Defining Initial/Boundary Conditions for an External Pathway

There are two ways by which species mass can enter an External pathway:

• Through a mass flux link from another pathway to the pathway; and/or

• By defining an initial condition and/or a boundary condition (in the form of a cumulative input) for the pathway.

In this section, we discuss defining an initial and/or boundary condition for an External pathway.

An initial condition/boundary condition for an External Pathway is specified using the drop-list toward the bottom of the External Pathway dialog. This drop-list provides three options:

• Initial Inventory. This is the default, and represents the initial inventory of each species. This field only accepts vectors by species with dimensions of mass. Moreover, this input cannot be specified as a function of time (if it is, GoldSim will display a fatal error when you try to run the model).

• Cumulative Input. This option provides a mechanism for simultaneously specifying an initial condition and a rate of mass addition (that may change with time). This field only accepts vectors by species with dimensions of mass. It represents the cumulative amount of mass of each species added to the External Pathway at a given time. Hence, if it is constant, it represents an initial condition. If it increases with time, its rate of change represents a specified rate of addition. Correct use of the "Cumulative Input" is summarized in the table below:

To specify this:	Enter the following into the "Cumulative Input" field:
An initial condition	A constant vector with dimensions of mass (in this case, however, it would be more transparent to use the “Initial Inventory” option).
An initial condition and a constant rate of addition	An expression such as: Initial + Rate Etime*, where Initial is a constant vector (with dimensions of mass) and Rate is a constant vector (with dimensions of mass/time).
An initial condition and a time-variable rate of addition	The output of a Pool, Reservoir or Integrator element (with output dimensions of mass) with a specified Initial Value and (time-variable) Rate of Change.

Warning: The Cumulative Input must stay constant or increase with elapsed time. That is, you cannot remove mass from an External Pathway using the Cumulative Input field. If the value ever begins to decrease with time, a fatal error occurs.

• Input Rate. The third option is used to specify a rate of mass addition (that may change with time). This field only accepts vectors by species with dimensions of mass/time. It represents the rate at which mass of each species is added to the Pathway over the next timestep.

Note: One way to easily enter a vector of data into an input field without having to create a separate element is to use GoldSim’s vector constructor function. For example, entering “vector(1g)” into the Cumulative Input field results in an initial condition of 1 g of each species being present in the Pathway.

Note: GoldSim passes the initial condition/boundary condition to the External pathway function by adding it to the first advective Inflow (or in the absence of Inflows, the first Diffusive Influx) to the External pathway. A separate vector is only sent to the External pathway function if there are no Inflows or Influxes into the pathway.