While a reach element conceptually represents a segment of stream or river, optional modeling of interactions with the subsurface is performed by a Loss/Gain Method contained within the reach. A Loss/Gain Method represents losses from the channel, additions to the channel from groundwater, or bi-directional water movements depending on the specific implementation of a method. A total of three different Loss/Gain Methods are provided. Each method included in the program provides a different level of detail and not all methods are equally adept at representing a particular stream. Further, because of differing data requirements, some Loss/Gain Methods are only compatible with certain Routing Methods.

The Loss/Gain Method for a reach is selected on the Component Editor for the reach element. Access the Component Editor by clicking the reach element icon on the Components tab of the Watershed Explorer. You can also access the Component Editor by clicking on the element icon in the Basin Map, if the map is currently open. You can select a Loss/Gain Method from the list of three available choices. If you choose the None method, the reach will perform routing calculations without including any losses or gains to the channel. Use the selection list to choose the method you wish to use. Each reach may use a different method or several reaches may use the same method.

When a new reach is created, it is automatically set to use the default Loss/Gain Method specified in the Program Settings. You may change the Loss/Gain Method for a reach at any time using the Component Editor for the reach element. Since a reach can only use one Loss/Gain Method at a time, you will be warned when changing methods that the old parameter data will be lost. You can turn off this warning in the Program Settings. You can change the Loss/Gain Method for several reaches simultaneously. Click on the Parameters menu and select the Loss/Gain | Change Method command. The Loss/Gain Method you choose will be applied to the selected reaches in the Basin Model, or to all reaches if none are currently selected.

The parameters for each Loss/Gain Method are presented on a separate Component Editor from the Reach Element editor. The Loss/Gain editor is always shown next to the Routing editor. The information shown on the Loss/Gain editor will depend on which method is currently selected.

Constant Loss/Gain

The Constant Loss/Gain Method uses an empirical relationship to calculate channel loss using a fixed flow rate reduction and a ratio of the flow.

This method does not include the ability to represent gaining streams.

A fixed flow rate is subtracted from the routed flow and then the remainder is multiplied by a ratio. The reduced flow becomes the outflow for the reach. This method is compatible with all Routing Methods. The Component Editor is shown below.

The Flow Rate parameter specifies the amount of flow to be subtracted from the inflow. It may be zero or greater than zero; if the specified value is zero then no flow rate reduction will occur. If the specified value is greater than the inflow for a time step, then no outflow will result for that interval.

The Fraction is used to reduce the inflow in linear proportion to the flow rate. After the constant value is subtracted from the routed inflow, the remainder is multiplied by the value one minus the fraction. The fraction must be between zero and one.

Deficit Constant Loss/Gain

The Deficit Constant Loss/Gain Method is very similar to the subbasin Deficit and Constant Loss Method. This method allows for channel losses using an initial volume followed by a constant infiltration rate in combination with the inundated area in the reach. 

This method does not include the ability to represent gaining streams.

This method is only compatible with the Normal Depth, Modified Puls, Kinematic Wave, and Muskingum-Cunge Routing Methods. The inundated area is calculated differently depending on the Routing Method:

  • When used with the Modified Puls method, the optional Elevation-Discharge Function must be specified in the routing parameters. The current storage and outflow are combined with the elevation-discharge curve to compute inundated area.
  • When used with the Normal Depth, Kinematic Wave, and Muskingum-Cunge methods, the wetted perimeter is multiplied by the reach length in order to compute the inundated area. 

The Component Editor is shown below.

Deficit Constant Loss/Gain Component Editor

The Initial Deficit defines the volume of water that is be required to fill the soil layer at the start of the simulation while the Maximum Deficit specifies the total amount of water the soil layer can hold. The maximum deficit is typically defined using the product of the effective soil porosity and an assumed active layer depth, but it should be calibrated using observed data. 

The Percolation Rate defines the rate at which water will be infiltrated into the soil layer after the initial deficit has been satisfied. Typically, this parameter is equated with the saturated hydraulic conductivity of the soil.

Green Ampt Loss/Gain

The Green Ampt Loss/Gain Method is very similar to the subbasin Green and Ampt Loss Method. This method models channel losses using an exponentially decreasing function. Channel losses are assumed to be initially high and then decrease over time eventually equaling the saturated hydraulic conductivity. Similar to other methods, the inundated area in the reach is used to compute channel loss volumes.

This method does not include the ability to represent gaining streams.

This method is only compatible with the Normal Depth, Modified Puls, Kinematic Wave, and Muskingum-Cunge Routing Methods. The inundated area is calculated differently depending on the Routing Method:

  • When used with the Modified Puls method, the optional Elevation-Discharge Function must be specified in the routing parameters. The current storage and outflow are combined with the elevation-discharge curve to compute inundated area.
  • When used with the Normal Depth, Kinematic Wave, and Muskingum-Cunge methods, the wetted perimeter is multiplied by the reach length in order to compute the inundated area. 

The Component Editor is shown below.

Green Ampt Loss/Gain Component Editor

The Initial Deficit defines the saturation of the soil layer at the start of the simulation. This parameter must be specified as a volume ratio (i.e., porosity) and may be estimated in the same manner as the initial abstraction for other loss models.

The Wetting Front Suction describes the movement of water downwards through the soil column. 

The Hydraulic Conductivity defines the minimum rate at which water will be infiltrated into the soil layer after the soil column is fully saturated.

Percolation Loss/Gain

The Percolation Method uses a constant infiltration rate in combination with the inundated area in the reach to compute channel loss.

This method does not include the ability to represent gaining streams.

This method is only compatible with the Normal Depth, Modified Puls, Kinematic Wave, and Muskingum-Cunge Routing Methods. The inundated area is calculated differently depending on the Routing Method:

  • When used with the Modified Puls method, the optional Elevation-Discharge Function must be specified in the routing parameters. The current storage and outflow are combined with the elevation-discharge curve to compute inundated area.
  • When used with the Normal Depth, Kinematic Wave, and Muskingum-Cunge methods, the wetted perimeter is multiplied by the reach length in order to compute the inundated area. 

The Component Editor is shown below.

The Rate parameter specifies percolation in terms of a flow rate per area. The inundated area is multiplied by the rate to determine the channel loss for each time interval.