The worked example
With 800 mm of precipitation, 450 mm of evapotranspiration and 300 mm of runoff over the same period, the storage residual is 800 − 450 − 300 = 50 mm. Over 10 km², that equals 50 × 10 × 1,000 = 500,000 m³.
Why the unit conversion works
1 mm = 0.001 m and 1 km² = 1,000,000 m². Their product is 1,000 m³. Use one time period and one catchment boundary for all inputs.
Read the residual carefully
A negative balance can indicate storage depletion or incomplete inputs; it is not automatically an error. A zero residual means inputs and outputs balance in the simplified model. Water imports, abstractions, groundwater exchange and measurement uncertainty may require additional terms.
Assumptions and source
This is a closed-boundary teaching example, not a flood prediction, water availability forecast or engineering design tool. Its inputs are illustrative. The conservation of water follows the water-budget framework described by the USGS Circular 1308: Water Budgets. For a real assessment, define boundaries, measure flows and evaluate uncertainty.