Overview of configuring constituents
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There are two types of constituent routing available, Lumped and Marker routing. (Figure 1). Both assume that the constituents are conservativeof these are conservative routing models, which means that they do not change the total mass of constituent in the system.
Lumped routing is the simplest approach, where constituents are routed within a link based on kinematic wave theory. Assuming fully-mixed conditions within a link, the constituent flux and concentration simply move from the top of a link to the downstream end of a link within a time step, preserving the mass balance. Constituent concentrations in a link can be altered by the addition of constituents generated from sub-catchments, external inflows, and losses within a reach; and
- Marker routing considers constituents as particles and tracks their movement within a link, which can be divided into divisions for hydrologic routing purposes. Initially, the model will start with a marker at the end of each division in every link. At every time step, a new marker for each constituent will be created for each division, and the distance a marker moves is driven by the velocity in the division over the current time step. While the flow rate is assumed constant over the timestep, the velocity within the division will change as a result of a change in reach storage and cross-sectional area. Markers will travel through the river network until they are either merged with adjoining markers or leave the river network (ie. via extractions, decay within the reach, evaporation, groundwater inflows/losses and rainfall). Refer to Marker routing (Particle tracking) - SRG for more information.
For marker routing, you must specify two additional parameters:- Minimum Marker Gap – defines the spacing between markers as either a fraction of the model time-step or fraction of the reach division. This parameter can improve model efficiency by reducing the number of markers that require processing at each model time step. The allowable range is from 0 to 1, with 0 not deleting any markers, while a value of 1 will ensure that at the end of each time-step, there is only one marker defined for each reach division; and
- Minimum volume – volume to maintain constituent mass balance within the links.
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