Ecology & Environment
Rainwater Harvesting Forecast Calculator
Grow rainfall-driven collection and non-potable demand independently, then compare cumulative supply, demand, balance, coverage, and tank turnover.
Opening-month harvest potential (L)-
Harvest potential through horizon (L)-
Demand through horizon (L)-
Forecast harvest minus demand-
Forecast harvest as share of demand-
Average monthly harvest-
Forecast harvest divided by usable tank capacity-
Decision view
Rainwater supply-demand storage outlook
Rainwater supply-demand storage outlookHarvest and demand follow independent entered monthly paths before cumulative coverage is calculated.
Exact scenario comparisonMonthly rainfall change (%) changes while all other entered assumptions remain constant.
| Monthly rainfall change (%) | Opening-month harvest potential (L) | Harvest potential through horizon (L) | Demand through horizon (L) | Forecast harvest minus demand | Forecast harvest as share of demand | Average monthly harvest | Forecast harvest divided by usable tank capacity |
|---|
Period-by-period detail
Monthly rainwater supply-demand forecast
How to use Rainwater Harvesting Forecast Calculator
- Use effective roof catchment and realistic runoff efficiency.
- Treat rainfall growth as a scenario, not a forecast.
- Run a storage simulation before sizing the tank.
Calculator guide
Understanding Rainwater Harvesting Forecast Calculator
Cumulative harvest potential and cumulative demand are not the same as a month-by-month tank simulation.
Calculation method
How the calculation works
Forecast catchment yield and entered non-potable demand independently across each month, then compare cumulative supply, demand, and tank-capacity turnover. Calculate opening harvest from roof area, rainfall, and runoff efficiency; grow monthly harvest and demand separately; sum both paths and compare totals.
System design
Move from potential to reliability
Storage reliability depends on sequence, not only totals.
Worked situations
Practical examples
- One millimeter over one square meter equals one liter before losses.
- Positive cumulative balance can still coexist with dry months.
- Tank turnovers measure throughput, not assured availability.
Better inputs
Useful tips
- Use historical monthly rainfall.
- Model starting storage and overflow.
- Check water-quality regulations.
Before relying on the result
Limitations and common mistakes
- Rain timing, first flush, contamination, starting volume, overflow, drought sequences, and regulations are excluded.
- Percentage paths are constant scenarios.
- Tank capacity does not constrain harvest totals.
Reference
Key terms
- Runoff efficiency
- Share of roof rainfall collected after losses.
- Harvest potential
- Theoretical collected volume before storage constraints.
- Coverage
- Cumulative harvest divided by cumulative demand.
- Tank turnover
- Cumulative harvest divided by usable capacity.
Important note
Calculated from the entered environmental values using the displayed model. Measurement quality, local conditions, system boundaries, and source data affect interpretation.
Frequently asked questions
Does 100% coverage guarantee no shortages?
No; timing and storage can still create shortages.
Is overflow deducted?
No.
Does tank capacity limit harvest?
Not in this cumulative model.
Can this forecast weather?
No.