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Water User Water Quality
Water Quality
Water User Water Quality
The Water User currently models only salinity.
Water Users have water quality methods to model salt as either a stand alone water user or as an element of a “No Structure” or “Sequential” Agg Diversion site. Water Users on “Lumped” Agg diversions sites do not model salt as salt is lumped and modeled by the aggregate.
User Methods
Water User Water Quality
This category is available on the stand alone, no structure, or sequential water user.
* None
No Water Quality is modeled. No slots are added.
* Salinity
All of the following salinity slots are added.
Note:  The slots are technically added by a non-visible method that indicates whether the water user is stand alone, lumped or sequential. Therefore, the methods tab on the Open Object dialog does not show the slots, but the Slots tab does.
Note:  It is an error if a Return Flow Routing method other than “None” is selected and salinity is modeled. In other words, you cannot route return flows and model salinity on the same water user.
Diversion Salt Concentration
Type: Series Slot
Units: Concentration
Description: Salt concentration associated with the Diversion
I/O: Input, set by a rule, or propagated via a link
Diversion Salt Mass
Type: Series Slot
Units: Mass
Description: Salt mass associated with the Diversion
I/O: Output only
Return Flow Salt Mass
Type: Series Slot
Units: Mass
Description: Salt mass associated with the Return Flow
I/O: Output only: solved by dispatch and user methods
Return Flow Salt Concentration
Type: Series Slot
Units: Concentration
Description: Salt concentration associated with the Return Flow
I/O: Output only: solved by dispatch and user methods
Salt Storage and Loading
This category is available on the stand alone, no structure, or sequential water user.
* None
No salt is stored in the water user. No additional slots are added. This method is available for all Return Flow methods.
Diversion Salt Mass is set by multiplying the Diversion volume by the Diversion Salt Concentration.
(16.1)    
Return Flow Salt Concentration is computed from the Return Flow Salt Mass and Return Flow volume.
* Salt Loading
No salt is stored in the water user. No additional slots are added. This method is available for all Return Flow methods.
Salt Loading
Type: Series Slot
Units: Mass
Description: The additional salinity that is to be added to the return flow.
Information: This represents dry salt that is added possibly due to fertilizer or other salt additions.
I/O: Input or set by a rule
Diversion Salt Mass is set by multiplying the Diversion volume by the Diversion Salt Concentration. Then, if Salt Loading is valid,
(16.2)    
Otherwise:
(16.3)    
Return Flow Salt Concentration is computed from the Return Flow Salt Mass and Return Flow volume.
* Salt Pickup Concentration
This method determines the Return Flow Salt Mass as a function of Diversion Salt Concentration, Return Flow Pickup and Return Flow Volume. The following slots are added by this method.
RETURN FLOW SALINITY PICKUP CONC
Type: Series Slot
Units: CONCENTRATION
Description: The additional salinity that is to be added to the return flow.
Information: Optional input.
I/O: Not Linkable
The method first converts Return Flow and Diversion from a flow to a volume (ReturnFlowVol and DiversionVol).
Next, the local variable concentrated is computed as follows:
(16.4)    
Note:  If ReturnFlowVol is zero, concentrated is set to zero.
If both of the local variables concentrated and ReturnFlowVol are less than zero, then Return Flow Salt Concentration equals the following:
(16.5)    
Else, Return Flow Salt Concentration equals the following:
(16.6)    
At the end of this method, Return Flow Salt Mass is calculated as follows:
(16.7)    
* Salt Pickup Mass
This method determines the Return Flow Salt Mass as a function of Diversion Salt Concentration, Return Flow Salinity Pickup Mass and Return Flow Volume. The user specified salt mass can be negative or positive and can add salt even when Return Flow equals zero.
Slots Specific to This Method
Return Flow Salinity Pickup Mass
Type: SeriesSlot
Units: Mass
Description: slot for salinity pickup mass
Information: user specified salinity pickup mass is entered in the slot.
I/O: Required Input
Return Flow Salinity Pickup Conc
Type: Series Slot
Units: CONCENTRATION
Description: The additional salinity that is to be added to the return flow.
Information: Output only.
I/O: Not Linkable
Note:  In the Agg Diversion Site method of the same name, there are two additional slots: Annual Salinity Pickup Mass and Annual Return Flow Volume. Neither of these slots are used or accessed by the method. These are not added to the Water User by this method. You could add these as custom slots if you want to use them for comparison.
The method first converts Return Flow and Diversion from a flow to a volume (ReturnFlowVol and DiversionVol). Next, the local variable concentrated is computed as follows:
(16.8)    
Note:  If ReturnFlowVol is zero, concentrated is set to zero.
The Salt Pickup Mass method next checks if Return Flow Salinity Pickup Mass is valid. If it is not valid, the run will abort and an error will be posted. If Return Flow equals zero, Return Flow Salinity Pickup and Return Flow Salt Concentration is set equal to zero and Return Flow Salt Mass is calculated as follows:
(16.9)    
If Return Flow does not equal zero, Return Flow Salinity Pickup Conc is calculated as follows:
(16.10)    
Then, Return Flow Salt Mass is calculated as follows:
(16.11)    
Return Flow Salt Concentration is calculated as follows:
(16.12)    
* Soil Moisture Salt Storage
When Soil Moisture methods are selected (see “Irrigation Requests with Soil Moisture” in Objects and Methods), water quality is enabled, the Water User Water Quality method is set to Salinity, and the water user is either a Sequential, No Structure, or Stand Alone water user, the Soil Moisture Salt Storage is available.
This method selection is necessary on both stand alone water users and on those that are elements of an aggregate diversion site as the method is only valid if soil moisture is being modeled.
Slots Specific to This Method
Delivered Flow Salt Concentration
Type: Series
Units: Concentration
Description: The salinity concentration of the delivered flow. This is the concentration after incidental loss.
Information:  
I/O: Output only
Delivered Flow Salt Mass
Type: Series
Units: Mass
Description: The salinity mass of the delivered flow. This is the salt mass after incidental loss.
Information:  
I/O: Output only
Incidental Loss Salt Concentration
Type: Series
Units: Concentration
Description: The salt concentration of water lost incidentally.
Information: When this slot is valid, the value is used. For example, the user could set this to zero, indicating the loss is evaporation and the salt remains. Otherwise, Incidental Loss Salt Concentration is assumed to be the same as Diverted Flow Salt Concentration indicating the salt disappears too, as would happen with seepage.
I/O: Optional Input
Soil Moisture Gain Loss Salt Concentration
Type: Series
Units: Concentration
Description: The concentration of Soil Moisture Gain Loss water.
Information: If the Soil Moisture Gain Loss is positive, water is entering the soil moisture. This slot can be specified via input or rule. If not specified, it defaults to zero. If the Soil Moisture Gain Loss is negative, water is leaving the soil moisture. In this case the concentration is set to the previous Soil Moisture Salt Concentration.
I/O: Optional Input, set by a rule, or computed
Soil Moisture Gain Loss Salt Mass
Type: Series
Units: Mass
Description: The mass of salt moving with the Soil Moisture Gain Loss
Information:  
I/O: Output only
Soil Moisture Salt Concentration
Type: Series
Units: Concentration
Description: The concentration of the entire soil moisture including the portion that is unavailable
Information: It is assume the Soil Moisture is well mixed
I/O: Output only, except initial timestep must be input
Soil Moisture Salt Mass
Type: Series
Units: Mass
Description: The mass of salt in the entire soil moisture.
Information:  
I/O: Output only
Soil Moisture Unavailable to Crops
Type: Scalar
Units: Length
Description: The depth of water that is in the soil that is not available for the crops but contributes to the salinity concentration.
Information:  
I/O: Required Input
Soil Moisture Unavailable Gain Loss
Type: Series
Units: Volume
Description: Miscellaneous gain or loss of unavailable soil moisture volume during the timestep due to irrigated area changes.
Information: This slot represents and gains (positive) or loss (negative) from the unavailable soil moisture. It is used to track the soil moisture volume gained or lost when the irrigated acreage is changed from the previous timestep. It is a computed output, assuming the current unavailable zone is full.
I/O: Output only
Soil Moisture Unavailable Gain Loss Salt Mass
Type: Series
Units: Mass
Description: The salt mass associated with the Soil Moisture Unavailable Gain Loss
Information:  
I/O: Output Only
Soil Moisture Unavailable Gain Loss Salt Concentration
Type: Series
Units: Concentration
Description: The salt concentration associated with the Soil Moisture Unavailable Gain Loss
Information: If the Soil Moisture Unavailable Gain Loss is positive, water is entering the soil moisture. This slot can be specified via input or rule to indicate the concentration of gains when the Irrigated Area increases. If not specified, it defaults to the previous timestep’s Soil moisture Salt Conc.
I/O: Input, set by a rule, or computed
Surface Runoff Salt Concentration
Type: Series
Units: Concentration
Description: The concentration of the Surface Runoff
Information:  
I/O: Output only
Surface Runoff Salt Mass
Type: Series
Units: Mass
Description: The mass of salt associated with the Surface Runoff slot
Information:  
I/O: Output only
Method Details
Figure 16.1 shows the water balance necessary to model salt on the Water User. Items in blue are default flow values. Pink indicates optional gains/losses. Green is supplemental water (see the method “Soil Moisture Salt with Supplemental Flow” for more on supplemental flow). Brown indicates split return flows.
Note:  Not all values shown are separate slot values.
Figure 16.1  Water balance to model salt
When dealing with salinity concentrations, an additional term is included that is not part of the Soil Moisture. The Soil Moisture Unavailable to Crops is a depth of water that contributes to salinity concentrations but does not contribute to soil moisture volume.
If the Irrigated Area changes, the unavailable volume changes, so must be computed. If the current Irrigated Area does not equal the Irrigated Area(t‑1):
(16.13)    
Figure 16.2 shows the salt mass components.
Note:  Depletion and Excess Effective Precipitation are grayed out as they do not contribute or remove salt mass. In addition, the Soil Moisture and Soil Moisture Unavailable to Crops have been combined into one well mixed volume. The dotted line is the boundary used to compute the salt mass balance on the soil, see Equation 16.30.
Figure 16.2  Salt balance
If Incidental Loss Salt Concentration is valid, use that value. For example, the user could set this to zero, indicating the loss is evaporation and the salt remains. Otherwise, Incidental Loss Salt Concentration is unknown, so set it equal to Diversion Salt Concentration, as follows:
(16.14)    
Next, compute the Delivered Flow Salt Mass, as follows:
(16.15)    
Recompute the temporary variable flowToIrrigatedArea from slot values, as follows:
(16.16)    
(16.17)    
(16.18)    
Soil Moisture Gain Loss Salt
Next, compute Soil Moisture Gain Loss Salt Mass. Remember that Soil Moisture Gain Loss is a term that can be specified by the user as a miscellaneous addition or subtraction from the soil moisture. In addition, it is used to track the change in Soil Moisture volume if the irrigated area changes from one timestep to the next. As a result, the salt calculations must mirror these uses.
If Soil Moisture Gain Loss is positive, water is entering the soil moisture. The Soil Moisture Gain Loss Concentration can be specified or it is assumed to be zero.
Otherwise, the Soil Moisture Gain Loss is negative; water is removed from the soil moisture, The Soil Moisture Gain Loss Concentration is assumed to be the concentration of the soil moisture at the previous timestep, as follows:
(16.19)    
The Soil Moisture Unavailable Gain Loss Concentration can be specified via rule or input. Otherwise, the Soil Moisture Unavailable Gain Loss Concentration is assumed to be the concentration of the soil moisture at the previous timestep, as follows:
(16.20)    
Compute the Soil Moisture Gain Loss Salt Mass as follows. This could be positive or negative depending on the sign of the gains or losses.
(16.21)    
(16.22)    
Return Flow Salt
Next, compute the Return Flow salt mass and concentration. First, the dispatch method will now compute any flows that are greater than the max infiltration rate. This water runs off directly without entering the soil.
(16.23)    
We assume this runoff does not pick up any salt, but instead has the concentration of the applied water.
(16.24)    
(16.25)    
If Surface Runoff is linked, the Surface Runoff is not included in the total Return Flow. Therefore, Return Flow Salt Concentration is as follows:
(16.26)    
(16.27)    
Otherwise, Surface Runoff is not linked and it is included in the total Return Flow. Therefore,
(16.28)    
(16.29)    
Salt Mass Balance
Compute the mass balance as follows:
(16.30)    
Soil Moisture Salt Concentration is the mass divided by the total volume
(16.31)    
* Soil Moisture Salt with Supplemental Flow
This method is the same as the Soil Moisture Salt Storage method (see “Soil Moisture Salt Storage”), in terms of slots added and calculations, but also accounts for Supplemental Flow and the corresponding salt concentrations.
This method must be selected when using the Supplemental Diversion including Soil Moisture and you wish to model salt storage in the soil moisture. It is usually auto-selected but if you select methods out of standard order, it can get into a state where you must select it yourself.
Slots Specific to This Method
Supplemental Flow Salt Concentration
Type: Series
Units: Concentration
Description: The concentration of Supplemental Flow water
Information:  
I/O: Input, set by a rule or more likely linked to a groundwater object
Supplemental Flow Salt Mass
Type: Series
Units: Mass
Description: The salt mass associated with the Supplemental Flow
Information:  
I/O: Output only
Supplemental Return Flow Salt Conc
Type: Series
Units: Concentration
Description: The salinity concentration of the Supplemental Return Flow
Information:  
I/O: Output only
Supplemental Return Flow Salt Mass
Type: Series
Units: Mass
Description: The salt mass of the Supplemental return flow.
Information:  
I/O: Output only
Supplemental Runoff Salt Conc
Type: Series
Units: Concentration
Description: The salt concentration of supplemental runoff (supplemental water applied that is larger than max infiltration)
Information:  
I/O: Output Only
Supplemental Runoff Salt Mass
Type: Series
Units: Mass
Description: The salt mass of supplemental runoff.
Information:  
I/O: Output only
Total Return Flow Salt Concentration
Type: Series
Units: Concentration
Description: The salinity concentration of the Total Return Flow. This could include both Return Flow Salt and Supplemental Return Flow Salt, depending on the linking.
Information: When this slot is linked, the Outgoing Salt Concentration (on a sequential element) does not include the salt from the Total Return Flow. Because it is linked, it is assumed the flow/salt goes elsewhere and not to the next element.
I/O: Output Only
Method Details
This method is the same as the Soil Moisture Salt Storage method (see “Soil Moisture Salt Storage”), in terms of slots added and calculations up through Equation 16.29, where Return Flow Salt Concentration is computed. This method adds in the supplemental salt as described below:
Supplemental Salt
When Supplemental water is in use, compute Supplemental Salt. Supplemental Flow Salt Concentration must be specified or linked, with the value provided by the other object.
(16.32)    
Also, compute Supplemental Return Flow salt mass and concentration. First, the water dispatch method will now compute any supplemental flows that are greater than the max infiltration rate.
(16.33)    
(16.34)    
If Supplemental Runoff is linked, the Supplemental Runoff is not included in the total Supplemental Return Flow. Therefore, Supplemental Return Flow Salt Concentration is:
(16.35)    
(16.36)    
Otherwise, Supplemental Runoff is not linked and it is included in the total Supplemental Return Flow. Therefore,
(16.37)    
(16.38)    
The Total Return Flow Salt Concentration is computed as follows:
(16.39)    
Salt Mass Balance
Compute the mass balance, as follows:
(16.40)    
Soil Moisture Salt Concentration is the mass divided by the total volume, as follows:
(16.41)    
Dispatch Methods
Following are the available dispatch methods for salinity.
* Solve Return Flow Salt Conc Sequential
This dispatch method is available for the Salinity method when the water user is on a Sequential Agg Diversion Site. This method computes the Return Flow Salt Concentration (and Mass) and Outgoing Salt Concentration.
Required Known slots
• Diversion Salt Concentration
• Return Flow
• Diversion
• Outgoing Available Water
Required Unknown slots
• Return Flow Salt Concentration
• Outgoing Salt Concentration
Method Details
First, execute the selected method in the Salt Storage category. This computes Return Flow Salt Mass and Concentration.
Next, the portion of the outgoing water that is from Return Flow is computed, called outgoingFromRF. The purpose is to determine which water has been used and which has not been used as “used” water has the Return Flow Salt Concentration while “unused” water has the Diversion Salt Concentration. This computation depends on the selected Return Flow Split method as described on the Water User documentation and the linking of various slots. Basically, if the return flow slot (Surface Return Flow, Return Flow, or Routed Return Flow) is linked, it is going elsewhere and not available to the next element. If it is not linked, it is considered outgoingFromRF. Thus:
(16.42)    
(16.43)    
If using Soil Moisture Salt with Supplemental Flow, the equation is as follows:
(16.44)    
A variable with “Vol” appended is the same variable converted to a volume. Finally, Outgoing Salt Concentration is computed based on the Outgoing Salt Mass and Outgoing Water Available.
* Solve Return Flow Salt Conc
This dispatch method is available for the Salinity method when the water user is stand alone or on a “No Structure” Agg Diversion Site. This method computes the Return Flow Salt Concentration (and Mass).
Required Known slots
• Diversion Salt Concentration
• Return Flow
• Diversion
Required Unknown slots
• Return Flow Salt Concentration
Method Details
Execute the selected method in the Salt Storage category. This method computes the Return Flow Salt Concentration (and Mass).
 
Revised: 06/03/2019