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Environmental science · Grades 9–12

Environmental stocks, flows and system boundaries

A stock is an amount present at a time; a flow is an amount transferred per time. Mixing their units can produce invalid comparisons.

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Prerequisites

Relevant school subject foundations and careful reasoning.

Study notes

Stock and flow

A stock is an amount present at a time; a flow is an amount transferred per time. Mixing their units can produce invalid comparisons.

Balance

Change in stock equals inputs minus outputs under a stated boundary and period, with transformations accounted for consistently.

Boundaries

An environmental comparison depends on which stages, locations and time horizons are included. A shifted burden outside the boundary is not necessarily eliminated.

Uncertainty

Report measurement uncertainty and assumptions. An accounting model is a tool for reasoning, not automatically a verified full lifecycle assessment.

Worked examples

A reservoir starts with 100 units, receives 30 and loses 20 over the period. Find its ending stock.

100+30−20=110.

110

Which is a flow unit?

A flow measures transfer per time.

Tonnes per year

Check your understanding

  1. Why state a system boundary?

    1. It makes uncertainty disappear
    2. It guarantees zero emissions
    3. It determines which inputs, outputs and processes are counted
    4. It replaces all measurements

  2. A constant flow of 4 units/day continues for 7 days. Find transferred amount.

  3. Why can an improvement inside one boundary increase a burden elsewhere?

  4. Does an uncertain estimate become exact when shown with many decimal places?

    1. No
    2. Yes, always
    3. Only in a long report
    4. Only in a spreadsheet

Answers and marking guidance

Answers and marking notes
  1. It determines which inputs, outputs and processes are counted

    Accounting depends on inclusion rules.

  2. 28 units

    4×7=28.

  3. A process may shift energy, materials or emissions to another location or stage. A broader assessment can reveal the tradeoff.

    Credit burden shifting.

  4. No

    Displayed precision is not measurement accuracy.

Further reading and review references

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