System Diagrams in IB ESS: Stores, Flows and Feedback

09 Jun System Diagrams in IB ESS: Stores, Flows and Feedback

System diagrams in IB ESS show how components of an environmental system interact. A clear diagram can represent stores of matter or energy, flows between stores, and exchanges across a system boundary. It should explain a process, rather than simply decorate an answer.

Start with the system boundary

Decide what your model includes. A pond, a forest or a drainage basin can be treated as a system at different scales. Draw a boundary and identify exchanges across it. An open system exchanges matter and energy with its surroundings; a closed system exchanges energy but not matter; an isolated system exchanges neither.

The boundary is a modelling choice. Sunlight may enter a pond system as an energy input, while water enters through rainfall or inflow. Fish living in the pond are components of that system, not automatically external inputs.

Distinguish stores, flows and changes

• Stores: quantities held within the system, such as water in a lake or carbon in plant biomass.
• Flows: movement between stores or across the boundary. Label each arrow with what moves and its direction.
• Transfers: movement without a change of form, such as water moving from a river into a lake.
• Transformations: a change of form or state, such as liquid water becoming water vapour through evaporation.

Where quantities are given, distinguish an amount from a rate. A water store might be measured in cubic metres, while an inflow is measured in cubic metres per day. Comparing them without accounting for time is misleading.

Feedback requires a causal loop

An arrow returning to its starting point does not by itself establish feedback. Explain how a change influences other components and then influences the original change.

Positive feedback amplifies an initial change; negative feedback counteracts it. These terms describe the direction of the response, not whether the outcome is beneficial or harmful. A carbon cycle is not automatically a feedback loop simply because carbon moves through several stores.

See the positive feedback guide for a worked explanation.

Original practice: a lake water budget

This is an original exercise, not an official IB question or a graded example. Draw a lake water store. Add rainfall, river inflow and groundwater inflow as inputs, and evaporation, river outflow and water abstraction as outputs. State which flows you have simplified or omitted.

If inputs exceed outputs over the chosen period, the store increases. If outputs exceed inputs, it decreases. Explain how seasonal measurements could change your interpretation. A diagram alone cannot show that the lake is sustainable or that a particular human action caused a change.

Check the diagram against the question

Use readable labels, consistent symbols and a key if needed. For a requested sketch, include the relevant relationships rather than unnecessary detail. Software container diagrams and UML sequence diagrams are not standard ESS requirements. Hand-drawn diagrams can communicate the science clearly.

For IA work, a diagram can clarify the investigation’s scope, but it does not replace the method, data or analysis. Practise interpreting diagrams alongside ESS data questions.

Practise with individual ESS support

In Marija’s online IB ESS SL and HL lessons, you can work on concepts, data interpretation and clear explanations. Request a free 20-minute trial lesson to discuss your priorities. For IA work, the investigation and submitted writing remain your own.

Course reference: Official IB ESS guide, first assessment 2026 and IB course updates. Reviewed 7 October 2026.