Description
Rainwater harvesting refers to water collected from roofs. The water is stored and used for beneficial purposes, usually onsite. The contributing area must be the roof area.
Rain is a source of relatively clean, soft water. As rain falls on non-roof surfaces such as pavement and grass, it contacts more contaminants than it would from a roof. Thus, harvested roof runoff can often be used with minimal treatment.
Harvesting rainwater from roof runoff is a method of disconnecting impervious surfaces and capturing water before it contacts many potential contaminants. Runoff as a fraction of rainfall was calculated for ten different roof surfaces and slopes allowing an estimate of runoff for future use (Wanielista, M.P., Chopra, M., Hardin, M., Kakuturu, S., and Runnenbaum, N. (2011). Evaluating Runoff and Abstraction from Impervious Surfaces as Components Affecting Recharge. University of Central Florida, Orlando Florida).
They calculated abstraction of rain by various materials and slopes, and these values are built into BMPFast.
One popular use for capturing and reusing roof runoff is replacing potable water. Harvested rainwater is commonly used for:
- Outdoor irrigation
- Car washing
- Toilet flushing
- Clothes washing
- Cooling tower make-up
- Irrigation of indoor planters
- Hose bibs
- Rarely, potable use after treatment
Most indoor uses require approval of the local Health Department or Planning Department.
Types of Rainwater Harvesting Systems in Florida
There are four systems typically used in the State of Florida:
- Small residential systems storing rainwater in rain barrels for supplemental irrigation
- Large residential or commercial systems storing rainwater in cisterns for irrigation, vehicle washing, dust control, or other outdoor non-potable uses
- Large residential or commercial systems storing rainwater in cisterns for indoor graywater uses such as:
- Toilet flushing
- Urinal flushing
- HVAC make-up water
- Laundry wash water
- Outdoor non-potable uses
- Rare systems for purification of stored rainwater as a potable water source
The annual capture effectiveness calculations within BMPFast assume the stored water is used every day.
This differs from stormwater harvesting BMPs, which remove water from the cistern twice per week.
Input Data
Input to calculate average annual removal consists of:
- Type of roof surface
- Contributing area (SF – must equal the catchment area)
- Irrigation area (SF)
- Cistern size (gallons)
- Average annual harvest rate (inches per week)
Example: 2-Acre Light Industry Shop
Shown below are the input values for a 2-acre light industry shop.
Note:
- Contributing area is calculated from the roof area
- The harvesting system collects 55% of the average annual runoff
This equals:
41,613 gallons of water each week
Calculated as:
44,500 SF × 1.5 inches/week ÷ 12 inches/ft × 7.481 gallons/cubic foot
This calculation is provided to translate between:
- Average water used per week
- Irrigation area input
- Harvest rate input
