Editing Source Node Properties

You can edit the properties of the source node by: 

You will then be presented with a dialogue box,  which contains different tabs containing parameters regarding the hydrologic and water quality characteristics of the source node.

You can click on any tab and  can edit any of the data presented on the active dialogue box. 

The Source Tab contains Land Use/Zoning type, the total area, the percentage of impervious and pervious area for that source node. It also contains the hydrologic parameters used by the rainfall runoff model. For more information on model structure, operation and suggested parameter ranges see Rainfall Runoff Modelling.

Land Use/Zoning Type

For source nodes, select the zoning or surface type. This will change the water quality parameters used in the generation of pollutants from the source node to values appropriate for that zoning or surface type.  See below for more details. 

Total Area

Enter the total area of this source node in hectares.

The area information forms part of the input to MUSIC’s rainfall runoff model, derived from a model developed by the CRC for Catchment Hydrology (Chiew & McMahon, 1997). For a diagram of the model structure and a brief description of its operation, see Appendix A: Rainfall-Runoff Modelling. Further guidance on setting appropriate parameters for the rainfall runoff model is contained in the Appendices, especially for parameters specific to particular regions.

Pervious and Impervious Proportions

To edit the proportions of impervious and pervious areas, type the percentage of impervious/pervious area. Change in one will automatically adjust the other making the sum 100%.

Impervious Area

The impervious area has depression storage only and no infiltration, and quickly produces surface runoff during an event. Water in depression storage is lost to evapotranspiration daily.

Rainfall Threshold

The rainfall threshold represents the effect of depression storage in the impervious area and defines the minimum daily rainfall (mm) before surface runoff would occur from the impervious area.

Pervious Area

The pervious area represents the fraction of the source node in which infiltration occurs. Water that is stored in the pervious storage can be lost to evapotranspiration at any time, and to groundwater when the volume in store exceeds the field capacity.

Soil Storage Capacity

Soil Storage Capacity is the maximum storage depth of the pervious area store in millimetres.

Initial Storage

Initial Storage represents the level of storage in the pervious area store at the start of the run, expressed as a percentage of the storage capacity.

Field Capacity

Field capacity is the amount of the soil capacity that, when exceeded, water in the soil stores can drain by gravity to the groundwater store. If the amount of water in the soil stores is less field capacity value, water can be removed only by evapotranspiration. Field capacity should normally be less than the soil storage capacity of the pervious store (see above).

Infiltration Capacity Coefficient (a) and Exponent (b)

These parameters are used to calculate the maximum infiltration rate into the soil store in each time interval. The maximum infiltration rate is dependent on the soil moisture level with the coefficient a defining the infiltration rate for a dry soil condition and the exponent b defining the exponential rate in which the maximum infiltration rate decreases as soil moisture level increases. Rainfall in excess of the calculated rate becomes surface runoff.

Groundwater

Groundwater is modelled as a storage beneath the pervious area, and is the only source of baseflow. Recharge is by gravity drainage from the pervious area store whenever their volume exceeds the field capacity.

Initial Depth

The Initial Depth represents the volume of groundwater at the start of the simulation, expressed as a depth in millimetres.

Daily Recharge Rate

The Daily Recharge Rate represents the amount of water that drains daily to groundwater from the soil store, expressed as a percentage of the volume above field capacity in the store.

Daily Baseflow Rate

The Daily Baseflow Rate represents the amount of water that leaves the groundwater daily as baseflow, expressed as a percentage of the current groundwater volume.

Daily Deep Seepage Rate

The Daily Deep Seepage Rate represents the amount of water that leaves the groundwater daily as seepage, expressed as a percentage of the current groundwater volume. The seepage from the groundwater store represents water that is lost from the catchment to a deep groundwater storage. This seepage is permanently lost from the model and cannot return to the catchment.

Each component of runoff is calculated using the information above and a daily time-step. Each component is then disaggregated to the required time-step, using the detailed rainfall pattern and a technique appropriate to that component. For more information on disaggregation see Appendix A: Rainfall-Runoff Modelling.

Water Quality Parameter Dialogue Boxes

There are different tabs as Total Suspended Solids, Total Phosphorus and Total Nitrogen for modelling these pollutants.

The pollutants are generated as concentrations in mg/L, and can be defined for both the storm flow and base flow components of runoff generated from the rainfall runoff model. 

The default values displayed for Mean and Standard Deviation (Std Dev) for both base flow and storm flow (Table 1) are adapted from a range of sources, particularly Duncan (1999) and are discussed in more detail in Urban Runoff Generation. Wherever possible, these values have been modified in the MUSIC6.ini file to be representative of the region in which MUSIC is being applied. You can modify the pollutant concentrations as desired, by changing the values displayed in the Mean and Std Dev boxes. 

 There are two options available for defining pollutant concentration in both the surface and baseflow components of the runoff:

• Mean: A constant value set at the value displayed in the Mean text box; or
• Stochastically Generated: A stochastically generated concentration whose mean and standard deviation will be consistent with those displayed in the Mean and Standard Deviation text boxes.

A review of instantaneous water quality data has been undertaken to examine the ‘cross-correlation’ between pollutant concentrations, under both baseflow and stormflow conditions. No significant correlations were found during baseflow, however in urban catchments, a strong correlation can exist between TSS and TP during stormflow. In previous versions of music, this correlation was hard-wired, however in Version 4 and above of music, this has been removed to allow greater flexibility in the configuring of constituents.

You can specify serial correlation (autocorrelation) for both baseflow and stormflow data. The R2 for each should be derived from data in the log domain. The purpose of the stochastic generation option is to provide more realistic temporal variations in concentration; in other words, the concentration predicted by music at time t will be related to (correlated with) the concentration at the previous time-step (t-1). This results in pollutographs (plots of pollutant concentration over time) which are more realistic.

The default autocorrelation coefficient is set to zero to allow the same model run by different users to produce the same magnitude of loads, however you can specify the auto correlation coefficient if required (say if needing to calibrate against measured concentration data) and should use the values as set out in the table below:

It is important to note that the autocorrelation coefficient will not significantly affect the treatment train effectiveness produced by music, but simply ensures that the variation over time in concentrations during storm events and baseflow conditions is more ‘realistic’.  Depending on the time-step and coefficient used, there can be variations in mean annual loads for the same model run on different computers, however the maximum difference is usually within 10% of the previous run.