One of the drawbacks of the Probability Distributed Model (PDM) model is that many of the parameters are not physically meaningful. Therefore, it is often used in conjunction with the Simplified Runoff Model (SRM). Like the PDM the SRM is a total catchment model which relies on a runoff coefficient and 2 time constraints.
The SRM model determines an effective rainfall based on the following equation:-
Effective Rainfall=Catchment Rainfall*Runoff Coefficient*(1-Soil Moisture Deficit).
To set up a catchment to use a SRM model, first a runoff surface needs to be set up to use the SRM runoff volume model. Like the PDM, this requires no additional parameters to be defined in the runoff surface properties as the rest of parameters can be specified in the subcatchments property sheet.
The runoff coefficient is a constant runoff coefficient which is usually based on land cover and soil type. If GIS layers are available for land cover and soil type, then a look up table can be used in InfoWorks ICM to assign these values automatically using the Model-Subcatchments-CN/Runoff Coefficient lookup… dialogue.
It is also possible to define the SRM linear time constants to represent the routing of flow in the double linear reservoir model which controls the timing of the resulting flow. A time delay can also be added if required.
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. If baseflow is required then it is possible to represent this using the PDM model.
The Soil Moisture Deficit can be observed, in which case it is added to the rainfall event editor, or it can be calculated using the PDM model based on the PDM descriptors.
If the PDM is used to simulate baseflow and/or SMD then it is necessary to add the PDM descriptors in the subcatchment property sheet. These can be set for a single subcatchment or it is possible to use the same set of PDM descriptors for a number of subcatchments.