Draw storages as labelled rectangular boxes and flows as labelled arrows pointing in the direction of movement. Show the system boundary, inputs, outputs, and any important transfers or transformations. This is core content for SL and HL in IB ESS subtopic 1.2 Systems.
The Method
- Define the system boundary. Decide what is inside the system. For a lake ecosystem, the lake and its organisms might be inside, while the surrounding watershed is outside.
- Identify storages. Draw boxes for quantities held within the system, such as lake water, fish biomass, dissolved nutrients, or sediment.
- Add inputs and outputs. Inputs enter across the boundary; outputs leave it. Examples include precipitation entering and evaporation leaving.
- Connect storages with flows. Draw arrows in the direction that matter or energy moves. Label every arrow with the process, not merely the material.
- Show relative magnitude where data permit. Larger storages should have larger boxes, while larger flows should have thicker arrows.
| Feature | Correct representation | Lake example |
|---|---|---|
| Storage | Rectangular box | Water stored in the lake |
| Input | Arrow entering the boundary | Precipitation |
| Output | Arrow leaving the boundary | Evaporation |
| Transfer | Movement without a change in form | Surface run-off carrying water |
| Transformation | Change in state, chemistry, or energy | Photosynthesis producing biomass |
A transfer changes location, while a transformation changes the state, chemical nature, or energy form. For example, water moving into a lake is a transfer, whereas photosynthesis converting light energy into chemical energy is a transformation.
A common misconception is that every labelled component should be a box. Processes such as evaporation and decomposition are flows, so they belong on arrows, not inside storage boxes.
Exam Technique
For a draw question, use clear boxes, directional arrowheads, accurate labels, and an identifiable boundary. Examiners may also expect proportional box or arrow sizes when magnitudes are provided; never use unlabelled arrows or confuse a flow with a storage.
