Selecting a Baseflow Method

While a subbasin element conceptually represents infiltration, surface runoff, and subsurface processes interacting together, the actual subsurface calculations are performed by a Baseflow Method contained within the subbasin. A total of six different Baseflow Methods are provided. Some of the methods are designed primarily for simulating events while others are intended for continuous simulation.

The Baseflow Method for a subbasin is selected on the Component Editor for the subbasin element as shown in the following figure. Access the Component Editor by clicking the subbasin 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 Baseflow Method from the list of six available choices. If you choose the None method, the subbasin will not compute baseflow and the outflow will only include direct runoff from the Transform Method. Use the selection list to choose the method you wish to use. Each subbasin may use a different method or several subbasins may use the same method.

When a new subbasin is created, it is automatically set to use the default Baseflow Method specified in the Program Settings. You may change the Baseflow Method for a subbasin at any time using the Component Editor for the subbasin element. Since a subbasin can only use one Baseflow 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 Baseflow Method for several subbasins simultaneously. Click on the Parameters menu and select the Baseflow | Change Method command. The Baseflow Method you choose will be applied to the selected subbasins in the Basin Model, or to all subbasins if none are currently selected.

The parameters for each Baseflow Method are presented on a separate Component Editor from the Subbasin Element editor. The Baseflow editor is always shown near the Transform editor. The information shown on the baseflow editor will depend on which method is currently selected.

Bounded Recession Baseflow

The Bounded Recession Baseflow Method is intended primarily for real-time forecasting operations. The method is very similar to the Recession Method, and like it, does not conserve mass in the subbasin. The principal difference is that monthly baseflow limits can be specified. The baseflow is computed according to the Recession Methodology and then the monthly limits are imposed. Though there are many similarities with the Recession Method, one important difference is that this method does not reset the baseflow after a storm event. The Component Editor is shown below.

The Initial Baseflow at the beginning of a simulation must be specified. Two methods are available for specifying the initial condition: Initial Discharge and Initial Discharge Per Area. Using the first method, you must specify the Initial Baseflow as a discharge with units of volume per time. This method is particularly good when there is observed streamflow data at the outlet of the subbasin for determining the initial flow in the channel. In the second method you specify the Initial Baseflow as a volume per area per time. This method is better suited when general guidelines for watershed yield must be used to estimate the initial flow.

The Recession Constant describes the rate at which baseflow recedes between storm events. It is defined as the ratio of baseflow at the current time, to the baseflow one day earlier.

A Baseflow Value must be entered for the month of January. Likewise, a value must also be entered for each of the remaining months from February to December. This value is used to limit the computed baseflow.

Constant Monthly Baseflow

The Constant Monthly Baseflow Method allows the specification of a constant baseflow for each month of the year. It does not conserve mass within the subbasin. It is intended primarily for continuous simulation in subbasins where the baseflow is nicely approximated by a constant flow for each month. This Baseflow Method can be combined with the 2D Diffusion Wave Transform Method.  The Component Editor is shown below.

A Baseflow Value must be entered for the month of January. Likewise, a value must also be entered for each of the remaining months from February to December.

Linear Reservoir Baseflow

The Linear Reservoir Baseflow Method, as its name implies, uses a linear reservoir to model the recession of baseflow after a storm event. It is the only Baseflow Method that conserves mass within the subbasin. Infiltration or percolation computed by the Loss Method is connected as the inflow to the linear reservoirs. It can be used with one, two, or three reservoirs. Partition fractions are used to split the inflow to each of the reservoirs. The inflow is multiplied by the partition fraction to determine the amount of inflow going to each reservoir. The sum of the partition fractions must be less than or equal to one.  If the sum of the fractions is less than one, the remaining percolated water is considered as aquifer recharge. If the sum of the fractions is exactly equal to one, then all percolation will become baseflow and there will be no aquifer recharge. All losses from the SCS Curve Number, Exponential, Initial and Constant, Green and Ampt, and Smith-Parlange Loss Methods are routed to the Linear Reservoir Baseflow model. Only percolation losses (during saturated conditions) in the Soil Moisture Accounting, Layered Green and Ampt, and Deficit and Constant Loss Methods are routed to the Linear Reservoir Baseflow model. 

Using the Linear Reservoir Baseflow Method with the Soil Moisture Accounting or Gridded Soil Moisture Accounting Loss Methods results in special behavior. In this case, the number of baseflow reservoirs should be consistent with the number of groundwater layers in the Loss Method. The lateral outflow from upper groundwater is connected as the inflow to baseflow reservoir 1. The lateral outflow from lower groundwater is connected as the inflow to baseflow reservoir 2. The percolation out of lower groundwater is connected as inflow to baseflow reservoir 3. Partition fractions are not used for baseflow reservoirs 1 and 2 because their inflow is determined by the respective lateral outflow. A partition fraction should be used with the percolation in order to define the split between aquifer recharge and inflow to baseflow reservoir 3.  The Component Editor is shown below.

The Linear Reservoir Baseflow Method can be combined with the 2D Diffusion Wave Transform Method. When it is, the Flow Type can be either Interflow or Baseflow. If the user chooses the Baseflow flow type, then the routed subsurface water is added to the total flow at the subbasin outlet (outlet(s) of the 2D area). If the user chooses the Interflow flow type, then the subsurface water is only routed within the 2D cell, where the infiltration happened. The routed interflow is added to the cell's surface flow and routed on the 2D surface to downstream grid cells (where it can be infiltrated again). The new Interflow Linear Reservoir Baseflow option can only be activated when the 2D Diffusion Wave Transform option is selected.

The Number of Reservoirs (number of groundwater layers) must be specified. The minimum is one and the maximum is three. Each groundwater layer requires separate initial conditions and parameters. 

The Initial Baseflow at the beginning of a simulation must be specified for each groundwater layer. Two methods are available for specifying the initial condition: Initial Discharge and Initial Discharge Per Area. Using the first method, you must specify the initial baseflow as a discharge with units of volume per time. This method is particularly good when there is observed streamflow data at the outlet of the subbasin for determining the initial flow in the channel. In the second method you specify the initial baseflow as a volume per area per time. This method is better suited when general guidelines for watershed yield must be used to estimate the initial flow. The same method must be used for specifying the initial condition for all groundwater layers.

The Fraction determines how water from the Loss Method is split to the groundwater layers. Each fraction must be greater than zero and less than or equal one. When the sum of the fractions is exactly one then there will be no aquifer recharge. When the sum of the fractions is less than one, the remainder of the percolation becomes aquifer recharge.

The Groundwater Storage Coefficient is the time constant for each groundwater layer (linear reservoir coefficient). Because it is measured in hours, it gives a sense of the response time for a component of subsurface flow within a subbasin.

The Number of Steps can be used to subdivide the routing through a reservoir and is related to the amount of attenuation during the routing. Minimum attenuation is achieved when only one routing step is selected. Attenuation of the baseflow increases as the number of steps increases.

Nonlinear Boussinesq Baseflow

The Nonlinear Boussinesq Baseflow Method is designed to approximate the typical behavior observed in watersheds when channel flow recedes after an event. It is similar to the Recession Baseflow Method, but by assuming an unconfined groundwater layer and invoking the Boussinesq assumptions, it is possible to parameterize the method using measurable field data. This method is intended primarily for event simulation. However, it does have the ability to automatically reset after each storm event and consequently may be used for continuous simulation. It does not conserve mass within the subbasin. The Component Editor is shown in the following figure.

The Initial Baseflow at the beginning of a simulation must be specified. Two methods are available for specifying the initial condition: Initial Discharge and Initial Discharge Per Area. Using the first method, you must specify the initial baseflow as a discharge with units of volume per time. This method is particularly good when there is observed streamflow data at the outlet of the subbasin for determining the initial flow in the channel. In the second method you specify the initial baseflow as a volume per area per time. This method is better suited when general guidelines for watershed yield must be used to estimate the initial flow.

There are two different methods for determining how to reset the baseflow during a storm event: Ratio to Peak and Threshold Flow. When using the Ratio to Peak method, you must specify the flow ratio to the peak. The baseflow is reset when the current flow divided by the peak flow is falls to the specified value. For example, if a ratio of 0.2 is selected, the baseflow will be reset on the receding limb of an event hydrograph when the flow has decreased to 20% of the event peak flow. With the Threshold Flow method, the baseflow is always reset when the receding limb of the hydrograph falls to a specified flow value, regardless of the peak flow during the previous storm event.

The Characteristic Subsurface Flow Length must be specified. This could be estimated as the mean distance from the subbasin boundary to the stream.

The Conductivity of the soil must be specified. This could be estimated from field tests or from the soil texture.

The Drainable Porosity must be specified in terms of volume ratio. The upper limit would be the total porosity minus the residual porosity. The actual drainable porosity depends on local conditions.

Recession Baseflow

The Recession Baseflow Method is designed to approximate the typical behavior observed in watersheds when channel flow recedes exponentially after an event. This method is intended primarily for event simulation. However, it does have the ability to automatically reset after each storm event and consequently may be used for continuous simulation. It does not conserve mass within the subbasin. The Component Editor is shown in the following figure.

The Initial Baseflow at the beginning of a simulation must be specified. Two methods are available for specifying the initial condition: Initial Discharge and Initial Discharge Per Area. Using the first method, you must specify the initial baseflow as a discharge with units of volume per time. This method is particularly good when there is observed streamflow data at the outlet of the subbasin for determining the initial flow in the channel. In the second method you specify the initial baseflow as a volume per area per time. This method is better suited when general guidelines for watershed yield must be used to estimate the initial flow.

The Recession Constant describes the rate at which baseflow recedes between storm events. It is defined as the ratio of baseflow at the current time, to the baseflow one day earlier.

There are two different methods for determining how to reset the baseflow during a storm event: Ratio to Peak and Threshold Flow. When using the Ratio to Peak method, you must specify the flow ratio to the peak. The baseflow is reset when the current flow divided by the peak flow is falls to the specified value. For example, if a ratio of 0.2 is selected, the baseflow will be reset on the receding limb of an event hydrograph when the flow has decreased to 20% of the event peak flow. With the Threshold Flow method, the baseflow is always reset when the receding limb of the hydrograph falls to a specified flow value, regardless of the peak flow during the previous storm event.