Functional units (FUs) refer to areas of particular hydrological response. In Source, Functional Unit classes are initially defined in the Geographic Wizard (Specify functional units). This page explains how to manipulate FU areas outside the Geographic Wizard.

Note: Although you can edit the list of FUs using the Edit » Functional Units » List menu item, this is not recommended as it may result in the loss of any model definition and settings of the deleted FU(s).

Functional Unit names cannot be modified once they have been assigned in the Geographic Wizard

Specify functional unit areas

You can assign areas to every functional unit in each sub-catchment in one of two ways:

Manually using Edit » Functional Units » Assign Area Manually (Figure 1); or
From a raster such as a land use map (Figure 2).

Areas can also be imported into the Functional Unit table from a .csv file.

Note: Areal percentages per sub-catchment must sum to 100%. An error message is shown if Functional Unit areas do not add up to 100%:

A 'Normalize Area Ratios' Button has been introduced in versions of Source after version 4.2.6. This button can be used to manage small errors resulting from decimal rounding. It will automatically fixed the sum of FU areas to equal the sub-catchment area.

This is especially useful when areas are calculated from a shapefile or raster, where resolution differences between sub-catchments and FUs can result in small area differences.

Manual assignment

Manual assignment of functional units is done in the Function unit area configuration dialog (Figure 1). This table lists all combinations of FUs and sub-catchments. For each sub-catchment, enter the area that is allocated to each FU. You can specify this as an absolute value in the Area column, or as a percentage in the Area Ratio column. Values can be entered into individual cells, or in multiple cells as follows:

Use the contextual menu to assign a single value to all cells in the selected column;
Use the panel below the table to re-allocate a percentage of land cover from one pre-defined FU to another. For example, if you have already specified that 50% of each sub-catchment is of FU type "Forest", and 50% is of type "Urban", you could model increases in urban encroachment (by say 5%) as follows:
- Specify the Auto change by field to 5%;
- Specify the From Functional Unit and To Functional Unit fields to Forest and Urban respectively; and
- Click the Auto-increase button (Figure 1).

Figure 1. Assigning FU areas manually

Note: Using filters in the Functional Unit Area Configuraiton dialog is similar to using them in the Feature Table (see Working with filters in the Feature Table). However, there is also a subcatchment filter to help you find subcatchments either by name or by using the subcatchment map, see Subcatchment filter.

Using a raster

Choose Edit » Functional Units » Assign Area Via Raster to open the FU raster dialog (Figure 2);
Click Load to upload a raster file;
If the land use map does not include a mapping of grid code to land use, click Assign Mapping;
In the resulting dialog (Figure 3), assign a FU to each grid code using the drop-down menu in the FU column. Hover over the map to view the code which corresponds to each sub-catchment; and
Click OK to close the window.

Specify functional unit areas

Manual assignment

Figure 1. Assigning FU areas manually

Using a raster

Figure 2. Assigning FU areas using a land use raster

Figure 3. Assigning FU areas using a land use raster, assign mapping

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