Return Flow
The Return Flow category is dependent on the user selecting either the No Structure or Sequential Structure linking structure or the Water User being standalone. The user methods within the Return Flow category specify how the return flow is to be calculated for each Water User. The user has the option of linking the return flow back to the diverted object or elsewhere along the system.
These methods are executed after the Diversion slot has been set.
NoneNone is an invalid method. An error is posted and the run terminated if the user tries to run a model with None selected in the Return Flow category.
Note: This is the default method for the Return Flow category.
Fractional Return FlowThe Fractional Return Flow method calculates the return flow based on a user input fraction. The input fraction (Fractional Return Flow) is representative of the amount of flow that is not consumed by the Water User.
Slots Specific to This Method
Fraction Return Flow
Type: Series Slot
Units: Fraction
Description: Fraction of the diverted flow that is not consumed by the Water User
Information:
I/O: Required input
Links: Not linkable
Return Flow
Type: Series Slot
Units: Flow
Description: Amount of diverted water not consumed by Water User
Information:
I/O: Output only
Links: May be linked to the Return Flow slot on any object, or the Inflow slot on a Groundwater Storage object.
Method Details
This user method performs very simple calculations. The Return Flow is calculated as follows:
The Depletion (or amount of water consumed by the water user) is calculated as follows:
The above calculations are only performed if the Fractional Return Flow is known. If the Fractional Return Flow is not known, a RiverWare error is posted and the run terminates.
Proportional ShortageThe Proportional Shortage Method computes the Return Flow based on the ratio of Diversion to Diversion Requested and the Depletion Requested.
Slots Specific to This Method
Return Flow
Type: Series Slot
Units: Flow
Description: Amount of diverted water not consumed by the Water User
Information:
I/O: Output only
Links: May be linked to the Return Flow slot on any object, or the Inflow slot on a Groundwater Storage object.
Method Details
This method uses a set of simple calculations and logical expressions to determine the Return Flow.
The delivered fraction (a local variable) is determined as follows. If the Diversion Requested is equal to 0.0, the delivered fraction is set equal to 1.0. If the Diversion Requested is not equal to 0.0, the delivered fraction is set as follows:
After the delivered fraction is determined, determine the Depletion, as follows:
The Return Flow is then calculated as follows:
Proportional Shortage with Soil MoistureThis method uses the diverted water to first meet Depletion Requested. If that is not sufficient, then Soil Moisture can be used to meet the Depletion Requested. This method also tracks the volume of water stored in the soil moisture.
This method is only available for standalone water users and water user elements that are part of a No Structure or Sequential structure Agg Diversion site. Also, you must select the Irrigation Requests with Soil Moisture or Irrigation Requests with Soil Moisture method.
See Irrigation Requests with Soil Moisture for a diagram of the Soil Moisture slots.
Slots Specific to This Method
Available Soil Moisture Fraction
Type: Series
Units: Fraction
Description: Fraction (decimal or percentage) of moisture available. This is computed as the moisture in the soil divided by the maximum soil moisture volume
Information: 
I/O: Output only
Links: Not linkable
Crop Usage
Type: Series
Units: Flow
Description: Amount of water used by the crops. This includes depletion of delivered water and soil moisture usage, but not supplemental flow.
Information:
I/O: Output only
Links: Not linkable
Crop Shortage
Type: Series
Units: Flow
Description: Amount of crop demand that is not met by either diversions or soil moisture.
Information:
I/O: Output only
Links: Not linkable
Consumptive Use from Delivered Flow
Type: Series Slot
Units: Flow
Description: Portion of the consumed water that came from the Diversion.
Information: This is identical to Depletion.
I/O: Output only
Links: Not linkable
Consumptive Use from Soil Moisture
Type: Series Slot
Units: Flow
Description: Portion of the consumed water that came from the soil moisture
Information:
I/O: Output only
Type: Not linkable
Delivered Flow
Type: Series Slot
Units: Flow
Description: Portion of the diverted flow that is delivered to the fields and is available for crops or to refill the soil moisture.
Information: This is computed as the Diversion minus the Incidental Loss.
I/O: Output only
Links: Not linkable
Diversion Request for Crop
Type: Series Slot
Units: Flow
Description: Requested diversion to meet crop depletion
Information: This is computed by the Irrigation Requests with Soil Moisture method. See Irrigation Requests with Soil Moisture.
I/O: Output only
Links: Not linkable
Efficiency including Soil Moisture
Type: Series Slot
Units: Fraction
Description: Efficiency calculation that includes the flow to the soil moisture.
Information: It shows how water is used beneficially since water stored in soil moisture will be consumed later by the crop. This slot is only used for informational purposes and is not used in further calculations.
I/O: Output only
Incidental Loss
Type: Series Slot
Units: Flow
Description: Portion of the Diversion that is lost due to the incidental loss rate.
Information:
I/O: Output only
Links: Not linkable
Incidental Loss Rate
Type: Table Slot
Units: Fraction
Description: Losses in the water distribution system.
Information: Expressed as a fraction of the flow.
I/O: Optional; if not specified, zero is assumed.
Links: Not linkable
Irrigated Area
Type: Series Slot
Units: Area
Description: Surface area of the land to be irrigated
Information:
I/O: Specified according to the selected method in the Irrigation Acreage and Evapotranspiration rates category. See Irrigation Acreage and Evapotranspiration Rates.
Links: Not linkable
Maximum Soil Moisture
Type: Scalar
Units: Length
Description: Depth of water that the soil can hold.
Information:
I/O: Required input
Links: Not linkable
Maximum Infiltration Rate
Type: Scalar
Units: Velocity (Length per Time)
Description: Maximum depth of water that can be absorbed by the soil
Information: A valid value must be specified or an error will occur at the beginning of run.
I/O: Required input
Links: Not linkable
Return Flow
Type: Series Slot
Units: FLOW
Description: Amount of diverted water not consumed by the Water User
Information:
I/O: Output only
Links: May be linked to flow slots on any object
Soil Moisture
Type: Series Slot
Units: Volume
Description: This slot tracks the volume of soil moisture water in the soil at each timestep in the run.
Information: For the initial timestep, this slot must be valid. If this slot is not input, it is computed as the Maximum Soil Moisture times the Irrigated Area at the initial timestep. That is, the soil moisture starts full.
I/O: Typically output for run timesteps.
Links: Not linkable
Soil Moisture Demand
Type: Series Slot
Units: Flow
Description: Flow necessary to refill the soil moisture to capacity.
Information:
I/O: Output only
Links: Not linkable
Soil Moisture Fill Efficiency
Type: Series Slot with Periodic Input
Units: Fraction
Description: Fraction of the flow applied for soil moisture refill that is not absorbed.
Information: For example, if you are only refilling the soil moisture, you may apply 100 cfs, but 5 cfs is not absorbed even though max infiltration indicated it could absorb 200cfs. The Soil Moisture Fill Efficiency would be 0.95.
I/O: Optional input. If not specified, it is assumed to be zero
Links: Not linkable
Soil Moisture Flow
Type: Series
Units: Flow
Description: Flow into or out of the soil moisture.
Information: A positive number indicates flow into the soil moisture storage. A negative number indicates the soil moisture is being used to meet crop demands.
I/O: Output only
Links: Not linkable
Surface Runoff
Type: Series
Units: Flow
Description: Water that runs off the field because the maximum infiltration rate has been exceeded. This water never enters the soil.
Information: If the Surface Runoff slot is linked to a slot on another object it is not included in the total Return Flow. If the Surface Runoff is not linked, the Surface Runoff is included as part of the total Return Flow.
I/O: Output only
Links: Linkable to another object’s Return Flow slot
Excess Effective Precipitation
Type: Series with Periodic Input
Units: Velocity (length/time)
Description: Excess means effective precipitation that typically cannot be used by the crops. Normally, effective precipitation is subtracted from the crop demand to compute a net Depletion Requested. However, in some instances the effective precipitation is greater than what the crops could use within a given time step. Therefore, a value for excess effective precipitation would likely be input only when input Depletion Requested is zero (because the crop demand would have been met by the non-excess portion of the effective precipitation). Excess Effective Precipitation goes towards filling the soil moisture column until the available capacity is reached. Any remaining excess effective precipitation that cannot be stored in the soil moisture column is added to return flow
Information: This value is multiplied by the irrigated acreage to compute a flow.
I/O: Input or set by a rule. It is assumed to be zero if not input.
Links: Not linkable
Excess Effective Precipitation Flow
Type: Series
Units: Flow
Description: Flow rate that represents the excess effective precipitation
Information: Excess Effective Precipitation Flow = Excess Effective Precipitation * Irrigated Area
I/O: Output only
Links: Not linkable
Method Details
The Proportional Shortage with Soil Moisture Return Flow method does the following.
First, compute how much of diversion is lost due to conveyance or incidental loss.
Calculate how much of the diverted flow is actually delivered, as follows:
Calculate the Excess Effective Precipitation flow as follows:
Calculate the temporary variable flowToIrrigatedArea as follows. This is the flow applied to the field.
Compute the maximum infiltration flow intermediate variable, as follows:
Compute the flow the is above the maxInfiltrationFlow. This water does not absorb and runs directly off the field’s surface.
Compute the flow that infiltrates to the soil, as follows:
Compute the Depletion as follows. Depletion is limited by the maxInfiltrationFlow as it cannot use more than is infiltrated.
Calculate the initial return flow intermediate value as follows. This will occur for any diversion even when storing soil moisture.
Calculate the initial crop shortage as follows. This is the amount of the depletion requested that is not met by diverted water.
Calculate Soil Moisture Flow as follows:
IF (cropShortageInitial > 0)
This is negative indicating water is moving from the soil moisture to the crops.
ELSE
Extra water may have been diverted (if the demand factor was 0, or Diversion was specified) or there may be excess effective precipitation. The above equation uses the Soil Moisture Demand, so it can refill the soil moisture, even if the demand factor is zero. When water is applied for soil moisture, there is some that is returned, according to the Soil Moisture Fill Efficiency.
END IF
Calculate final Return Flow as follows. If the Surface Runoff slot is linked to another object it is not included in the total Return Flow.
If this Surface Runoff is not linked, the Surface Runoff is included as part of the total Return Flow, as follows:
Calculate Soil Moisture and Available Fraction as follows:
Diversion Shortage:
Depletion Shortage:
Compute total crop usage and shortage as follows:
Finally compute summary variables, as follows:
Variable EfficiencyThe Variable Efficiency method computes the Return Flow based on an efficiency driven approach. This method (or Variable Efficiency with Soil Moisture) must be selected if the user wishes to use the SW GW Efficiency Split method in the Return Flow Split category.
Slots Specific to This Method
Efficiency
Type: Series Slot
Units: Fraction
Description: Amount of water depleted per unit water diverted for a given timestep
Information: Expressed as a fraction.
I/O: Output only
Links: Not linkable
Maximum Efficiency
Type: Table Slot
Units: Fraction
Description: Maximum amount of water depleted per unit water diverted by the Water User
Information: Expressed as a fraction.
I/O: Required input
Links: Not linkable
Return Flow
Type: Series Slot
Units: Flow
Description: Amount of diverted water not consumed by the Water User
Information:
I/O: Output only
Links: May be linked to the Return Flow slot on any object, or the Inflow slot on a Groundwater Storage object.
Method Details
The engineering algorithm associated with this method is given below.
The first step in this algorithm is to check that the Maximum Efficiency is greater than the ratio of Depletion Requested to Diversion Requested. If it is not, a RiverWare error is posted and the run terminates. If Depletion Requested is not known, it is assumed to be the same as Diversion Requested.
If the Maximum Efficiency is greater than the ratio of Depletion Requested to Diversion Requested, the Efficiency is calculated as follows:
If the calculated Efficiency is greater than the Maximum Efficiency, the Efficiency is reset to value of the Maximum Efficiency.
The Depletion is calculated as follows:
After the Depletion has been calculated, it is used to calculate the Return Flow.
Note: This method must be selected to use the Split Return Efficiency method in the Return Flow Split category.)
Variable Efficiency with Soil MoistureThe Variable Efficiency with Soil Moisture method computes the Return Flow based on an efficiency driven approach. Diverted water first meets Depletion Requested. If that is not sufficient, then Soil Moisture can be used to meet the Depletion Requested. This method also tracks the volume of water stored in the soil moisture.
This method is only available for standalone water users and water user elements that are part of a No Structure or Sequential structure Agg Diversion site. Also, you must select the Irrigation Requests with Soil Moisture method.
See Irrigation Requests with Soil Moisture for a diagram of the use of Soil Moisture slots.
Slots Specific to This Method
Efficiency
Type: Series Slot
Units: Fraction
Description: Amount of water depleted per unit water diverted for a given timestep
Information: Expressed as a fraction.
I/O: Output only
Links: Not linkable
Efficiency including Soil Moisture
Type: Series Slot
Units: Fraction
Description: Efficiency calculation that includes the flow to the soil moisture.
Information: It shows how water is used beneficially since water stored in soil moisture will be consumed later by the crop. This slot is only used for informational purposes and is not used in further calculations.
I/O: Output only
Links: Not linkable
Available Soil Moisture Fraction
Type: Series
Units: Fraction
Description: Fraction (decimal or percentage) of moisture available. This is computed as the moisture in the soil divided by the maximum soil moisture volume
Information: 
I/O: Output only
Links: Not linkable
Crop Usage
Type: Series
Units: Flow
Description: Amount of water used by the crops. This includes depletion of delivered water and soil moisture usage, but not supplemental flow.
Information:
I/O: Output only
Links: Not linkable
Crop Shortage
Type: Series
Units: Flow
Description: Amount of crop demand that is not met by either diversions or soil moisture.
Information:
I/O: Output only
Links: Not linkable
Consumptive Use from Delivered Flow
Type: Series Slot
Units: Flow
Description: Portion of the consumed water that came from the Diversion.
Information: This is identical to Depletion.
I/O: Output only
Links: Not linkable
Consumptive Use from Soil Moisture
Type: Series Slot
Units: Flow
Description: Portion of the consumed water that came from the soil moisture
Information:
I/O: Output only
Links: Not linkable
Delivered Flow
Type: Series Slot
Units: Flow
Description: Portion of the diverted flow that is delivered to the fields and is available for crops or to refill the soil moisture.
Information: This is computed as the Diversion minus the Incidental Loss.
I/O: Output only
Links: Not linkable
Diversion Request for Crop
Type: Series Slot
Units: Flow
Description: The requested diversion to meet crop depletion
Information: This is computed by the Irrigation Requests with Soil Moisture method; see Irrigation Requests with Soil Moisture.
I/O: Output only
Links: Not linkable
Incidental Loss
Type: Series Slot
Units: Flow
Description: Portion of the Diversion that is lost due to the incidental loss rate.
Information:
I/O: Output only
Links: Not linkable
Incidental Loss Rate
Type: Table Slot
Units: Fraction
Description: Losses in the water distribution system.
Information: Expressed as a fraction of the flow.
I/O: Optional; if not specified, zero is assumed.
Links: Not linkable
Irrigated Area
Type: Series Slot
Units: Area
Description: Surface area of the land to be irrigated
Information:
I/O: Specified according to the selected method in the Irrigation Acreage and Evapotranspiration Rates category; see Irrigation Acreage and Evapotranspiration Rates.
Links: Not linkable
Maximum Soil Moisture
Type: Scalar
Units: Length
Description: Depth of water that the soil can hold.
Information:
I/O: Required input
Links: Not linkable
Maximum Infiltration Rate
Type: Scalar
Units: Velocity (Length per Time)
Description: Maximum depth of water that can be absorbed by the soil
Information: A valid value must be specified or an error will occur at the beginning of run.
I/O: Required input
Links: Not linkable
Return Flow
Type: Series Slot
Units: FLOW
Description: Amount of diverted water not consumed by the Water User
Information:
I/O: Output only
Links: May be linked to flow slots on any object
Soil Moisture
Type: Series Slot
Units: Volume
Description: This slot tracks the volume of soil moisture water in the soil at each timestep in the run.
Information: For the initial timestep, this slot must be valid. If this slot is not input, it is computed as the Maximum Soil Moisture times the Irrigated Area at the initial timestep. That is, the soil moisture starts full.
I/O: Typically output for run timesteps.
Links: Not linkable
Soil Moisture Demand
Type: Series Slot
Units: Flow
Description: Flow necessary to refill the soil moisture to capacity.
Information:
I/O: Output only
Links: Not linkable
Soil Moisture Fill Efficiency
Type: Series Slot with Periodic Input
Units: Fraction
Description: Fraction of the flow applied for soil moisture refill that is not absorbed.
Information: For example, if you are only refilling the soil moisture, you may apply 100 cfs, but 5 cfs is not absorbed even though max infiltration indicated it could absorb 200cfs. The Soil Moisture Fill Efficiency would be 0.95.
I/O: Optional input. If not specified, it is assumed to be zero
Links: Not linkable
Soil Moisture Flow
Type: Series
Units: Flow
Description: Flow into or out of the soil moisture.
Information: A positive number indicates flow into the soil moisture storage. A negative number indicates the soil moisture is being used to meet crop demands.
I/O: Output only
Links: Not linkable
Surface Runoff
Type: Series
Units: Flow
Description: Water that runs off the field because the maximum infiltration rate has been exceeded. This water never enters the soil.
Information: If the Surface Runoff slot is linked to a slot on another object it is not included in the total Return Flow. If the Surface Runoff is not linked, the Surface Runoff is included as part of the total Return Flow.
I/O: Output only
Links: Linkable to another object’s Return Flow slot
Excess Effective Precipitation
Type: Series
Units: Velocity (length/time)
Description: Excess means effective precipitation that typically cannot be used by the crops. Normally, effective precipitation is subtracted from the crop demand to compute a net Depletion Requested. However, in some instances the effective precipitation is greater than what the crops could use within a given time step. Therefore, a value for excess effective precipitation would likely be input only when input Depletion Requested is zero (because the crop demand would have been met by the non-excess portion of the effective precipitation). Excess Effective Precipitation goes towards filling the soil moisture column until the available capacity is reached. Any remaining excess effective precipitation that cannot be stored in the soil moisture column is added to return flow
Information: This value is multiplied by the irrigated acreage to compute a flow.
I/O: Input or set by a rule. It is assumed to be zero if not input.
Links: Not linkable
Excess Effective Precipitation Flow
Type: Series
Units: Flow
Description: Flow rate that represents the excess effective precipitation
Information: Excess Effective Precipitation Flow = Excess Effective Precipitation * Irrigated Area
I/O: Output only
Links: Not linkable
Method Details
The Variable Efficiency with Soil Moisture Return Flow method proceeds as follows.
First, compute how much of diversion is lost due to conveyance or incidental loss.
Calculate how much of the diverted flow is delivered, as follows:
Calculate the Excess Effective Precipitation flow as follows:
Calculate the temporary variable flowToIrrigatedArea. This is the flow applied to the field:
Compute the maximum infiltration flow intermediate variable as follows:
;Compute the flow the is above the maxInfiltrationFlow. This water does not absorb and runs directly off the field’s surface.
Compute the flow that infiltrates to the soil, as follows:
The Efficiency is calculated as follows:
Thus, the Efficiency should always be between the Min and Max Efficiency, unless limited by max infiltration. If limited by max infiltration, the efficiency can be less than the minimum.
Compute the Depletion. Depletion is limited by the maxinfiltrationFlow as it cannot use more than is infiltrated.
Calculate the initial return flow intermediate value. This will occur for any diversion even when storing soil moisture. This water enters the soil but is not consumed.
Calculate the initial crop shortage as follows. This is the amount of the depletion requested that is not met by diverted water.
Calculate Soil Moisture Flow as follows:
IF (cropShortageInitial > 0)
This is negative indicating water is moving from the soil moisture to the crops.
ELSE
Extra water may have been diverted (if the demand factor was 0, or Diversion was specified). The above equation uses the Soil Moisture Demand, so it can refill the soil moisture, even if the demand factor is zero. When water is applied for soil moisture, there is some that is returned, according to the Soil Moisture Fill Efficiency.
END IF
Calculate final Return Flow as follows. If the Surface Runoff slot is linked to another object it is not included in the total Return Flow.
If this Surface Runoff is not linked, the Surface Runoff is included as part of the total Return Flow, as follows:
Calculate Soil Moisture and Available Fraction as follows:
Diversion Shortage:
Depletion Shortage:
Compute total crop usage and shortage, as follows:
Finally compute summary variables, as follows:
Pump Back Return FlowThe Pump Back Return Flow Method allows a percentage of return flow from a water user to be returned to the diverted water for reuse. This method requires that the user selects Impulse Response as a routing method and None in the Return Flow Split category.
Slots Specific to This Method
Return Flow
Type: Series Slot
Units: Flow
Description: Amount of diverted water that is not consumed by the user.
I/O: Output only
Links: May be linked to the Return Flow slot on any object, or the Inflow slot on a Groundwater Storage object.
Incidental Depletions
Type: Scalar Slot
Units: fraction
Description: Single value that describes the percentage of diverted water lost to seepage or leakage.
Information: Decimal Value
I/O: Required input
Links: Not linkable
Irrigated Area
Type: Series Slot
Units: Area
Description: Area of land that is irrigated by the user.
I/O: Required input
Links: Not linkable
Consumptive Irrigation Requirement
Type: Series Slot
Units: velocity
Description: Rate of water consumption per unit area of irrigated land.
I/O: Required input
Links: Not linkable
Application Efficiency
Type: Scalar Slot
Units: fraction
Description: Fraction of applied water that is consumed by the crops
I/O: Required input
Links: Not linkable
Pump Back Flow
Type: Series Slot
Units: Flow
Description: Amount of Routed Return Flow that is pumped back into use.
Information: This slot may be set by a rule.
I/O: Optional; if not input, Pump Back Flows are not considered
Links: Not Linked
Pump Back Depletion
Type: Scalar Slot
Units: Flow
Description: Table slot describing the percentage of Pump Back Flow that is depleted by the user.
I/O: Required input
Links: Not Linked
Pump Back Return
Type: Series Slot
Units: Flow
Description: Amount of water (from Pump Back Flow) that is available for return after Pump Back Depletions are considered.
I/O: Output only
Links: Not Linked
Project Efficiency
Type: Scalar Slot
Units: Fraction
Information: Expressed as a fraction
I/O: Required input
Links: Not Linked
Non Applied Water
Type: Series Slot
Units: Flow
Description: Unused diverted water
I/O: Input or Set by a Rule
Links: May be Linked
Method Details
The routing lag coefficient is checked first to be sure that the initial value has been set to zero. This prevents an iterative loop from occurring between Return Flow, Pump Back Return, and Routed Return Flow. The Project Efficiency is also checked to make sure it is not zero. This prevents division by zero when calculating the Non Applied Water. Non Applied Water can then be input or set by a rule.
If Non Applied Water is a value less than zero, it is set equal to zero. Otherwise the value calculated above is used.
Return flow is then computed as follows:
Next, Depletion is calculated as follows:
After Return Flow and Depletion are calculated, the method checks to see if there are pump back flows. If so, return flow from above is calculated with the following equations:
The Routed Return Flow is adjusted as follows:
Revised: 01/05/2024