Formula & Calculator
Rainwater Harvesting Potential
Estimates the volume of rainwater that can be collected from a catchment surface based on rainfall and a runoff coefficient.
Interpretation
Rainwater harvesting potential: V = A × R × C, where A is catchment area, R is rainfall depth, C is runoff coefficient. Example: A=100 m², R=50 mm, C=0.8 → V = 4 m³ = 4000 litres.
Variables
| Symbol | Quantity | Unit |
|---|---|---|
| V | Harvestable rainwater volume | liters |
| A | Catchment (roof) area | m2 |
| R | Rainfall depth | mm |
| C | Runoff coefficient (0-1, ~0.8 for roofs) |
What it means
This formula estimates the volume of rainwater that can be collected from a given catchment surface (e.g., a roof). The catchment area A is in square metres, rainfall depth R is in metres (or mm converted to metres), and C is the runoff coefficient (0.7‑0.9 for hard surfaces, 0.1‑0.3 for green areas). The product gives the volume in cubic metres, which can be converted to litres. This calculation is used in sustainable building design, water conservation projects, and stormwater management. It helps determine the size of storage tanks needed and the feasibility of rainwater harvesting. In practice, losses due to evaporation, first‑flush diversion, and overflow are considered, so a factor of safety is often applied. The formula is also used for designing drainage systems and for assessing the water balance of a site. It is a key tool for green building certifications like LEED.
Worked example
Rainwater Harvesting – Two Examples
Real‑World| Parameter | Value |
|---|---|
| A | 100 m² |
| R | 50 mm = 0.05 m |
| C | 0.8 |
| Parameter | Value |
|---|---|
| A | 200 m² |
| R | 30 mm = 0.03 m |
| C | 0.85 |
Common mistakes
- Rainfall depth R: Convert mm to metres (divide by 1000).
- Runoff coefficient C: Depends on surface type (e.g., 0.8‑0.9 for roofs, 0.2‑0.4 for grassy areas). Use the correct value.
- Catchment area A: The plan area (horizontal projection) of the catchment, not the surface area.
- Losses: First‑flush losses, evaporation, and infiltration reduce collected volume – this formula gives gross potential.
- Units: A in m², R in m → V in m³.
Applications
Rainwater harvesting potential estimates the volume of rainwater that can be collected from a catchment area, using V = A × R × C, where A is the catchment area, R is the rainfall depth, and C is the runoff coefficient. This formula is crucial for sustainable water management, helping to design systems that capture and store rainwater for non‑potable uses such as irrigation, flushing, and washing. Architects, environmental engineers, and urban planners use it to assess the viability of rainwater harvesting in new and existing buildings. The runoff coefficient accounts for losses due to evaporation, infiltration, and surface retention. By estimating the potential yield, professionals can size storage tanks, select filtration systems, and integrate the system with building services, contributing to water conservation and reduced stormwater runoff.
- Feasibility studies for rainwater harvesting systems
- Design of roof‑water collection systems
- Sizing of storage tanks and filters
- Stormwater management and sustainability initiatives
- Integration with greywater recycling systems
Frequently Asked Questions
The potential volume is V = A × R × C, where A is the catchment area (m²), R is the rainfall depth (m), and C is the runoff coefficient (dimensionless). The result is in cubic metres (m³).
- Metal or tile roof: 0.8‑0.95
- Concrete/tarmac: 0.7‑0.9
- Gravel or permeable paving: 0.4‑0.6
- Grass: 0.1‑0.3
- Vegetated roof: 0.3‑0.5
- Using an overly optimistic C – assuming C=1 for all surfaces leads to overestimation.
- Using the wrong rainfall units – if rainfall is in mm, convert to metres (divide by 1000).
- Ignoring the first flush – the initial runoff may be contaminated and should be diverted.
- Not accounting for system losses – filtration, overflow, and evaporation reduce the usable volume.
Annual rainfall in metres = 800/1000 = 0.8 m. Use C = 0.85 (typical for roof). Volume = 100 × 0.8 × 0.85 = 68 m³ per year = 68,000 litres. This is a rough estimate.
The first flush is the initial runoff that washes off dust, bird droppings, and debris. It is usually diverted (not collected) to improve water quality. A common practice is to divert the first 1‑2 mm of rainfall.
Determine the average daily water demand (e.g., gardening, flushing). Size the tank to store enough water to cover the dry period (days without rain). Use the formula to estimate how much you can harvest per event.
In regions with distinct wet and dry seasons, you may need a larger tank to store water for the dry period. Use monthly rainfall data to size the tank.
The effective catchment area is the horizontal projection of the roof, not the sloped area. For a roof with pitch, the horizontal area is the footprint area (length × width).
- Irrigation (gardens, lawns).
- Toilet flushing.
- Washing cars.
- Laundry (with appropriate filtration).
- Potable use (with treatment) in some systems.
Filtration removes debris and sediment before water enters the storage tank. A simple filter (e.g., mesh or sand) is essential to maintain water quality and prevent tank contamination.