Formulas
yield L = rainfall mm × roof m² × runoff coefficient × efficiency
One millimetre of rain on one square metre equals one litre before losses.
Tank balance
Each month starts with the previous storage. Inflow is added, demand is removed, excess above capacity becomes overflow and unmet demand becomes shortfall.
Sources
- IMGW-PIB — Normy klimatyczne 1991–2020
- US DOE FEMP — Rainwater Harvesting Calculator
- US EPA WaterSense — Water Budget Tool
- FAO CROPWAT / FAO 56
Limitations
The monthly model simplifies the timing of rainfall and use within a month. A detailed design should use daily data, local measurements and professional engineering.