Working with the Generic Rainfall-Runoff Boundary

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 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. Calculations to evaluate the total flow are performed for each sub-basin, and the total flow from this unit is determined as the summation over all the sub-basins.

Why use multiple sub-basins?

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 to the sub-basin will also apply to the contribution from the DCIA to the total flow summation. Please see the Results Tab and Generic Rainfall-Runoff Boundary - Impervious Areas and DCIA sections of the help for further details.
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.

Advantages of using multiple sub-basins

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 separately, 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. More information on the differences between impervious areas and directly connected impervious areas can be found in the Generic Rainfall-Runoff Boundary - Impervious Areas and DCIA section of the help.

How to use the Generic Rainfall-Runoff unit

Add a Generic Rainfall-Runoff boundary (GERRbdy node) to your network by selecting Hydrographs > Generic Rainfall/Runoff.

Click anywhere on the map to place the node, and then double-click the node to fill the details of your hydrological unit as follows:

Catchment Details Tab

On this tab, enter the details of your catchment area. Click in the 'Area' column to add the area. Your total catchment can be split into multiple sub-basins as you deem fit.

The outlet location can also be given on this tab. Please note this is for information only and is not used within the calculations.

Using multiple sub-basins

Click 'Add' on the right-hand-side to add a sub-basin to your catchment. Double-click the 'Area' field to give the area of the sub-basin and click the green 'tick' symbol at the bottom left of the table to update the total catchment area.

To delete a sub-basin, highlight the relevant row in the table and click 'Remove' on the right-hand-side.

A sub-basin name will automatically be given. Double-click in the 'Sub Basin Name' field to adjust this if necessary.

Adding DCIA to a sub-basin

A DCIA (directly connected impervious area) proportion can be given for each sub-basin. This area will not be considered when calculating rainfall infiltration; all the rainfall on this area will end up in the outlet of the sub-basin. Note that if DCIA is set to 1, the entire sub-basin is assumed to be DCIA and as such, all rain on the sub-basin will go straight to the outlet.

More information on the differences between impervious areas and directly connected impervious areas, and how to enter the required details into a Generic Rainfall Runoff unit can be found in the Generic Rainfall-Runoff Boundary - Impervious Areas and DCIA section of the help.

For more details about entering catchment details, including the advantages of splitting areas into smaller sub-basins, please see the Generic Rainfall-Runoff Boundary - Catchment Details Tab section of the help.

Rainfall Profiles Tab

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

At least one rainfall profile must be defined.

Selecting a rainfall profile

To add a new rainfall profile, select 'Add new profile' to open the rainfall profile wizard. You will be prompted to enter a profile name and select a method from the following:

• SCS Profiles
  For this option, you will also be prompted to select a storm type in the rainfall profile wizard. This will then create your profile. Your total storm depth must be given. You can also double-click to adjust values for the data interval and storm duration
  
  For more details about entering SCS profiles, please see the Generic Rainfall-Runoff Boundary - Rainfall Details Tab: SCS Profiles section of the help.
• User Entered Profiles
  Selecting this option will create your (blank) profile. Click on the table to enter the depths at each time interval and the software will automatically update the total depth given at the bottom of the table. You will need to also enter this total storm depth to the field on the left-hand-side before you can save this profile.
  To increase/decrease the rows in the table, adjust the total storm duration and/or data interval fields given on the left-hand-side. The arial reduction factor (ARF) is editable. Double-click to adjust these values.
  
  For more details about entering user entered rainfall profiles, please see the Generic Rainfall-Runoff Boundary - Rainfall Details Tab: User Entered Profiles section of the help.
• Library Entered Profiles
  For this option, you will also be prompted to select a library and event in the rainfall profile wizard. The profile will then be created. None of the values for these profiles are adjustable aside from the arial reduction factor (ARF), which can be adjusted by double-clicking in the relevant field.
  

For more details about entering rainfall profiles from library events, please see the Generic Rainfall-Runoff Boundary - Rainfall Details Tab: Library Profiles section of the help.

Using multiple rainfall profiles

Multiple rainfall profiles can be entered for assignment to the sub-basins defined. You can assign each sub-basin the same profile, assign different profiles for each sub-basin, or a mix of these.

To delete a rainfall profile, highlight the relevant row in the table on the left-hand-side and click 'Delete Current Profile'. To ensure you do not miss any necessary parameters or find the wrong parameters are applied to a profile, we recommend you always start a new profile (and therefore go through the rainfall profile wizard), rather than adjusting the method given for a current profile.

A rainfall profile name will automatically be given. Double-click in the 'Rainfall Profile Name' field to adjust this if necessary.

Once you have defined a rainfall profile, fields are given to delay the start of the storm. Please note that entering a value here will update the master timestep given on the model summary tab. For more details about the multiple time intervals involved in the unit and how they fit together, please see the Generic Rainfall-Runoff Boundary - Time Interval Details section of the help.

For more details about entering rainfall profiles in general, please see the Generic Rainfall-Runoff Boundary - Rainfall Details Tab section of the help.

Loss profiles Tab

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

At least one loss model must be defined, but you can select to apply no loss model to any sub-basin on the 'Model Summary' tab.

Selecting a loss profile

To add a new loss profile, click 'Add New Profile' and select a method from the following:

• SCS Curve Number method
  Based on methodology used by NRCS (Natural Resources Conservation Service, previously SCS), mass rainfall is converted to mass runoff by using a runoff curve number (CN). A curve number must lie between 1 and 100, with higher curve numbers giving greater runoff. The software will issue a warning if you select a CN lower than 40 or higher than 98, as the SCS CN methodology is not recommended in these cases. The method also requires the ratio between initial abstraction and potential maximum retention. The default relationship is Ia = 0.2S, where S = potential maximum retention after runoff begins = 1000/CN - 10. Please see the Generic Rainfall-Runoff Boundary - Loss Models Tab: SCS Curve Number Methodsection of the help for further details. The following methods are available for selection of the curve number:
  • User Specified
    If this method is selected, the software will provide a field for you to manually enter the CN.
    
  • Published Data
    This selection allows you to create a composite CN based on multiple land types within your sub-basin. Add anything from the tables 2.2a, 2.2b, 2.2c or 2.2d in TR-55: Select a land type, cover description and soil group. This will add the row to the table and calculate the relevant curve number.
    An area proportion must be given. After the table is filled, the area column (over all rows) must sum to 1. The calculated curve number (weighted by area) will be shown in the 'Composite Curve Number' field.
    The AMC (antecedent moisture condition) will have the default value of AMC = 2 (average). This can be adjusted to AMC = 1 (dry conditions) or AMC = 3 (wet conditions). Adjust this value and the CN will update automatically based on the following relationship [Hawkins, et al. 1985]:
    
    where CN1, CN2 and CN3 are the curve numbers for AMC 1, 2 and 3, respectively.
    
  • Event Data
    A composite CN can be calculated given the rainfall and runoff of previous events. Select this option to open a table of events and click the 'Add' button to add a new row to this. The event date can be adjusted, and the rainfall and runoff can be manually entered. The software will then calculate the composite CN based on these events and the relationship Ia = 0.2S, where S = potential maximum retention after runoff begins. Add new events with the 'Add' button and remove events with the 'Remove' button.
    
• Green & Ampt method
  Green & Ampt infiltration is an approach based on fundamental physics that gives results that also match empirical equations. Details are required for the effective saturated water content, saturated hydraulic conductivity and wetting front suction of the land type in the sub-basin. The total runoff is then calculated based on these details, together with the initial water content of the soil, and the area proportion.
  If either of the Green & Ampt methods ('user specified' or 'published data') are selected, a table will appear to enter your data. 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.
  
  The area proportion must be given, and the initial water content can be manually entered (this will default as 0). Click the 'Update' button at the bottom of the table to update the table with the new row of data. Repeat the process to add multiple rows. The 'Remove' button can be used to delete highlighted rows from the table.
  
  For further details about Green & Ampt infiltration and the equations used, please see Generic Rainfall-Runoff Boundary - Loss Models Tab: Green & Ampt Infiltration.

Using multiple loss profiles

Multiple loss models can be defined. These can then be applied to any of your defined sub-basins in the 'Model Summary' tab. The option of "none" is also provided in the 'Model Summary' tab - this would apply no loss (and thus all rain falling on the sub-basin would be converted to runoff).

To add a new profile, click 'Add New Profile' and to delete a profile, ensure it is highlighted on the left-hand-side and click 'Delete Current Profile'.

The profile name can be manually edited by clicking in the 'Profile Names' field.

For more details on the loss models available and the calculations performed, please see the Generic Rainfall-Runoff Boundary - Loss Models Tab of the help.

Transformation model details - Unit hydrograph selection

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

A method of transformation is required and a drop-down is provided to choose from the SCS unit hydrograph or Clark unit hydrograph methodologies. A timing method must be selected for both the SCS and Clark methods.

Selecting a transformation profile: unit hydrograph method

For the SCS method, the dimensionless unit hydrograph peak rate factor is required. The default value is 484 but this can be adjusted by clicking in the 'DUH peak rate factor' field. For further details on the SCS methodology and calculations used, please see the Generic Rainfall-Runoff Boundary - Transformation Profiles: SCS Method section of the help.

For the Clark method, a storage coefficient must be provided (in hours). A time/area method is also required; current implementation only allows for the default method, for more details regarding this, please see Generic Rainfall-Runoff Boundary - Transformation Profiles: Clark Method.

Selecting a transformation profile: timing method

A timing method must be selected for both the SCS and Clark methods. Timing details can be selected from the following options:

• Specifying a time of concentration
  A time of concentration can be specified and the time of lag is calculated based on the relationship
  Time of lag = toc/tol multiplier * time of concentration
  The toc/tol multiplier defaults to 0.6, as per the NRCS Part 630 Hydrology National Engineering Handbook. This value can be edited to adjust the above relationship. Choose from the following time of concentration methods:
  • User specified
    The 'Time of Concentration' field should be filled manually if this option is selected.
    
  • NRCS (Watershed Lag)
    If this option is selected, details should be provided on the flow length and slope, and the curve number of the land type. The software will automatically calculate and populate the 'Time of Concentration' field based on these values.
    
  • TR-55 (Velocity)
    For this method, parameters are required related to sheet flow, shallow concentrated flow and open channel flow. The travel time will be computed for each of these components individually. These are then summed to provide a total time of concentration.
    
  • Kerby
    The Kerby method finds the time of concentration based on overland flow. For this method, the flow length and overland slope should be manually entered. The terrain can be selected from a drop-down list, and this will automatically calculate the retardance coefficient.
    
  • Kerpich
    The Kerpich method calculates the time of concentration based on in-channel flow. The channel flow length and channel slope should be manually entered in the fields provided. A terrain can be selected from a drop-down list, and this will automatically populate the 'Adjustment Factor' field. To enter your own adjustment factor, select the terrain 'Other' and then click in the 'Adjustment Factor' field to manually edit the value.
    
  • Kerby/Kerpich Combined
    For this method, Kerby parameters describing overland flow and Kerpich parameters describing in-channel flow are both required. The time of concentration is calculated as the sum of the travel times found by both these methods.
• Specifying a time of lag
  A time of lag can be specified and the time of concentration is calculated based on the relationship
  Time of lag = toc/tol multiplier * time of concentration
  The toc/tol multiplier defaults to 0.6, as per the NRCS Part 630 Hydrology National Engineering Handbook. This value can be edited to adjust the above relationship. Choose from the following time of lag methods:
  • User specified
    If this option is chosen, the 'Time of Lag' field should be filled manually.
    
  • Snyder
    The Snyder method requires the length from the outlet to the divide, the length to the centroid, and the (positive) coefficient Ct. The software will automatically calculate the time of lag based on these values.
    

Using multiple transformation profiles

Multiple transformation models can be defined; use the 'Add New Profile' button to define a new profile.

To delete a profile, ensure it is highlighted on the left-hand-side and click 'Delete Current Profile'.

The profile name can be manually edited by clicking in the 'Profile Name' field.

For more details on the transformation models, both unit hydrograph selection and timing options available, please see the Generic Rainfall-Runoff Boundary - Transformation Models Tab section of the help.

Baseflow Details Tab

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

At least one baseflow must be defined, although this can be a constant baseflow of zero.

Selecting a baseflow profile

Constant or recessive baseflow can be defined as follows:

• Constant baseflow
  Selection of this method allows you to provide a constant baseflow of your choice.
  
• Recessive baseflow
  For this option, the baseflow is governed by:
  Baseflow = initial baseflow * k^t
  where the constant k is the recession constant. A threshold can be provided and the baseflow will be recalculated if the total flow falls below this threshold.
  

Using multiple baseflow profiles

Multiple baseflow profiles can be defined to be applied to each sub-basin.

Add a new baseflow profile by clicking 'Add New Profile'. Highlight a profile and click 'Delete Profile' to delete the profile.

A name can be given to your baseflow profiles. Click in the 'Profile Name' field to manually edit this name.

For more detailed information on the baseflow profiles and how these are applied to the sub-basins, please see Generic Rainfall-Runoff Boundary - Baseflow Details Tab.

Model Summary Tab

Once you have defined your rainfall profiles, loss profiles, transformation profiles and baseflow profiles, these need to be applied to the sub-basins you defined in the 'Catchment Details' tab. The model summary tab is provided to allow you to do this.

When you first navigate to the model summary tab, a table will be shown containing all your defined sub-basins. Buttons are provided at the top to allow you to select from your defined models (rainfall, loss, transformation and baseflow) and apply these globally, i.e. to all defined sub-.

Additional parameters, for example additional lag on the hydrograph and scaling options, can be defined on both a sub-basin level, and globally.

Editing sub-basin details in the model summary

Double-click in an individual row to edit the profile details for a particular sub-basin. Here you can choose which rainfall, loss, transformation and baseflow profiles to apply.

In this edit mode, you can also add additional lag for a sub-basin. This additional lag will be applied to all flows - the quick (or runoff) flow and the baseflow, and therefore (by definition) the total flow. The initial baseflow value will be used for the baseflow at all times during this additional lag. The additional lag has to be a multiple of the master timestep, see Generic Rainfall-Runoff Boundary - Time Interval Details and Generic Rainfall Runoff Unit - Results Tab for further details.

Fields are also provided to choose to scale and/or adjust the total flow produced from this sub-basin prior to adding this to the total flow from the other defined sub-basins. Options can be selected from the following:

• Peak Flow Only - check this box to override the total flow from this sub-basin to instead be the single, constant, peak flow value over all times
• Minimum Flow - give a parameter in this field to override the total flow value with the parameter given at any times where the calculated flow is lower
• Scaling Option/Flag and Factor - these parameters scale the total flow from the sub-basin. The user can select to scale the entire flow (Full hydrograph option) or the Runoff (quick) flow only. The flow can be scaled by a specified factor or to a specified peak - select the appropriate radio button as your 'Scaling Flag' and manually type your 'Scaling Factor' of choice in the field provided.

When you have made your changes, click 'Update' to apply these adjustments to the sub-basin.

Editing global details in the model summary

Global parameters are given at the bottom of the model summary tab. Here a delay can be chosen, alongside options for a global minimum flow, peak flow only globally, and scaling options, identical to those available for the individual sub-basins.

Options can be selected from the following:

• Peak Flow Only - check this box to override the total flow from this sub-basin to instead be the single, constant, peak flow value over all times
• Minimum Flow - give a parameter in this field to override the total flow value with the parameter given at any times where the calculated flow is lower
• Scaling Option/Flag and Factor - these parameters scale the total flow from the sub-basin. The user can select to scale the entire flow (Full hydrograph option) or the Runoff (quick) flow only. The flow can be scaled by a specified factor or to a specified peak - select the appropriate radio button as your 'Scaling Flag' and manually type your 'Scaling Factor' of choice in the field provided.

An additional field for a manually entered parameter 'Delay' is also provided. This delays the contribution from the Gerrbdy unit as a whole in comparison to the simulation start time

The software will calculate the total flow from the Gerrbdy unit in the following order:

1. Calculate the flow obtained from each of your sub-basins in turn, based on rainfall, loss, transformation and baseflow parameters, at the time interval given in the (applied) rainfall profile;
2. Evenly divide depths, and interpolate flows, so data is provided at master timestep (if necessary);
3. Apply any scaling and/or adjustments to the total flow from the sub-basins at the master timestep;
4. Add the flows from the sub-basins to find the contribution to the flow from the whole unit;
5. The global parameters defined are applied to this summation.

For more detailed information on the 'Model Summary' tab and delays/scaling options, please see Generic Rainfall-Runoff Boundary - Model Summary Tab.

Results Tab

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.

Note that the calculations are always performed at the time interval of the rainfall profile applied to the sub-basin, and results are split or interpolated to the 'master timestep' if required for the summation. For more details on this, including a simplified example, please see the Generic Rainfall Runoff Unit - Results Tab section of the help for further details.

Sub-basin Results

Select a specific sub-basin from the drop-down provided to view the results from that sub-basin.

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. 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.

The following results are displayed:

• Gross Rain. The gross rain is your specified storm: the rain that enters the system.
• Net Rain. 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.
• Unit Hydrograph. The unit hydrograph ordinates (UH) are based on the transformation details selected.
• Quick Flow. 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 m3/s 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.
• Baseflow. The baseflow will be calculated based on the model chosen.
• Total Flow. The total flow is then evaluated as the addition of the baseflow and quickflow.

If we assume a time delay has been defined on the storm profile used in the sub-basin, the results will show the gross rain as 0 for this delay. This will then delay the net rain and quick flow accordingly. The baseflow model will start at the start time of the sub-basin, and therefore this will not be affected by any rainfall delay.

If we assume instead an additional lag is applied, the results for the sub-basin adjust by all flows being lagged by this time. 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 initial 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.

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

• 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.

Summation Results

Select 'Summation over all sub-basins' to see the final results produced by the Generic Rainfall-Runoff unit. This is the summation over all the sub-basins, after interpolation (if this has been required).

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

• The ‘Results’ tab allows the user to visualise 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 visualise 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.

For further details on the 'Results' tab, please see Generic Rainfall Runoff Unit - Results Tab.

Now you're ready to go with the Generic Rainfall-Runoff boundary providing inflow to your model.