Groundwater Outflow

This method category is dependent upon the selection of the Single Computed Outflow in the Solution Type category. The user-selectable methods in this category are used to calculate Storage and Outflow from the Groundwater object based on a given Inflow.

None

This is the default user method for the Groundwater Outflow category. It performs no calculations. If it is selected, RiverWare gives an error because it is unable to calculate either an Outflow from or a Storage for the Groundwater Storage object.

Table Flow

The Table Flow method calculates the outflow based on the previous timestep’s Storage and a table input by the user.

Outflow is calculated by a linear interpolation of the Storage Outflow Table, using the Storage calculated at the previous timestep.

Slots Specific to This Method

Storage Outflow Table

Type: TableSlot

Units: VOLUME vs. FLOW

Description: outflow for any given volume of storage

Information:

I/O: Required input

Links: Not linkable

Linear Flow

The Linear Flow method calculates Outflow as a linear function of the previous timestep’s Storage value.

Outflow is calculated by the empirical equation:

Slots Specific to This Method

GW Alpha Param

Type: TableSlot

Units: PER TIME

Description: an empirical parameter used in the Outflow equation

Information:

I/O: Required input

Links: Not linkable

Current Storage Linear Flow

The Current Storage Linear Flow method calculates Outflow as a linear function of the Average Storage at the current timestep.

Slots Specific to This Method

Average Storage

Type: SeriesSlot

Units: VOLUME

Description: the average storage over the timestep

I/O: Output only

Links: Usually not linked

GW Alpha Param

Type: TableSlot

Units: PER TIME

Description: an empirical parameter used in the Outflow equation

I/O: Required input

Links: Not linkable

Method Details

At each timestep, the method first solves for instantaneous storage, then Average Storage, and then Outflow. The equations are as follows.

Lagged Linear Flow

The Lagged Linear Flow method calculates Outflow linearly as a function of the alpha coefficient times some previous storage value.

Slots Specific to This Method

GW Alpha Param

Type: TableSlot

Units: PER TIME

Description: an empirical parameter used in the Outflow equation

I/O: Required input

Links: Not linkable

Lag Time

Type: TableSlot

Units: TIME

Description: the amount of time lag between a Storage and its corresponding Outflow

I/O: Required input

Links: Not linkable

Method Details

Outflow is computed as follows:

Depending on the length of the lag time, one or more storage values may be required prior to the required initial storage, to compute the first outflows. If these values are not input by the user, the method assumes that storage has been constant and uses the initial storage value, which is always required user input.

This method uses instantaneous end-of-timestep storage values.

Exponential Flow

The Exponential Flow method calculates Outflow as an exponential function of the previous timestep’s Storage value.

Note: This calculation is carried out in user units. The value set in the GW Beta Param slot, as set by the user, must be consistent with the user units for the Storage slot. Changing the user units on the Storage slot will produce a different result. There will not be an automatic conversion of the GW Beta Param value.

Slots Specific to This Method

GW Alpha Param

Type: TableSlot

Units: PER TIME

Description: an empirical parameter used in the Outflow equation

Information:

I/O: Required input

Links: Not linkable

GW Beta Param

Type: TableSlot

Units: NONE

Description: an empirical parameter used in the Outflow equation

Information: This value must be consistent with the user units for the Storage slot. If the user units on the Storage slot are changed, the value in this slot must be changed manually to correspond to the new units. Otherwise different results will be produced.

I/O: Required input

Links: Not linkable

Method Details

Outflow is calculated by the empirical equation:

The exponential term is computed in user units. That is, Storage(t‑1) is converted to user units, raised to the power of GW Beta Param, and then converted back to the standard RiverWare units before being multiplied by GW Alpha Param).

Impulse Response

The Impulse Response method calculates Outflow using a set of impulse response outflow coefficients.

Slots Specific to This Method

Impulse

Type: SeriesSlot

Units: FLOW

Description: the impulse is the inflow minus pumped flow

I/O: Output only

Links: Usually not linked

Number of Outflow Coeffs

Type: TableSlot

Units: NONE

Description: the number of impulse response coefficients to use

Information: This single integer value is the number of values that need to be entered in the Outflow Coeffs slot.

I/O: Required input

Links: Not linkable

Outflow Coeffs

Type: TableSlot

Units: NONE

Description: the impulse response coefficients

Information: The same number of coefficients must be input as the value in the Number of Outflow Coeffs slot. The input will be in rows.

I/O: Required input

Links: Not linkable

Method Details

This method first calculates the impulse to groundwater as a function of inflow and pumped flow:

Outflow is computed as a function of current and previous impulse values and the outflow coefficients:

Revised: 01/04/2021