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