Knowledge Bank

# Formulae to remember

## Total water consumption

The amount of water supplied through the different sources of water to the school for daily consumption. It can be calculated by the following two methods:

a. By using Water bills

Total water consumption (litre/day) = _________________

b. Without water bills
Open tap to normal flow. Hold a 5 litre bucket for 1 minute. Measure the quantity of water. It gives x litre/minute. Multiply this value by 1440. It gives quantity of water used in a day (x litre/day).

1) To calculate the maximum rainwater harvesting potential:

Roof top potential (cu.m) = Rooftop area (in sq m) x mm (if in mm then convert to m) of rainfall x 0.8

Paved potential (cu.m) = Paved area (sq m) x mm (if in mm then convert to m) of rainfall x 0.6

Unpaved potential (cu.m) = Unpaved area (sq m) x mm (if in mm then convert to m) of rainfall x 0.10

Maximum rainwater harvesting potential of the school = Maximum Rooftop potential ___ (cu.m) +

Maximum Paved potential __ (cu.m) + Maximum Unpaved potential  ______ (cu.m)

Run-off coefficient is the amount of rainwater retained in an area and mainly dependent on roof type and catchment.

 Area/Catchment Run-off coefficient Roof 0.85 Paved area Driveways/courtyards, roads 0.70 Unpaved area garden, playground 0.30

Note: For Average Rainfall Data refer to Knowledge Bank: www.greenschoolsprogramme.org

2) To calculate total volume of rainwater actually harvested in the school (cu.m):

(a) Volume of rain harvested from the rooftop (cu.m) = Area of the roof actually harvested (sq m) x rainfall in m x 0.8

(b) Volume of rain harvested from the paved surface (cu m) = Area of the paved surface actually harvested (sq m) x rainfall in m x 0.7

(c) Volume of rain harvested from the unpaved surface (cu m) = Area of the unpaved surface actually harvested (sq m) x rainfall in m x 0.3

Total volume of rainwater actually harvested: = a+b+c

Example: Say the school harvests 20 per cent of the area of rooftop/paved / unpaved, then automatically the actual rainwater harvested from the whole area becomes less than the maximum potential.

Or say the school does not harvest its unpaved area, then (c) becomes zero – the actual rainwater harvested from the whole area becomes less than the maximum potential.

3) Percentage of rainwater your school harvested =

Total rainwater actually harvested in the school
_____________________________________________________ X 100

Rainwater harvesting potential of the school