Rainfall runoff models

A rainfall runoff model is used to derive runoff for a particular area from inputs of rainfall and potential evapotranspiration (or areal potential evapotranspiration). All rainfall runoff models in Source are conceptual models that represent catchment hydrological response to rainfall as a series of mathematical relationships. They provide runoff output from each functional unit as total discharge, which is split into quick flow (surface flow) and slow flow (baseflow) proportions. Refer to the Source Scientific Reference Guide for more detail.

Choosing the right model

Choosing and calibrating your rainfall runoff model is essential. The quality of your rainfall runoff calibration can interact with, and affect output from other models. For example, where constituent load is calculated as the product of flow and concentration, any errors in flow estimates will be propagated through to load estimates.

Consider the following when choosing a rainfall runoff model:

What models do you and other people in your organisation have experience with;
Have any rainfall runoff models worked well for your catchment in the past;
What type of catchment are you modelling? (urban, forested, mixed land use);
What data do you have available? You need to parameterise any models you choose; and
Do you have any information on previous rainfall runoff model calibrations? If calibrated models already exist, you can re-use the models and parameter sets.

The following might also be useful:

Run a range of different models and determine which ones work best; and
Consider using different models or parameters, in different sub-catchments or functional units. What works best in one functional unit may not work well in another. For example, different models may be useful where sub-catchments/FUs have different soil types (different soil water holding capacity) or different proportions of urban areas (large differences in impervious runoff).

Refer to the Source Scientific Reference Guide for details on the types of rainfall runoff models.

Configuring rainfall runoff models

The total discharge generated from rainfall runoff depends on which model is specified for the sub-catchment/FU combination. Configuring rainfall runoff models in Source involves 3 steps: assigning the model, adding input data to it, and parameterising the data. These are described next.

Assign rainfall runoff models

In Source, a functional unit in a sub-catchment can be assigned a rainfall runoff model by choosing Edit » Rainfall-Runoff » Assign Models.... (Figure 1) shows the resulting window.

Choose the appropriate model using the drop-down menu in the Rainfall runoff column. There is an option to assign a rainfall runoff model, using either a table a map to select sub-catchments, then assigning a model to the FUs. These methods are similar to those outlined for assigning FU areas.

If runoff is not of interest, or not applicable (eg. you may have FUs that represent water bodies), leave the entries in the Rainfall runoff column set to Nil runoff. Figure 1 shows some sub-catchment/FU combinations assigned a model, whereas others have been left with Nil runoff.

Figure 1. Assigning rainfall runoff models

Assign inputs to rainfall runoff models

To assign inputs (PET and rainfall) to the selected rainfall runoff model, choose Edit » Rainfall-Runoff » Assign Inputs...(Figure 2).

Figure 2. Assigning inputs to rainfall runoff models

The Grid-Based Input Assignment method allows you to load a time series of input data for each FU and sub-catchment combination in the model. You can assign inputs using a table or map. To assign input data using a table, do the following:

ln the PET or Rainfall columns, select a sub-catchment or FU cell for which you have climate data;
From the drop-down menu in the cell, choose Load;
Navigate to the input file and click Open; and
Choose the input file, which now appears on the drop-down menu for all cells in that column.

If required, refer to Using the Apply-to options to assign data to multiple sub-catchments and FUs.

The Climate Data Import tool allows you to upload spatial, gridded input data using the import tool. Refer to the Climate Data Import Tool.

Parameterise rainfall runoff models

You can parameterise rainfall runoff models by choosing Edit » Rainfall-Runoff » Parameters....

A parameter set consists of a set of variables that you can assign values to. You can accept the defaults or change these values to create a new parameter set, which can be applied to each of the rainfall runoff models that you have previously selected. If the same model needs a different set of parameters because for example, some parameters are spatially dependent, simply create another parameter set with values appropriate for a different geographic region:

Click a cell in the Model column. This specifies the model you wish to create a parameter set for; and
Click New at the top right of the window (Figure 2) to create a new parameter set. This opens a window (Figure 3) showing the default values for the parameter set for that model. You can also edit or delete a parameter set using the respective buttons.

Figure 3. Editing a parameter set based on SURM's defaults

Click OK and use the Table or Map tab to assign the values from that parameter set to any or all of the functional units that have the relevant model assigned to them.

Note: Once you apply a parameter set to the models in a FU or sub-catchment, the models’ parameter values are fixed. Any subsequent changes to the parameter set values do not change the values applied to the models. If you have to change the parameters in a parameter set, you must re-apply the changed parameter set with the Using the Apply-to options.

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