Direct transfer mass flux links are used to transport mass directly from one pathway to another via a mechanism that cannot be appropriately represented as either advection or diffusion. They are defined by specifying a fractional rate at which mass is being moved from one pathway to another. Hence, a direct transfer mass flux is computed as the mass of the species in the pathway multiplied by a fractional transfer rate:
Direct Transfer Mass Flux = Cell Mass * Transfer Rate
The Transfer Rate has dimensions of inverse time. The Transfer Rate is specified as a vector by species, which allows the Transfer Rate to be species-specific.
As an example of how you might apply a direct transfer mass flux link, consider the following. Suppose that you were simulating the transfer of mass from soil to a plant. You want to represent the kinetics of the transfer process (i.e., you do not want to assume immediate equilibrium). The transfer rate is first order (i.e., proportional to the mass of species in the soil), but cannot be adequately represented as advection (no fluid is moving between the soil and the plant) or diffusion (for example, perhaps the transfer rate is not a function of the concentration of the species in the plant). You could use a direct transfer mass flux link to represent the kinetics in this case.
Learn more about:
Creating and Editing Direct Transfer Mass Flux Links
Inputs and Outputs Associated with Direct Transfer Mass Flux Links