Deficit and Constant Loss Model

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, with the excess rainfall going to runoff.

Using the Deficit and Constant Loss Model

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)
• Infiltration loss coefficient - infiltration rate applied when soil layer is saturated
• 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.)

Net rainfall is calculated as:

Net rainfall = Rainfall - Evaporation

If Net rainfall > 0 and saturation deficit > 0

If Net rainfall > saturation deficit: Available rainfall = Net rainfall - Deficit Deficit = 0 Runoff = max(0, Available rainfall - Constant infiltration rate)     If Net rainfall < saturation deficit: Deficit = Deficit - Net rainfall Runoff = 0

 

If Net rainfall > 0 and saturation deficit = 0

Runoff = max(0, Net rainfall - Constant infiltration rate)

 

If Net rainfall ≤ 0

Deficit = min(Maximum deficit, Deficit - Net rainfall)