Return Flow Split

The Return Flow Split category is dependent selecting either the No Structure or Sequential linking structure on the Agg Diversion Site or the Water User being standalone. This category contains methods which allow the user to split the return flow into surface water and groundwater components or multiple components. The None, SW GW Fractional Split and MultiReturn Fractional Split methods are available regardless of which Return Flow method is chosen. The SW GW Efficiency Split method is only available when the Variable Efficiency or Variable Efficiency with Soil Moisture methods are selected in the Return Flow category. If the user has selected Supplement Diversion in the Conjunctive Use category, the Total Return Flow, which is the sum of the Return Flow and the Supplement Return Flow, will be split into components. Otherwise, it is just the Return Flow that is split.

For each method below, the final step describes how the return flow split and the linked slots affect a sequential Agg Diversion Site. In each method, it lists the part of the return flow that contributes to the Outgoing Available Water based on linking of return flow slots.

None

This method performs no computations.

There is no split into surface and groundwater components. The return flow is treated as a single entity. This is the default method for the Return Flow Split category.

When part of a sequential Agg Diversion Site the Return Flow Split determines method determines how much contributes to the next water user element. If using a Conjunctive Use method:

• If the Total Return Flow is linked, then the Total Return Flow is not included in the outgoing available flow.

• If the Total Return Flow is not linked, Return Flow is included in the outgoing available flow.

If not using a Conjunctive Use method:

• If the Return Flow is linked, then the Return Flow is not included in the outgoing available flow.

• If the Return Flow is not linked, Return Flow is included in the outgoing available flow.

SW GW Fractional Split

The SW GW Fractional Split method splits the Return Flow (plus Supplement Return Flow if applicable) into surface water and groundwater components based on a user input fraction.

Slots Specific to This Method

Fraction GW Return Flow

Type: Series Slot

Units: Fraction

Description: Amount of groundwater return flow per unit total return flow

Information: Expressed as a fraction.

I/O: Optional; Required when GW Return Flow and Surface Return Flow are not input.

Links: Usually not linked.

GW Return Flow

Type: Series Slot

Units: Flow

Description: Amount of return flow to groundwater

Information:

I/O: Optional; Required when Fraction GW Return Flow and Surface Return Flow are not input.

Links: Inflow slot on a Groundwater Storage object, series slot on Data Object.

Surface Return Flow

Type: Series Slot

Units: Flow

Description: Amount of return flow to surface water

Information:

I/O: Optional; Required when Fraction GW Return Flow and GW Return Flow are not input.

Links: Return Flow slot on any object, series slot on Data Object.

Method Details

If the total return flow (sum of Return Flow and Supplement Return Flow) is equal to 0.0, both the GW Return Flow and the Surface Return Flow are set equal to 0.0. If the total return flow is not equal to 0.0 and Surface Return Flow is input (assuming both Fraction GW Return Flow and GW Return Flow are not input), GW Return is calculated by the following equation:

Fraction GW Return Flow is then calculated as follows:

If GW Return Flow is input and both Fraction GW Return Flow and Surface Return Flow are not input, Surface Return Flow is calculated as follows:

Fraction GW Return Flow is then calculated as follows:

If both Surface Return Flow and GW Return Flow are NOT input, the following steps are taken: If the total return flow is not equal to 0.0 and the Fraction GW Return Flow is input (and both GW Return Flow and Surface Return Flow are not input), the following equations are used to calculate the GW Return Flow and the Surface Return Flow. The GW Return Flow is calculated as follows:

The Surface Return Flow is the difference between the total return flow and the GW Return Flow, as follows:

When part of a sequential Agg Diversion Site, the Surface Return Flow is included in the outgoing available flow when Surface Return Flow is not linked. If Surface Return Flow is linked, it goes elsewhere and is not available for downstream elements.

SW GW Efficiency Split

The SW GW Efficiency Split method is available only when the Variable Efficiency or Variable Efficiency with Soil Moisture methods are selected in the Return Flow category. The Efficiency value calculated from the Variable Efficiency method is used to determine the portion of return flow that is groundwater and the portion that is surface water.

Slots Specific to This Method

GW Return Flow

Type: Series Slot

Units: Flow

Description: Amount of return flow to groundwater

Information:

I/O: Optional.

Links: Inflow slot on a Groundwater Storage object, series slot on a Data Object.

GW Split Adjust Factor

Type: Table Slot

Units: Fraction

Description: Amount of groundwater return flow per unit total return flow

Information:

I/O: Optional; Set to 1.0 if not input.

Links: Not linkable.

Surface Return Flow

Type: Series Slot

Units: Flow

Description: Amount of return flow to surface water

Information:

I/O: Optional.

Links: Return Flow slot on any object, series slot on Data Object.

Method Details

If Surface Return Flow is input and GW Return Flow is not input, GW Return Flow is calculated by the following Equation:

If GW Return Flow is input and Surface Return Flow is not input, Surface Return Flow is calculated by the following equation:

If both Surface Return Flow and GW Return Flow are NOT input, the following steps are taken: If the total return flow (sum of the Return Flow and Supplement Return Flow) is equal to 0.0, both the GW Return Flow and the Surface Return Flow are set equal to 0.0. If the total return flow is not equal to 0.0, the GW Return Flow and the Surface Return Flow are calculated by the following equations.

The GW Return Flow is calculated as follows:

The surface Return Flow is the difference between the total return flow and the GW Return Flow, as follows:

When part of a sequential Agg Diversion Site, the Surface Return Flow is included in the outgoing available flow when Surface Return Flow is not linked. If Surface Return Flow is linked, it goes elsewhere and is not available for downstream elements.

Irrigated Area GW Return Rate

The Irrigated Area GW Return Rate method partitions Return Flow into GW Return Flow using irrigated area and groundwater return rate, with any remaining return flow assigned to Surface Return Flow.

Slots Specific to This Method

Irrigated Area

Type: Series Slot

Units: Area

Description: The irrigated area contributing to return flow.

Information: Also used by the Irrigated Requests method, if active

I/O: Input or Output.

Links: Not linkable.

Groundwater Return Rate

Type: Series Slot

Units: Velocity

Description: Rate that water percolates into the ground.

Information: This represents the volume returning to groundwater per area of irrigated land per unit of time

I/O: Input only.

Links: Not linkable.

GW Return Flow

Type: Series Slot

Units: Flow

Description: Portion of the total Return Flow that percolates into the groundwater.

Information:

I/O: Output only.

Links: Linkable to the Inflow from Surface Water series slot on the connected Groundwater object.

Surface Return Flow

Type: Series Slot

Units: Flow

Description: Portion of the total return flow that remains surface water

Information:

I/O: Output only.

Links: Return Flow slot on any object, series slot on Data Object.

Method Details

If Diversion Requested is 0, meaning no irrigation is occurring:

Otherwise:

Using the user-input Groundwater Return Rate, which is constrained to be greater than or equal to 0, perform the following equation:

With the calculated total return flow (Return Flow plus Supplement Return Flow, where applicable) constraining GW Return Flow, if necessary:

When part of a sequential Agg Diversion Site, the Surface Return Flow is included in the outgoing available flow when Surface Return Flow is not linked. If Surface Return Flow is linked, it goes elsewhere and is not available for downstream elements.

Multi Return Fractional Split

This method is used to split the return flow into multiple return flow destinations. The user specifies the portion of the total return flow that goes to each destination.

Slots Specific to This Method

Return Flow Proportion

Type: Table Slot

Units: Fraction

Description: Table giving the proportion of the total return flow that goes to each Returned Flows subslot

Information: The values in this slot should add up to 1.0. For example, if there are two columns and the total return flow is split evenly, 0.5 would go in one column and 0.5 would go in the other column. The table will have the same number of columns as the number of return flows. The columns will be added and named (by RiverWare) whenever a link is added to the Returned Flows multislot. A decimal number will be input for each column representing each return flow slot.

I/O: Required input

Links: Not linkable

Returned Flows

Type: No Compute Multi Slot

Units: Flow

Description: Multiple (or split) return flows based on the Return Flow Proportion table

Information: The subslots of this multislot contain the multiple return flows. A subslot is created when the user links this slot to a destination object.

I/O: Output

Links: Linked to the return flow destination objects. If this slot is linked, the Return Flow slot should not be linked.

Method Details

The multiple return flows would then be calculated as follows:

where i is the column of the Return Flow Proportion table; and total return flow is the sum of Return Flow and Supplement Return Flow, where applicable.

The first column on the Returned Flows multislot is the multislot itself and is not one of the subslots. It is set as the sum of all the subslot values. Therefore it should be equal to the total return flow.

Each of the multiple return flows will be represented by a subslot in the Returned Flows multislot. A subslot is created by linking the Returned Flows slot to the destination object. Whenever a new subslot is created, a new column will be added to the Return Flow Proportion slot. The column will have the same name as the subslot so that the user can keep track of which subslot goes with which column.

When part of a sequential Agg Diversion Site, the Returned Flows are not included in the outgoing available flow. The Returned Flows are all linked, so flow goes elsewhere and is not available for downstream elements.

Revised: 08/02/2021