The Deficit and Constant Loss Model is based on the HEC-HMS model described in the HEC-HMS Users's Manual (2010).
This method models the surface as a single soil layer in which incident rainfall is initially stored and which is subject to evaporative loss (defined in the Rainfall Event). When the soil layer reaches saturation capacity, infiltration may occur. When used as a runoff volume model, any excess rainfall goes to runoff.
Using the Deficit and Constant Loss model for runoff volume
To use the Deficit and Constant Loss model, set the Runoff Volume Type field of the Runoff Surface to DefConLoss.
The following parameters are used by this model:
- Maximum deficit - the amount of water that the soil layer can hold (depth). Defined in the Runoff surface properties.
- Infiltration loss coefficient - infiltration rate applied when soil layer is saturated. Defined in the Runoff surface properties.
- Initial deficit value - initial condition indicating the amount of water required to saturate the soil layer. Defined in the initial conditions of the Rainfall event.
- Evaporation - rate of evaporation specified as a single value in sub-event or profile properties, or as a time varying profile in the Rainfall event.
To model the impervious proportion of the subcatchment, define a separate fixed runoff surface.
Total storage is calculated as:
| Total storage = Surface storage + Net rainfall |
Net rainfall is calculated as:
| Net rainfall = Rainfall - Evaporation |
If Total storage > 0 and saturation deficit > 0
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If Total storage > saturation deficit:
If Total storage ≤ saturation deficit:
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If Total storage > 0 and saturation deficit = 0
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If Total storage ≤ 0
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Using the Deficit and Constant Loss model for infiltration (2D)
To use Deficit and Constant Loss to model 2D infiltration, set the Infiltration type field of the Infiltration surface (2D) to DefConLoss.
The following parameters are used by this model:
- Maximum deficit - the amount of water that the soil layer can hold (depth). Defined in the Infiltration surfaces (2D) properties.
- Infiltration loss coefficient - infiltration rate applied when soil layer is saturated. Defined in the Infiltration surfaces (2D) properties.
- DefConLoss initial deficit - initial condition indicating the amount of water required to saturate the soil layer. Initial deficit values for specific zones are defined in the Initial Conditions 2D grid.
- Evaporation - rate of evaporation specified as a single value in sub-event or profile properties, or as a time varying profile in the Rainfall event.
When used for modelling 2D infiltration, the models behaves almost the same as it does for modelling runoff volume (described previously), except for the following:
- If depth in the 2D element > 0, Surface storage = depth, Net rainfall = 0
- If depth in the 2D element = 0, Surface storage = 0, Net rainfall = - evaporation
When infiltration is calculated, the 2D element depth will have already been updated to include any rainfall onto the element.