Ecology & Environment
Rainwater Harvesting Trend Calculator
Project annual rainfall, gross roof catchment, demand coverage, and cumulative harvested volume across the selected horizon.
First-year harvest potential (L)-
Final-year rainfall assumption-
Final-year harvest potential (L)-
Final-year gross demand coverage-
Final minus first-year harvest (L)-
Final rainfall versus starting year-
Decision view
Annual rainwater harvest projection
Annual rainwater harvest projectionProjection year is the x-axis; annual harvest potential in liters follows the exact compounded rainfall path.
Exact scenario comparisonAnnual rainfall trend (%) changes while all other entered assumptions remain constant.
| Annual rainfall trend (%) | First-year harvest potential (L) | Final-year rainfall assumption | Final-year harvest potential (L) | Final-year gross demand coverage | Final minus first-year harvest (L) | Final rainfall versus starting year |
|---|
Period-by-period detail
Annual rainfall and gross harvest projection
How to use Rainwater Harvesting Trend Calculator
- Enter starting rainfall, annual trend, roof area, efficiency, demand, and horizon.
- Read the final-year outputs.
- Use the annual table and cumulative curve to inspect the whole path.
Calculator guide
Understanding Rainwater Harvesting Trend Calculator
A rainfall trend compounds year by year, so harvest potential follows the same percentage path while roof area and runoff efficiency remain fixed.
Calculation method
How the calculation works
Project a rainwater-harvesting trend by compounding annual rainfall change and translating each year into catchment yield and demand coverage. Compound the entered rainfall change from the starting year, then multiply each year's rainfall depth by catchment area and runoff efficiency.
Detailed calculation process
Compound rainfall and translate it into annual harvest
The default starts at 900 mm, changes by -1.5% annually for 15 years, uses 180 m² of roof at 85% efficiency, and compares with 100,000 L annual demand.
General formula: g = 1 + r/100R_t = R_1 g^(t-1)H_t = R_t A etaC_t = 100H_t/DH_cum,N = sum(t=1..N) H_t One millimetre of rain on one square metre equals one litre. The trend factor compounds rainfall, and fixed area and efficiency convert each projected depth directly to liters.
What each symbol means
Worked substitution with the default inputs
The default annual potential falls from 137,700 L to 111,440 L while the full 15-year cumulative curve records every intervening year.
Purpose-built visual
Cumulative harvest area curve
Annual harvest forms a changing line while the shaded cumulative series shows how volume accumulates through the horizon.
Worked situations
Practical examples
- The default starts at 900 mm, changes by -1.5% annually for 15 years, uses 180 m² of roof at 85% efficiency, and compares with 100,000 L annual demand.
- The default annual potential falls from 137,700 L to 111,440 L while the full 15-year cumulative curve records every intervening year.
Better inputs
Useful tips
- Change one assumption at a time and compare the live result and visual.
- Keep all entered quantities on the units stated beside their fields.
- Retain extra precision through intermediate steps and round only reported results.
Before relying on the result
Limitations and common mistakes
- The smooth compound trend is not a climate forecast.
- Storage, overflow, first flush, seasonal timing, drought, and water quality are excluded.
- Gross coverage can exceed 100% without proving that demand can be met at every date.
Reference
Key terms
- Catchment yield
- Rainfall depth times receiving area before or after stated efficiency.
- Compound trend
- A percentage change applied to each preceding year.
- Gross coverage
- Annual potential divided by annual entered demand.
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
Why is the exponent t minus 1?
Year 1 is the starting value, so no annual change has occurred yet.
Why does mm times m² equal liters?
A 1 mm layer over 1 m² is 0.001 m³, which is 1 liter.
Does coverage include tank size?
No. It is an annual gross comparison only.
Can a negative trend reach negative rainfall?
No for valid rates above -100%; the compound factor stays nonnegative.