Simple Runoff Model (SRM)

The Simple Runoff Model (SRM) uses a soil moisture deficit time series in conjunction with a constant runoff coefficient to modify input rainfall to get an effective rainfall:

Catchment rainfall * RC * (1-SMD) (1) where: RC = Constant runoff coefficient SMD = Soil moisture deficit factor

The soil moisture deficit (SMD) time series can either be calculated using PDM or obtained from a profile in the Rainfall Event Editor.

Constant runoff coefficient (RC) for a subcatchment can be determined from land use and soil type GIS files and a user defined lookup table. (See the Setting Runoff Coefficients from GIS Data topic for details.)

The effective rainfall is then applied to a simplified PDM model in which soil moisture storage, evaporation, infiltration and drainage to groundwater are set close to zero. Direct runoff is routed through surface storage defined by a double linear reservoir expressed as a discretely coincident transfer function model (O'connor K.M., 1982).

A pure time delay to be added to the start time of the input rainfall may also be defined.

SRM only generates a surface flow component. Baseflow may be calculated using PDM if required.

Using the SRM model

To use the SRM runoff volume model, set the Runoff volume type field of the Runoff Surface to SRM.

For subcatchments referencing an SRM runoff surface, the following subcatchment data fields will be used and must have values specified:

• Soil moisture deficit - soil moisture deficit (SMD) profile used in effective rainfall calculation. Choose whether to calculate SMD using PDM, or to use SMD profiles defined in the Rainfall Event.
• SRM runoff coefficient - constant runoff coefficient used in effective rainfall calculation
• SRM linear time constant 1 - time constant of linear reservoir 1
• SRM linear time constant 2 - time constant of linear reservoir 2
• SRM time delay - time delay to be added to the start time of input rainfall
• Baseflow calculation - allows baseflow to be calculated using PDM