Diversion Salt Mass is set by multiplying the Diversion volume by the Diversion Salt Concentration. Then, if Salt Loading is valid,

Otherwise:

Return Flow Salt Concentration is computed from the Return Flow Salt Mass and Return Flow volume.

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:

If both of the local variables concentrated and ReturnFlowVol are less than zero, then Return Flow Salt Concentration equals the following:

Else, Return Flow Salt Concentration equals the following:

At the end of this method, Return Flow Salt Mass is calculated as follows:

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:

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:

If Return Flow does not equal zero, Return Flow Salinity Pickup Conc is calculated as follows:

Then, Return Flow Salt Mass is calculated as follows:

Return Flow Salt Concentration is calculated as follows:

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.

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):

Figure 16.2 shows the salt mass components.

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:

Next, compute the Delivered Flow Salt Mass, as follows:

Recompute the temporary variable flowToIrrigatedArea from slot values, as follows:

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:

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:

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.

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.

We assume this runoff does not pick up any salt, but instead has the concentration of the applied water.

If Surface Runoff is linked, the Surface Runoff is not included in the total Return Flow. Therefore, Return Flow Salt Concentration is as follows:

Compute the mass balance as follows:

Soil Moisture Salt Concentration is the mass divided by the total volume

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:

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.

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.

If Supplemental Runoff is linked, the Supplemental Runoff is not included in the total Supplemental Return Flow. Therefore, Supplemental Return Flow Salt Concentration is:

The Total Return Flow Salt Concentration is computed as follows:

Compute the mass balance, as follows:

Soil Moisture Salt Concentration is the mass divided by the total volume, as follows: