Understanding How Cells Manage Flow Balance

The quantity of a medium in a Cell is specified directly by the value in the media list for the Cell. It is important to understand that these values are not changed by the specified Outflows and/or Inflows of that medium from/to the Cell. That is, even though you specify the rate at which the medium leaves (and/or enters) the Cell when you create an advective flux, this has no affect the quantity of the medium in the Cell(s). GoldSim uses the advective rates only to transport mass and does not keep track of the movement of media (e.g., Water) through the pathway network.

So if you have specified that the quantity of medium is constant, but the Inflows are different than the Outflows, what does it mean and how does GoldSim treat this? The answer is that the specified volume is always respected. Conceptually, this implies that a quantity of "clean" medium (containing no species mass) enters (or leaves) the Cell in exactly the correct amount such that the quantity of medium in the Cell remains constant. For example, if a Cell had an Outflow flux link of Water of 10 m³/sec and Inflow flux link of Water from another Cell of 3 m³/yr, GoldSim would then assume that 7 m³/yr of "clean" Water also flowed into the Cell in order to conserve the Cell's volume.

If the volume in a Cell is constant, but the specified Outflows are greater than the specified Inflows (as in the example above), the effect on the species concentrations in the Cell is essentially one of dilution, since "clean" medium is assumed to be flowing into the Cell. If, however, the specified Outflows are less than the specified Inflows, the effect would be to concentrate mass in the Cell, since GoldSim would assume that a quantity of "clean" medium (carrying no species mass) flowed out of the cell in order to conserve the Cell's volume. Although there are some physical situations in which you may actually want to do this (e.g., in order to simulate the evaporation of Water from a Cell), typically such behavior would not be intended. As a result, if during a timestep the Inflows to a Cell exceed the Outflows, and the quantity of the Reference Fluid is defined in such a way that it cannot change with time (e.g. using a Data element), GoldSim writes a (non-fatal) warning message to the Run Log to alert you to this.

Note: Although this is a helpful warning, every model you build will have at least one Cell in which the Inflows exceed the Outflows. This is because all models have an “edge” or an “sink” where the model ends, and you are no longer tracking mass. Hence, to avoid the warning message in this case, if the name of the Cell starts with the letters “sink” (case-insensitive), GoldSim will not issue the warning message for that Cell.

Note: The warning message is only written for the Reference Fluid. If the Inflows to a Cell exceed the Outflows for any other medium in the Cell or the quantity.

Of course, in some cases, the quantities of media in the Cell may actually be changing (e.g., the Outflows are greater than the Inflows, and no “clean” water is entering, and hence the Cell volume is decreasing). In such a case, you should explicitly represent this by modeling the flows and volumes (typically using Reservoir or Pool elements) and coupling this to the Cells (e.g., the amount of Water in the Cell would be defined as the output of a Reservoir or Pool).

Note: If the Inflows to a Cell exceed the Outflows, and the quantity is specified as being able to change with time (e.g., it is represented using a Reservoir or a Pool), GoldSim does not write a warning message to the Run Log to alert you to this. In this case, it is assumed that you are properly tracking the quantity directly.