Source models the movement of water through a river reach using a routing link. Three link types are available:

Straight Through links
Lagged Flow links
Storage Routing links

Straight Through Routing

Straight Through links (no routing) directly transfer reach inflow to outflow in the same model time-step. This link type is provided mainly to allow modellers to connect model elements when no reach modelling is required

Lagged Flow Routing

Lagged Flow links directly translate reach inflow to outflow a specified number of model time steps later, without attenuation and without taking into account any lateral fluxes. This link type is provided for use when the lateral fluxes are insignificant, there is insufficient information available regarding reach processes, or these cannot be modelled due to constraints when integrating Source model elements with those from other products.

The lag time is specified by the modeller. If the specified lag time is not an integer multiple of the model time-step, it will be rounded to the nearest time-step.

Solution Methodology

Ordering Phase

For Lagged Flow links, the travel time is constant and known. The Order Travel Time is hence equal to the wave travel time and is calculated automatically by Source.

Flow Phase

The methodology for lagged flow is based on consideration of the average travel time of water in a river reach (ie. a routing link). Water entering the link during a given time-step exits the link at a user specified time in the future; this is rounded to the nearest number of model time-steps and is the number of time-steps of lag.

Internally, Source partitions the link into ndiv divisions (where ndiv is the lag measured in number of model timesteps). Water moves progressively through the link, one division at a time, without attenuation and ignoring any loss and gain fluxes and any dead storage, as follows:

Outflow from the link during the current time-step is the inflow to the most downstream division during the previous time-step;
Inflow enters the most upstream division during the current time-step; and
For intermediate divisions, the flow rate in any given division for the current time-step is the same as the flow rate in the division immediately upstream for the previous time-step.

The volume of water stored in each division at the end of the current time-step is calculated as the flow rate in the division for the current time-step multiplied by the model time-step. The minimum number of divisions, and the minimum number of time-steps of lag, is one.

Storage Routing Links

Storage Routing links model the storage and movement of water through a length of river using a hydrologic routing method. They can represent the travel time of water through a reach, the attenuation of flow rates due to channel shape and roughness and reach processes such as lateral fluxes. Examples of lateral fluxes include net evaporation from the water surface and exchanges between groundwater and surface water.

Storage Routing links offer a choice of several different hydrologic routing methods:

linear Muskingum routing,
non-linear Muskingum routing (using a power function),
variable parameter Muskingum routing

See the Link storage routing - SRG for a detailed description of storage routing.

Storage Routing links can be configured to represent lagged flow without attenuation (equivalent to a Lagged Flow link) but with the capacity to include lateral fluxes. See the Links user guide for how to configure the Storage Routing link to achieve this, and the Storage Routing Link SRG for information on the solution procedure.

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