# Lesson 3 – Exercise: Modeling a Pond with a Changing Volume

In this Lesson, we will work on a simple Exercise to simulate a Cell whose volume is changing with time.

The model consists of a single tank with a constant inflow and a constant outflow.  The outflow is greater than the inflow, and hence the volume will drop. Mass is enters the tank via the inflow at a constant concentration.

The input parameters describing this system are summarized below:

Variable Value
Initial Volume of in Tank 10 m3
Inflow Rate 0.1 m3/day
Outflow Rate 0.175 m3/day
Inflow Concentration 100 mg/l

To build this model, create a new model and follow these steps:

1. Edit the Species element to change the name of the single species from Species1 to X.
2. Create four Data elements for the inputs in the table above. Note that the inflow concentration should be defined as a vector of species.
3. Create a Pool element with the appropriate initial volume, inflow and outflow.
4. Create a Cell (named Tank).  The amount of water should be defined by the Pool.
5. Create a Cell (named Source) with an arbitrarily constant volume (e.g., 1 m3) and create an Outflow from this Cell to the Tank (with a flow rate defined by the Inflow Rate element).
6. Create a Cell (named Sink) with an arbitrarily constant volume (e.g., 1 m3).
7. Create an Outflow from the Tank Cell to the Sink Cell (with a flow rate defined by the Outflow output of the Pool).
8. We will run the model for 100 days with a 1 day timestep (the default settings).

Stop now and try to build the model.

Once you are done with your model, save it to the “MyModels” subfolder of the “Contaminant Transport Course” folder on your desktop (call it ExerciseCT7.gsm). If, and only if, you get stuck, open and look at the worked out Exercise (ExerciseCT7_Changing_Volume.gsm in the “Exercises” subfolder) to help you finish the model.

Let’s walk through the model now.

The system should look like this:

A critical point is how the Outflow from Tank to Sink is defined:

The rate is defined as the output of the Pool element.  This is the actual outflow (as opposed to the requested outflow). As discussed in Unit 7 of the Basic Course, requested withdrawals/outflows from a Pool or Reservoir are not necessarily the same as the actual withdrawals/outflows.  In particular, if the Pool or Reservoir hits its Lower Bound, the actual withdrawal/outflow can be lower than the requested value.  Hence, it is critical when linking Pools and Reservoirs to Cells that you specify flow rates appropriately. In this case, the outflow from the Pool is defined as follows:

The output of the Pool named “Outflow1” (which was the default name) was defined by the Data element named Outflow (which represents the requested outflow).  When we created the Outflow from the Pond Cell to the Sink Cell, we referenced this output (Tank_Volume.Outflow1), which always represents the actual outflow, rather than referencing the requested outflow.

Run the model and plot the volume in the tank.  It should look like this: