Time of concentration: hours (hours)
• Time of Lag: hours (hours)
• Flow length: m (ft)
• Flow slope: m/m (ft/ft)
• Retardance factor: dimensionless

Time of concentration - TR-55 (velocity method)

Theory and Guidance

Select the method 'TOC - TR-55' to enter parameters to calculate the time of concentration by the TR-55 (velocity) methodology. Later, in the 'Model Summary' tab, you will have the option to apply this transformation profile (or any other profiles defined) to any of your defined sub-basins.
Key features of the TR-55 time of concentration method are:

• User selects radio button of ‘Specify time of concentration’
• Select ‘TR-55 (velocity)’ option.
• This disables the ‘Time of Lag’ field and the ‘Time of concentration’ field at the bottom of the page. Neither are editable by the user.
• User enters parameters for sheet, shallow and open channel flow. Default values all zero aside from the drop-down box ‘Velocity Coefficient’.
• Once these parameters have been given, the ‘Time of concentration’ field should be automatically updated with the calculated time of concentration.
• The ‘Time of Lag’ field should be automatically updated. This should contain the value
  TOC/TOL multiplier * Time of concentration
• The TOC/TOL multiplier is editable by the user. If this is edited, clicking outside of the field should automatically update the ‘Time of Lag’ field.

Default (and US customary) units are given as:

• Time of concentration: hours (hours)
• Time of Lag: hours (hours)
• Sheet flow – Flow length: m (ft)
• Sheet flow – Flow slope: m/m (ft/ft)
• Sheet flow – Manning’s n: dimensionless
• Sheet flow – Rainfall depth: mm (inches)
• Shallow concentrated flow – Flow length: m (ft)
• Shallow concentrated flow – Slope: m/m (ft/ft)
• Shallow concentrated flow – Velocity coefficient: m/s (ft/s)
• Open channel flow – Flow length: m (ft)
• Open channel flow – Slope: m/m (ft/ft)
• Open channel flow – Manning’s n: dimensionless
• Open channel flow – Cross-sectional area: m²(ft²)
• Open channel flow – Wetted perimeter: m (ft)

Time of concentration - Kerby method

Theory and Guidance

Select the method 'TOC - Kerby' to enter parameters to calculate the time of concentration by the Kerby methodology. Later, in the 'Model Summary' tab, you will have the option to apply this transformation profile (or any other profiles defined) to any of your defined sub-basins.

Key features of the Kerby time of concentration method are:

• User selects radio button of ‘Specify time of concentration’
• Select ‘Kerby’ option.
• This disables the ‘Time of Lag’ field and the ‘Time of concentration’ field at the bottom of the page. Neither are editable by the user.
• User enters parameters for flow length and overland slope. Default values are zero.
• User selects a ‘Terrain’ from the drop-down box provided. This automatically fills the ‘Retardance coefficient’ field as follows:

The ‘retardance coefficient’ field is not editable unless the terrain option of ‘other’ is chosen. In this case the user can enter the retardance coefficient in the field provided.

• Once these parameters have been given, the ‘Time of concentration’ field will be automatically updated with the calculated time of concentration.
• The ‘Time of Lag’ field should be automatically updated. This should contain the value
  TOC/TOL multiplier * Time of concentration
• The TOC/TOL multiplier is editable by the user. If this is edited, clicking outside of the field will automatically update the ‘Time of Lag’ field.

Default (and US customary) units are given as:

• Time of concentration: hours (hours)
• Time of Lag: hours (hours)
• Flow length: m (ft)
• Overland slope: m/m (ft/ft)
• Retardance coefficient: dimensionless

Time of concentration - Kerpich method

Theory and Guidance

Select the method 'TOC - Kerpich' to enter parameters to calculate the time of concentration by the Kerpich methodology. Later, in the 'Model Summary' tab, you will have the option to apply this transformation profile (or any other profiles defined) to any of your defined sub-basins.
Key features of the Kerpich time of concentration method are:

• User selects radio button of ‘Specify time of concentration’
• Select ‘Kirpich’ option.
• This disables the ‘Time of Lag’ field and the ‘Time of concentration’ field at the bottom of the page. Neither are editable by the user.
• User enters parameters for channel flow length and channel slope. Default values are zero.
• User selects a ‘Terrain’ from the drop-down box provided. This automatically fills the ‘Adjustment Factor’ field as follows:

The ‘adjustment factor’ field is not editable unless the terrain option of ‘other’ is chosen. In this case the user can enter the adjustment factor in the field provided.

• Once these parameters have been given, the ‘Time of concentration’ field should be automatically updated with the calculated time of concentration.
• The ‘Time of Lag’ field should be automatically updated. This should contain the value
  TOC/TOL multiplier * Time of concentration
• The TOC/TOL multiplier is editable by the user. If this is edited, clicking outside of the field should automatically update the ‘Time of Lag’ field.

Default (and US customary) units are given as:

• Time of concentration: hours (hours)
• Time of Lag: hours (hours)
• Channel flow length: m (ft)
• Channel slope: m/m (ft/ft)
• Adjustment factor: dimensionless

Kerby and Kerpich combined

This method is simply using both the Kerby and Kirpich methods at the same time. The time of concentration is calculated as the sum of the times of concentration found via the Kerby and Kirpich methods. For detailed information of the parameters needed, please see the sections on the Kerby method  and Kerpich method .

We now discuss the options for specifying a time of lag.

Time of lag - user specified

Theory and Guidance

Select the method 'TOL - user specified' to manually enter a time of lag of choice. Later, in the 'Model Summary' tab, you will have the option to apply this transformation profile (or any other profiles defined) to any of your defined sub-basins.
Key features of the user specified time of lag method are:

• User selects radio button of ‘Specify time of lag’
• Select ‘User Specified’ option.
• This disables the ‘Time of concentration’ field and enables the ‘Time of lag’ field at the bottom of the page.
• The time of lag can now be entered directly into the field provided.
• On clicking outside of the ‘Time of lag’ field, the ‘Time of concentration’ field will be automatically updated. This will contain the value
  Time of lag / TOC-TOL multiplier
• The TOC/TOL multiplier is also editable by the user. If this is edited, clicking outside of the field should automatically update the ‘Time of concentration’ field.
• The ‘Time of concentration’ field is not editable by the user.

Default (and US customary) units are given as:

• Time of concentration: hours (hours)
• Time of Lag: hours (hours)

Time of lag - Snyder

Theory and Guidance

Select the method 'TOL - Snyder' to enter the Snyder parameters to calculate the time of lag. Later, in the 'Model Summary' tab, you will have the option to apply this transformation profile (or any other profiles defined) to any of your defined sub-basins.
Key features of the Snyder time of lag method are:

• User selects radio button of ‘Specify time of lag’
• Select ‘Snyder’ option.
• This disables the ‘Time of concentration’ field and the ‘Time of lag’ field at the bottom of the page. Neither are editable by the user.
• User enters Snyder parameters. The coefficient C_t should be positive.
• Once these parameters have been defined, the ‘Time of lag’ field should be automatically filled and the ‘Time of concentration’ field will be automatically updated. This will contain the value
  Time of lag / TOC-TOL multiplier
• The TOC/TOL multiplier is also editable by the user. If this is edited, clicking outside of the field should automatically update the ‘Time of concentration’ field.
• The ‘Time of concentration’ field is not editable by the user.

Default (and US customary) units are given as:

• Time of concentration: hours (hours)
• Time of Lag: hours (hours)
• Length (outlet to divide): km (miles)
• Length to centroid: km (miles)
• Coefficient C_t : dimensionless

Transformation Profiles - Data file format

Transformation profiles create the following data file boundary identifiers. Please note that the following lines are written automatically to the .dat file from the user interface when you save your GERRBDY unit - the format is only given here as a reference.

Data file format - start of transformation profiles

where FLP is the last line data of the final loss model profile

Transformation methods: Data file format - SCS UH method

The SCS unit hydrograph transformation profiles create the following data file boundary identifiers. Please note that the following lines are written automatically to the .dat file from the user interface when you save your GERRBDY unit - the format is only given here as a reference.

where, for profile N , PTP is the last line of data for the previous transformation profile (including time of concentration/lag details), profile N-1 ; for profile 1, PTP is the last line of the multiple transformation models profile data (FLP+3)

Transformation methods: Data file format - Clark method

The Clark transformation profiles create the following data file boundary identifiers. Please note that the following lines are written automatically to the .dat file from the user interface when you save your GERRBDY unit - the format is only given here as a reference.

where, for profile N , PTP is the last line of data for the previous transformation profile (including time of concentration/lag details), profile N-1 ; for profile 1, PTP is the last line of the multiple transformation models profile data (FLP+3)

Timing methods: Data file format - TOC user specified

The 'time of concentration - user specified' transformation profiles create the following data file boundary identifiers. Please note that the following lines are written automatically to the .dat file from the user interface when you save your GERRBDY unit - the format is only given here as a reference.

where, for profile N , ATP is the last line of data for the associated (N^th) transformation profile details

Timing methods: Data file format - TOC NRCS method

The 'TOC - NRCS' transformation profiles create the following data file boundary identifiers. Please note that the following lines are written automatically to the .dat file from the user interface when you save your GERRBDY unit - the format is only given here as a reference.

where, for profile N , ATP is the last line of data for the associated (N^th) transformation profile details

Timing methods: Data file format - TR-55 method

The 'TOC - TR-55' transformation profiles create the following data file boundary identifiers. Please note that the following lines are written automatically to the .dat file from the user interface when you save your GERRBDY unit - the format is only given here as a reference.

where, for profile N, ATP is the last line of data for the associated (N^th) transformation profile details.

Timing methods: Data file format - Kerby method

The 'TOC - Kerby' transformation profiles create the following data file boundary identifiers. Please note that the following lines are written automatically to the .dat file from the user interface when you save your GERRBDY unit - the format is only given here as a reference.

where, for profile N, ATP is the last line of data for the associated (N^th) transformation profile details

Timing methods: Data file format - Kerpich method

The 'TOC - Kerpich' transformation profiles create the following data file boundary identifiers. Please note that the following lines are written automatically to the .dat file from the user interface when you save your GERRBDY unit - the format is only given here as a reference.

where, for profile N, ATP is the last line of data for the associated (N^th) transformation profile details

Timing methods: Data file format - Kerby/Kerpich combined method

The 'TOC - Kerby/Kerpich combined' transformation profiles create the following data file boundary identifiers. Please note that the following lines are written automatically to the .dat file from the user interface when you save your GERRBDY unit - the format is only given here as a reference.

where, for profile >N , ATP is the last line of data for the associated (N^th) transformation profile details

Timing methods: Data file format - TOL user specified

The 'TOL - user specified' transformation profiles create the following data file boundary identifiers. Please note that the following lines are written automatically to the .dat file from the user interface when you save your GERRBDY unit - the format is only given here as a reference.

where, for profile N , ATP is the last line of data for the associated (N^th) transformation profile details

Timing methods: Data file format - TOL Snyder method

The 'TOC - Snyder' transformation profiles create the following data file boundary identifiers. Please note that the following lines are written automatically to the .dat file from the user interface when you save your GERRBDY unit - the format is only given here as a reference.

where, for profile N , ATP is the last line of data for the associated (N^th) transformation profile details

Multiple Transformation profile data entry

Please find below an example of multiple transformation profiles written to a data file. Please note that the .dat file is written automatically from the user interface when you save your GERRBDY unit - the format is only given here as a reference.

Baseflow Details Tab

On this tab, enter the details of baseflow profiles to be applied to your defined sub-basins.

Two methods are provided for defining baseflow models to apply to your sub-basins. Click a link below to jump to the relevant section within this help file.

1. Constant baseflow
2. Recessive baseflow

Key features of the 'Baseflow Profiles' tab are:

• At least one baseflow profile must be given (although this can be constant baseflow of 0, and the option of 'none' can be selected in the 'model summary' tab);
• A second baseflow profile can be added by clicking 'Add New Profile';
• The user can enter their own name for the rainfall profile(s). Default name is
  baseflowprofile_m, where m is sequential numbering;
• A ‘Delete Profile’ button is provided. This cannot be used unless you have multiple profiles defined (i.e. you cannot delete all baseflow profiles at once).

More information on how to use the baseflow profiles tab can be found here: How to setup a Generic Rainfall Runoff Boundary

Constant baseflow

Theory and Guidance

Select the method 'Constant baseflow' to manually enter a baseflow to be applied to your sub-basin. Later, in the 'Model Summary' tab, you will have the option to apply this baseflow profile (or any other profiles defined) to any of your defined sub-basins.
Key features of the constant baseflow profile are:

• User selects ‘Constant’ as baseflow method from drop-down box provided.
• A value can be given for the constant baseflow. This defaults at zero but is editable by the user.

Default (and US customary) units are given as:

• Constant baseflow: m³/s (ft³/s)

Recession baseflow

Theory and Guidance

Select the method 'Recessive baseflow' to manually enter parameters to apply a recessive baseflow model to your sub-basin. Later, in the 'Model Summary' tab, you will have the option to apply this baseflow profile (or any other profiles defined) to any of your defined sub-basins.
Key features of the recessive baseflow profile are:

• User selects ‘Recession’ as baseflow method from drop-down box provided.
• An initial value for baseflow is specified by the user;
• A recession constant value that defines the fraction of baseflow at time t+1 (i.e., <=1.0) is specified by the user;
• A baseflow threshold that defines a post quickflow peak flow magnitude at which the recession model defines the total flow is specified by the user;
  • If an ‘absolute’ baseflow threshold is selected, the baseflow threshold is an absolute value;
  • If a ‘relative to peak’ baseflow threshold is selected, the baseflow threshold is specified as a fraction of the peak flow;
• After the baseflow threshold flow, the total flow is given by the recession flow unless the quickflow and initial baseflow exceed the threshold.
• Default (and US customary) units are given as:
• Initial baseflow: m³/s(ft³/s)
• Recession constant: dimensionless
• Baseflow threshold (absolute): m³/s(ft³/s)

The recession baseflow model is illustrated here, where:

• Initial baseflow = 1 m³/s
• Recession constant = 0.8
• Baseflow threshold (absolute) = 1 m³/s
  
  If a recession model is chosen, the baseflow is calculated as:
  (1) B(t) = B₀ . K^(t - t), t ≥ t,
  where tis the time following ‘additional lag’ added to the sub-basin (we “drag back” the initial value so B(t) = B₀, for t ≤ t₀)
  The total flow is (by definition) the baseflow plus the quick flow:
  (2) T(t) = B(t) + Q(t), t > 0.
  If you have set a threshold, the total flow will decay no faster than the given recession model, once below the threshold, i.e.
  (3) T(t) ≥ T(t_h) . K^(t - t_h), t > t_h
  Where t_h is the first timestep at which T(t) as given by (2) is below the given threshold, and T(t_h) is the total flow as given by (2) at this timestep.

Baseflow Profiles - Data file format

Baseflow profiles create the following data file boundary identifiers. Please note that the following lines are written automatically to the .dat file from the user interface when you save your GERRBDY unit - the format is only given here as a reference.

Data file format - start of baseflow profiles

where FTP is the last line of the time of concentration data associated to the final transformation model profile

Data File Format - constant baseflow

The constant baseflow profiles create the following data file boundary identifiers. Please note that the following lines are written automatically to the .dat file from the user interface when you save your GERRBDY unit - the format is only given here as a reference.

where, for profile N , PBP is the last line of data for the previous baseflow profile, profile N-1 ; for profile 1, PBP is the last line of the multiple baseflow models profile data (FTP+3)

Data file format - recessive baseflow

The recession baseflow model creates the following data file boundary identifiers. Please note that the following lines are written automatically to the .dat file from the user interface when you save your GERRBDY unit - the format is only given here as a reference.

where, for profile Ni, PBP is the last line of data for the previous baseflow profile, profile N-1 ; for profile 1, PBP is the last line of the multiple baseflow models profile data (FTP+3)

Model Summary Tab

Theory and Guidance

All sub-basins defined in the ‘Catchment details’ tab are shown as a row in the ‘Model Assignment’ table. As default, the first profiles defined for each model are assigned to all sub-basins.
Key features of the 'Model Summary' tab are:

• The ‘Global Model Application’ section contains 4 drop-boxes (rainfall, loss model, transformation model, baseflow model) containing all profiles defined (the profile name given by the user) together with the option of “none”.
• The user can select a method here and click the ‘Apply Globally’ button directly underneath the selection they have made. This will apply the selection to all sub-basins. Note the ‘Apply Globally’ buttons are given, specifically applying rainfall, loss, transformation and baseflow models globally. This enables a user to define the rainfall (for example) for all sub-basins, and then select a loss model (for example) to apply globally without this affecting the previously made selections for rainfall.
• The user can also assign models individually to sub-basins in the ‘Model Assignment’ section. If the option of ‘none’ is selected as the rainfall model for a sub-basin, the remainder of the fields (i.e. loss model drop-box, transformation model drop-box and additional lag field) will all be disabled and contain ‘N/A’. As no rainfall is received on this sub-basin, no flow will be produced.
• The user can also choose to additionally lag sub-basins. The default value should be 0 for the ‘Additional lag’ field for each sub-basin but this is editable by the user. This will delay the flow from the sub-basin in question but without attenuation. If the user requires the hydrograph obtained from a sub-basin to be affected by both delay and attenuation, they must model from the outlet of the sub-basin to the main outlet (for example using Muskingum-Cunge routing methods or other functionality already available within Flood Modeller).
• Any rain fallen on area defined as DCIA (directly connected impervious areas) within a sub-basin will be transferred directly to the outlet without attenuation or delay, regardless of any additional lag specified for the sub-basin.
• Global options are to be applied to the total hydrograph, i.e. the summation over all the sub-basins. On each row in the summary table, define options for the individual sub-basins, and then the global parameters are applied to the summation over all sub-basins.

Default (and US customary) units are given as:

• SBINDEX, RFINDEX, LMINDEX, TPINDEX, BFINDEX: profile indices - Dimensionless
• ADDLAG: additional lag - hours (hours)
• PFONLY: peak flow flag - logical
• MINFLOW: minimum flow - m³/s (cfs)
• SCALING, SCALEBY: scaling option flags - dimensionless
• SCVALUE: scaling value - dimensionless if SCALEBY = FACTOR; m³/s (cfs) if SCALEBY = PEAK
• GDELAY: global delay - hours (hours)
• GPFONLY: global peak flow flag - logical
• GMINFLOW: global minimum flow - m³/s (cfs)
• GSCALING, GSCALEBY: global scaling options - flags, dimensionless
• GSCVALUE: global scaling value - dimensionless if GSCALEBY = FACTOR; m³/s (cfs) if GSCALEBY = PEAK

ADDLAG is additional lag (to push the calculated hydrograph along, if it has travelled some distance prior to joining the baseflow) and therefore must be a positive value for each sub-basin.

GDELAY is the global time “delay” applied to the unit time zero, in comparison to the simulation start time. This can therefore be a positive value (e.g. 2 would start the hydrology 2 hours after the simulation) or a negative value (e.g. -3 would start the hydrology 3 hours before the simulation).

Data file format

The model summary details create the following data file boundary identifiers. Please note that the following lines are written automatically to the .dat file from the user interface when you save your GERRBDY unit - the format is only given here as a reference.

where FBP is the last line data of the final baseflow profile.

Results Tab

On the Results Tab three sub-tabs are provided: i) Results; ii) Data; and iii) Hydrograph;

• The ‘Results’ tab allows the user to visualize tabular data of: i) Time; ii) Areal Rainfall; iii) Net rainfall; iv) Total Flow hydrograph; v) Quick Flow hydrograph; and vi) Baseflow. A “Plot” option is included in order to visualize the above mentioned data;
• The ‘Data’ tab shows a summary of the hydrologic parameters and calculated data obtained using the Generic Rainfall Runoff method; and
• The ‘Hydrograph’ tab provides a tabular summary of results (as per the ‘Results’ tab) and also provides a volumetric analysis of results.
  On first accessing the 'Results' tab, the results for the summation over all sub-basins will be shown. A drop-down is provided for the user to instead view results over a selected sub-basin.
  

Results - an example

For each sub-basin, the calculations are performed at the timestep of the rainfall profile applied to that sub-basin. If no rainfall is chosen (and so the contribution from the sub-basin is baseflow only), the baseflow is calculated at the master timestep. Note calculate the baseflow at an alternative timestep by applying a rainfall profile to the sub-basin in the desired timestep, and selecting the 'baseflow only' checkbox for that sub-basin in the model summary tab.
A simplified example is shown below of results for a single sub-basin with a 1 hour timestep. 

The gross rain is your specified storm: the rain that enters the system. The net rain is the rain still in the system after loss due to infiltration. For a sub-basin defined as all DCIA, or a sub-basin with loss model set to 'none', the net rain will be identical to the gross rain, i.e. no loss occurs.

The unit hydrograph ordinates (UH) are based on the transformation details selected.
The quick flow, sometimes referred to as the runoff flow, is the flow due to the net rain after transformation. In the table, we show the convolution of this net rain through the transformation model. Please note that the equation here is for representation only and does not account for the conversion from rainfall depth per unit area (units of the net rainfall column) to flow (in cumecs or ft3/s). Note also that the rain falling on DCIA is converted to quick flow without being transformed through a unit hydrograph. Similarly, a sub-basin with transformation model set to 'none' will convert all net rain to quick flow without convolution through a unit hydrograph.

The baseflow will be calculated based on the model chosen, and the total flow is then evaluated as the addition of the baseflow and quickflow.
If we assume a 2 hour time delay has been defined on the storm profile used in our example sub-basin, the results will adjust as follows:

Note that the quickflow values are identical (just delayed) but the total flow T is only identical to the original total flow t if the baseflow is constant.
If we assume instead a 3 hour additional lag is applied, the results for our example sub-basin adjust as follows:

Notice that in this case, the flow values are identical to the original (un-lagged) data, even if the baseflow is not constant, as all flows are delayed by the specified amount. The flow values for the inital flow are repeated ("pulled back" throughout the lag period) so the contribution from this sub-basin to the total flow summation does not "jump" in suddenly.

If the master timestep does not match the timestep for the rainfall profile assigned to a sub-basin, the rainfall depths will be equally divided, and the flows interpolated, to fit the master timestep, after the calculations have been performed at the original rainfall profile timestep.

The master timestep is the greatest common divisor of any applied rainfall profile time intervals and delays, and any additional lag added to a sub-basin. Therefore, the results will be divided (and interpolated) even if only considering a single sub-basin if the rainfall delays and additional lag are not multiples of the rainfall time interval.

We show below our example results with a 1.5 hour rainfall delay (so the master timestep has become 30 minutes) to illustrate this point further.
Stage 1 - calculate at rainfall timestep:

Stage 2 - split/interpolate to master timestep:

This shows the 2 stages of the process - the calculations are performed at the 1 hour timestep initially, and then depths are divided and flows are interpolated to the 30 minute timestep. Note that the baseflow is calculated for the initial times (during any rainfall delay added to the profile), and then at the (1 hour) timesteps required. For a recession baseflow model, the baseflow at the half hour intervals are defined (and so the values of bn etc. are well-defined and calculable for all n). 

Note also that the quickflow values (q1, q2, etc.) are identical to in our original example. For the transfer into the master timestep, the rainfall depths have all been divided (by two in this case, since two 30-minute timesteps fit into our 1-hour initial timestep) and the flows have all been interpolated. Note that the values in bold font are these interpolated values. The unit hydrograph ordinates also get interpolated - please note this is for illustrative purposes only and NOT indicative of an adjustment in the values used for the convolution of the quickflow (as can be seen by the identical values q for the (un-interpolated) quickflow).

A global lag field is also provided on the model summary tab. This delays the entire (summed) data calculated by the GERRBDY unit in comparison to the start time of your Flood Modeller simulation. This delay does not have any affect on timing set up within the sub-basins, and therefore in the example above, if only a single sub-basin was being considered for the summation, setting the global lag to 1.5 hours (and leaving the rainfall delay as 0) would have avoided the data divide and interpolation (assuming also any additional lag applied to the sub-basin was a multiple of the 1-hour time interval given for the rainfall). Alternatively, a 1 hour rainfall delay together with a 0.5 hour global lag would provide the same results, again avoiding the splitting/interpolation required for the 30-minute timestep.

Timing details

A time interval must be selected for each defined rainfall profile. For each sub-basin, the calculations (i.e. application of selected loss model and transformation model to calculate the 'quick' or 'runoff' flow; the calculation of the baseflow; and thus the calculation of the total flow) are performed at the timestep of the rainfall profile applied to that sub-basin.

A rainfall delay can be defined for each profile - this delays the start of te rainfall (and thus will delay the start of the runoff flow accordingly), but will not delay the start of any baseflow model defined (see the Results Tab section for further details).

Additional lag can also be defined for each sub-basin. This is to delay the contribution from that sub-basin to the total flow summation. As such, adding an additional lag has the effect of delaying allflow: the quick (runoff) flow andthe baseflow (and so, by definition, the total flow). Note that the baseflow will be assumed as the initial baseflow value throughout this lag period so the contribution to the total flow summation from this sub-basin doesn't "jump in" after the lag. Please see the Results Tab section for further details.

We recommend you select the same time interval for all rainfall profiles used in a Generic Rainfall Runoff unit at any one time, and that the delays (rainfall delays and additional lag on sub-basins) are all multiples of this common time interval. It is, however, possible to select different time intervals for rainfall profiles (and apply both at once to different sub-basins) or to select delays that are not multiples of the time interval. In this case, a so-called 'master timestep' will be required that divides all timesteps and delay times so that the summation over contributions from all sub-basins can be calculated. The calculation of the quick (runoff) flow (and so the total flow) is still performed at the time interval of the rainfall profile applied to the sub-basin, following which, any rainfall depths are divided evenly, and any flow data interpolated, to evaluate the contribution from the sub-basin at the master timestep. Please see the Results Tab section for further details.

MASTERT is the master time interval. This must divide ALL time intervals given. This includes:

• All time intervals used in all defined rainfall profiles
• All delays added to storms (defined on rainfall tab)
• All additional lags added to hydrographs in model summary tab

For example, if your data interval for your storm is 1 hour, but you choose to delay the start by 15 minutes, MASTERT will reduce to 15 minutes.

The Master time interval MASTERT will be calculated from the interface after the rainfall profiles have been entered. This will be the greatest common divisor of all the rainfall time intervals and time delays entered. Note a time delay can be given by the user per defined rainfall profile, and (later in the model summary) two additional time delays can be given per defined sub-basin. All of these time delays, together will all rainfall time intervals, must be multiples of MASTERT.

Impervious Areas and DCIA

The generic rainfall runoff boundary (GERRbdy) unit enables you to specify multiple land or soil types within a catchment as separate components. For each of these you can define different combinations of hydrological models that are combined to calculate an overall outflow from the catchment. This is then utilised within a 1D network (or 2D model) to provide a boundary inflow.

Catchments may contain impervious areas, i.e. areas where run-off will be quicker (i.e. possibly not transformed in any way) and unaffected by losses. In this case, the GERRbdy unit offers multiple options for defining these areas. These are explained in this section:

• Option 1: Directly Connected Impervious Areas (DCIA)
  
  A catchment can be subdivided in the GERRbdy unit into multiple sub-basins. For each of these you can specify a different combination of hydrological models (i.e. rainfall profile, loss model, etc.). In addition, you can specify a proportion of each sub-basin as a directly connected impervious area (DCIA).
  When a simulation is run, the specified rainfall for a sub-basin will be distributed between the DCIA and the remainder of the sub-basin. However, the proportion of rain falling on the DCIA will be converted to an immediate outflow, without applying the specified loss or transformation models for the sub-basin. The rain on the remainder of the sub-basin will be processed through the loss and transformation models. This processed runoff is then combined with the immediate runoff from the DCIA and any defined baseflow to compute the overall runoff from the sub-basin. Note that any rainfall delay or lag times defined for the sub-basin will apply to the entire sub-basin area including the DCIA (lag times are defined for each sub-basin on the Model Summary tab of the GERRbdy properties window).
  The DCIA is set on the Catchment Details tab of the GERRbdy properties window, as shown below. The value entered must be a number between 0 and 1 (inclusive).
  It should be noted that specifying impervious areas via the DCIA setting the resulting runoff will be immediate (upon the rain falling). If this is unrealistic for your catchment then you will either need to incorporate a lag time for the sub-basin (which will then be applied to both the DCIA and non-DCIA components of your sub-basin) or utilise one of the other methods described in this section.
  
  
  
• Option 2: Specify an impervious area as a separate sub-basin
  
  A catchment can be subdivided in the GERRbdy unit into multiple sub-basins. For each of these you can specify a different combination of hydrological models (i.e. rainfall profile, loss model, etc.). Thus, you have the option of dividing the catchment into an impervious sub-basin and a permeable sub-basin.
  In this case you would ignore the DCIA setting for each sub-basin, leaving this set to the default of 0.00. In the permeable sub-basin there is no impervious proportion and in the impervious sub-basin you will define the behaviour of the impervious area using a combination of the available loss and transformation models (and not DCIA). Thus, the sub-basin setup would look like this:
  
  Note, you have the option to further sub-divide the catchment into multiple sub-basin if a more detailed definition of different land types is required.
  You can then proceed to define a rainfall profile for your catchment. This can be used by all sub-basins or you can add different rainfall profiles for each sub-basin.
  For the impervious sub-basin, you can specify an SCS loss model that uses published data. This enables you to select impervious land types to represent the sub-basin (thus minimising losses and maximising runoff). The example below has utilised a combination of two different impervious land types, each representing 50% of the sub-basin area:
  
  With this option you also have the option of applying some degree of transformation by defining a transformation model and assigning this to your impervious sub-basin in the model summary. Alternatively (or as well) you have the option on the Model Summary tab to incorporate a degree of lag for your impervious runoff.
  It should be noted that if you need to include a baseflow in your catchment then this will need to be applied to all sub-basins if it is to be calculated for the entire catchment area.
  The definition of this option detailed above is quite simplistic. The GERRbdy properties window provides the capability of defining your catchment with greater levels of complexity if required. You can define multiple permeable and impervious sub-basins, each with the option to have different rainfall profiles, loss models, transformation models, etc.
  
  
• Option 3: Specify an impervious area as a component of a loss model
  
  The previous options describe how a catchment can be subdivided in the GERRbdy unit into multiple sub-basins. An alternative option is to represent the catchment using a single sub-basin but using a loss model to define the combined effect of the different land types present within the catchment.
  In this case you would ignore the DCIA setting for the sub-basin, leaving this set to the default of 0.00. The behaviour of the permeable and impervious proportions of the sub-basin can then be defined using a combination of the available loss and transformation models (and not DCIA).
  For the loss model, consideration of impervious areas can be achieved by defining a new loss profile set to use the “SCS curve number – published data” method. This enables you to define multiple land types, specifying what proportion of the sub-basin area they take up (sum of proportions must total 1.0). The published data list includes multiple options for both impervious and permeable land type. Thus, you can define impervious areas within your sub-basin (i.e. catchment) and the loss model will determine a composite SCS curve number that takes these into consideration, as shown below:
  
  With this option you also have the option of applying some degree of transformation (in the Transformation Models tab) and/or incorporate a degree of lag (in the Model Summary tab).
  The definition of this option detailed above is quite simplistic. The GERRbdy properties window provides the capability of defining your catchment with greater levels of complexity if required. You can define multiple sub-basins, each with the option to have different rainfall profiles, loss models, transformation models, etc.

References and Tabulated Data

Cover description relationships

US NRCS Part 630 Hydrology National Engineering Handbook Chapter 9 Hydrologic Soil-Cover Complexes Land type ID – Land type and Cover description ID – Cover description relationships are given as follows:
Land type ID (T9-1) – Land type (Agricultural lands):

Land type ID (T9-2) – Land type (Arid and semiarid rangelands):

Land type ID (T9-5) – Land type (Urban areas):

Green-Ampt infiltration parameter estimates

Green-Ampt infiltration parameter estimates based on US Army Corps of Engineers Engineer Manual 1110-2-1417 and 'The Handbook of Hydrology', Maidment, 1992 are given as follows:

Note: The wetting front suction head values provided assume dry initial conditions (i.e. residual saturation). The user is recommended to refer to original source material to understand possible variation in parameter values, e.g., according to different adopted initial conditions.

Time of concentration look-up tables - sheet flow

US NRCS Part 630 Hydrology National Engineering Handbook Chapter 15 Time of Concentration Velocity method sheet flow look-up tables are given as follows:
Manning’s roughness coefficients for sheet flow:

Maximum sheet flow lengths:

Time of concentration look-up tables - shallow concentrated flow

US NRCS Part 630 Hydrology National Engineering Handbook Chapter 15 Time of Concentration Velocity method shallow concentrated flow look-up table is given as follows:

Velocity equation coefficient:

DUH peak rate factors

The NOAA National Weather Service Office of Hydrology Hydrologic Research Laboratory & National Operational Hydrologic Remote Sensing Center provides the following guide values for estimating the DUH peak rate factor: