A residual can change sign when an input changes
https://water-msc.org/tools/

Keep the arithmetic separate from confidence in the inputs. The calculator’s positive storage residual is a result of the numbers entered. It does not establish that a real catchment is gaining water when those numbers are uncertain or incomplete.

Try the same boundary with three runoff estimates

Retain precipitation of 800 mm, evapotranspiration of 450 mm and area of 10 km² over one common period. Use illustrative runoff estimates of 250, 300 and 400 mm. Change only that input so you can see its effect.

Runoff Q800 − 450 − QEquivalent residual over 10 km²

250 mm+100 mm+1,000,000 m³

300 mm+50 mm+500,000 m³

400 mm−50 mm−500,000 m³



Run a one-input sensitivity check

Write the catchment boundary and the common observation period before entering values.

Record a baseline and one defensible alternative for the input under review. These classroom alternatives are not statistical confidence limits.

Recalculate, keeping every other input fixed. Report which input changed and the size of the residual change.

If the sign changes, say that the conclusion depends on the selected input. Do not average away disagreement without a measurement rationale.

List missing transfers and measurement uncertainties before interpreting a real residual as storage change.



Each extra 1 mm of runoff reduces the residual by 1 mm in this simplified equation. At 10 km², that is 10,000 m³. A negative result is permitted: do not clamp it to zero. The USGS discusses uncertainty arising from variability and measurement; a full assessment needs uncertainties for all terms and their relationships. This sensitivity exercise changes one number and is neither a flood forecast nor a probability interval.



Text equivalent: Q = 250 mm → ΔS = +100 mm → Q = 300 mm → ΔS = +50 mm → Q = 400 mm → ΔS = −50 mm

Boundary: __________________________

Common period: __________________________

Baseline P/ET/Q/area and units: __________________________

Changed input and rationale: __________________________

Alternative value: __________________________

Residual in mm: __________________________

Residual in cubic metres: __________________________

Sign changed?: __________________________

Missing terms: __________________________

Limits: not a confidence interval or forecast: __________________________

Original signed-residual plot for a fictional sensitivity exercise. The horizontal zero line separates positive and negative results.
https://www.usgs.gov/publications/evaluation-methods-and-uncertainties-water-budget