# 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:**

Area = length X breadth

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 | 0.8 |

Paved area Driveways/courtyards, roads | 0.6 |

Unpaved area garden, playground | 0.1 |

**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 0.6

**(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 0.1

**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