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Accounting : Account Reference : Diversion Account : Return Flow Route or Split
Return Flow Route or Split
The Return Flow Route or Split category allows the user to specify if and how return flows will be split and/or routed. It has two methods: Simple Lag and Split and Route. When switching between these two methods, RiverWare must delete any existing supplies that link returnFlow or Returned Flows. See Simple Lag, Split and Route, returnFlow, and Returned Flows for details.
If there is only one supply, a new supply will be created to the new slot. If there are multiple supplies, then no supplies will be re-created. Any input data on the supply will be lost. When you switch methods, a warning dialog will be posted to describe what will happen.
* Simple Lag
This default method uses simple time lag that uses an integer number of timesteps.
Slots Specific to This Method
 returnFlow
Type: Series Slot
Units: Flow
Alternate Units: Volume
Description: Amount of diverted water that is not consumed. This water then leaves the Diversion account as returnFlow
I/O: Input, Rules, Object-level Accounting Method, or Account Solution
 Return Flow Lag
Type: Scalar Slot
Units: None (no of timesteps)
Description: The integer number of timesteps the returnFlow should be lagged from Diversion. See returnFlow.
I/O: If not input, zero will be used
Method Details 
In general, the method will lag the internal tempReturnFlow[t] by the specified number of timesteps and set the returnFlow slot in the future. See Account Solution Equations for a description of the full solution approach.
When any routing is considered, multiple dates (timesteps) are involved in the solution. Although tempReturnFlow[t] is a flow, it is converted to a volume in the computation to conserve mass. The result is converted back to flows for the lagged timestep. If the run timestep is a constant size, these flow/volume conversions are not needed nor performed as the length of t is the same as the length of the lagged timestep. The equations become much simpler.
    
* Split and Route
Any return flow will have Multi Return Lag Coeffs. Then, the routed return flow will be set according to the coefficients. See Multi Return Lag Coeffs.
Note:  The Split and Route method cannot be used by a diversion account in a computational subbasin that performs a water rights allocation using the prior appropriation method and water rights solver.
Slots Specific to This Method
 Returned Flows
Type: Multi Slot (Accounting Multi Slot)
Units: Flow
Alternate Units: Volume
Description: This slot stores the split and routed return flows.
As supply links are added, columns are added to this multi slot.
I/O: Output only
 Multi Return Lag Coeffs
Type: Table Slot
Units: Fraction
Description: There will be one column for each supply to Returned Flows.
The number of columns will be added automatically when the supply links are made. The number of valid values in each column determine the number of coefficients for that supply. These should sum to 1.0 down a column. Columns are not deleted if supplies are deleted.
I/O: Required Input.
 PreRouted Return Flows
Agg Series Slot
Units: Flow
Alternate Units: Volume
Description: The return flows before they are routed
This slot is an Agg Series Slot but appears similar to a Multi Slot. If there are multiple supplies to Returned Flows, it has one column for the Total and then one column for each supply to Returned Flows. If there is only one supply, this slot only has one column with no total. The slot will be automatically resized when supplies are added or deleted.
I/O: Output Only during simulation, Input at presimulation timesteps if necessary
 Return Flow Proportion
Type: Table Slot.
Units: Fraction
Description: This slot specifies how the split should be made. There is one column for each split/supply. The columns should be automatically added when the supply links to Returned Flows is made. Columns are not deleted if supplies are deleted.
See PreRouted Return Flows, Returned Flows, and Multi Return Lag Coeffs.
The sum of the values should equal 1.0.
I/O: Required input
Method Details 
The solution equation will set the PreRouted Return Flows slot for each column “i” (supply) based on the temporary value tempReturnFlow[t], as follows.
    
For each split; that is, each column “i” of the Returned Flows slot and corresponding columns of the PreRouted Return Flows and Multi Return Lag Coeffs slot, the return flow is computed as:
    
Where C is the appropriate coefficient in the Multi Return Lag Coeffs slot and n is the number of coefficients in the Multi Return Lag Coeffs slot. “ts()” indicates the timestep length of the specified timestep. In both cases, n is computed from the number of valid values in the appropriate column of the Multi Return Lag Coeffs slot.
Note:  Typically in the accounting system, routing sets the current value at an integer number of timesteps in the future. This method is different, here the equations are setting the current Returned Flows using values from previous timesteps. This method then requires that there are PreRouted Return Flows at t-n timesteps to solve. On presimulation timesteps, these should be input or set by a rule.
Revised: 08/03/2021