Note: This is documentation for version 4.11 of Source. For a different version of Source go to the Space Directory.
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The Export Rate model applies a fixed constituent generation rate to a functional unit (FU) to calculate total constituent load. It requires only a single parameter so is quick to use and therefore useful for exploring sensitivity.


This model is applied at the functional unit scale within a sub-catchment.

The model is a scaling factor and is independent of the basic time-step of the model. It is therefore more appropriate for estimating long term loads.

Principal developer

Cooperative Research Centre for Catchment Hydrology.


Source v4.3


Export rate is automatically installed with Source.

Flow phase

The Export Rate model is a simple area-based scaling factor for constituents from a functional unit. It defines a value between 0 and ∞, representing the physical amount of constituent exported per unit area per year.

For constituent C

Equation 1

Input data

A single value representing the export rate in units of tonne/ha/year is required. Area of the FU is taken from the model.


For export rates: 

Generic lookup tables of export rates are available but there can be large variabilit between areas, which can introduce considerable uncertainty (Fletcher et al, 2004).

Long-term data is required to derive reliable local values. 

Robust estimation of export rates is particularly difficult in environments where most of the load is delivered in a few events (eg. Tropics). The export rate model can therefore be appropriate for estimation of mean annual loads over long periods but generally provides poor estimates over shorter time-steps (a few years or less).


Parameters or settings

The parameters for this model are summarised in Table 1.

Table 1. Model parameters





Default Min

Default Max

Output data

Constituent load in kg per time-step.


Fletcher, T, Duncan, H, Poelsma, P & Lloyd, L 2004, Stormwater flow and quality, and the effectiveness of non-proprietary stormwater treatment measures - a review & gap analysis, Technical Report 04/8, Cooperative Research Centre for Catchment Hydrology.