Water Balance Estimator
A site water balance sums every stream that adds water to the managed store and subtracts every stream that removes it, over a defined period.
Enter Values
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter each inflow (rainfall onto ponds, catchment runoff, process make-up) in megalitres for your chosen period — a month, wet season or full year.
- Enter each outflow (evaporation, seepage, entrainment in tailings/product, controlled discharge) as positive numbers in megalitres.
- Read the net change: a positive value is a surplus (store filling), negative is a deficit (store drawing down). Leave any term blank if it does not apply.
How it works
A site water balance sums every stream that adds water to the managed store and subtracts every stream that removes it, over a defined period. The net change is: net = (rainfall + runoff + make-up) − (evaporation + seepage + entrainment + discharge). All terms are kept in the same volume unit (megalitres) so they add directly; 1 ML = 1,000 m³, which the tool also reports.
Inflows and outflows must cover the same time window (e.g. one wet season) for the result to be meaningful. A positive net signals accumulation — relevant to pond freeboard and spill risk — while a negative net signals a shortfall that may need additional make-up. This is a lumped mass-balance screen; it does not route storm hydrographs, model pond stage or account for storage carry-over between periods.
Worked example
Wet-season site water balance. A tailings storage receives 120 ML direct rainfall, 340 ML catchment runoff and 80 ML process make-up (540 ML total inflow). Losses are 210 ML evaporation, 45 ML seepage, 95 ML entrainment in tailings and 0 ML discharge (350 ML total outflow). Net = 540 − 350 = 190.000 ML surplus (190,000 m³), so the pond is accumulating water and freeboard should be checked.
Common mistakes
- Entering outflows as negative numbers — evaporation, seepage and discharge are all typed as positive volumes; the tool subtracts them for you.
- Mixing time periods, e.g. annual rainfall against a monthly evaporation figure — every term must span the same period.
- Confusing megalitres and cubic metres: 1 ML = 1,000 m³, so a 190 ML surplus is 190,000 m³, not 190 m³.
Frequently asked questions
What does a positive net water change mean?
It means inflows exceed outflows over the period, so the managed water store is accumulating. That is important for pond freeboard and spill risk — a growing surplus may need controlled discharge or extra storage. A negative net means the store is drawing down and may need make-up water.
What units should I use?
Enter every stream in megalitres (ML) for the same time period. The tool reports the net in both ML and cubic metres (1 ML = 1,000 m³). Keep rainfall, runoff, evaporation and losses consistent — all monthly, or all annual.
Does this replace a proper site water balance model?
No. This is a lumped screening estimate that adds and subtracts the streams you enter. A design-grade balance uses monthly climate data, pond stage-storage curves, storm routing and carry-over between periods, and should be prepared by a competent water engineer.
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Tip: Enter any known values to calculate the remaining results.
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