Rainwater Collection Yield

Estimate harvestable rainfall volume from projected catchment area and a collection-efficiency factor.

How this calculation works

Rainwater yield begins with the rainfall depth over a chosen period multiplied by the horizontal projected catchment area. Enter rainfall in millimetres and area in square metres, then supply a net collection efficiency as a percentage. The model converts the rainfall depth to metres to obtain gross volume in cubic metres and applies efficiency to estimate collected water. The difference is the represented collection loss. Include runoff, first-flush, leakage and filter effects once within the net efficiency rather than deducting them again from the result. The rainfall must be representative of the catchment and period. This is a volume estimate only: storage overflow, the sequence of rain events and water demand are not simulated, and drinking-water suitability is not assessed.

Inputs and units

Rainfall over the chosen period (mm)
Horizontal projected catchment area (m²)
Net collection efficiency (%): Include runoff, first-flush, leakage and filter losses once

Method and formula

Gross m³ = rainfall in mm × horizontal area in m²/1000; collected volume = gross volume × efficiency/100.

Worked example

Example inputs

Rainfall over the chosen period: 100 mm
Horizontal projected catchment area: 200 m²
Net collection efficiency: 80 %

Calculation steps

Convert 100 mm rainfall to depth: 100/1000 = 0.1 m.
Multiply by the 200 m² projected catchment: gross volume = 0.1 × 200 = 20 m³.
Apply 80% net collection efficiency: collected volume = 20 × 80/100 = 16 m³.
Calculate collection losses: 20 − 16 = 4 m³.

Example results

Gross rainfall volume: 20 m³
Estimated collected volume: 16 m³
Collection losses: 4 m³

Assumptions

Rainfall is representative of the specified catchment and period
Collection efficiency accounts for all entered pre-storage losses

Limitations

No storage sizing, event sequencing, tank overflow or demand-reliability analysis
Does not assess water quality or suitability for drinking

Sources

Texas Water Development Board: Rainwater Harvesting FAQ: Catchment-area × rainfall × collection-efficiency yield relationship