The total amount of water that is received in the form of rainfall over an area is called the rainwater endowment of the area. Out of this, the amount that can be effectively harvested is called the water harvesting potential.
Water harvesting potential = Rainfall (mm) x Collection efficiency
The collection efficiency accounts for the fact that all the rainwater falling over an area cannot be effectively harvested, because of evaporation, spillage etc. Factors like runoff coefficient and the first-flush wastage are taken into account when estimated the collection efficiency.
The following is an illustrative theoretical calculation that highlights the enormous potential for water harvesting. The same procedure can be applied to get the potential for any plot of land or rooftop area, using rainfall data for that area..
Consider your own building with a flat terrace area of 100 sq m. Assume the average annual rainfall in your area is approximately 600 mm (24 inches). In simple terms, this means that if the terrace floor is assumed to be impermeable, and all the rain that falls on it is retained without evaporation, then, in one year, there will be rainwater on the terrace floor to a height of 600 mm.
- Area of plot = 100 sq. m. (120 square yards)
- Height of the rainfall = 0.6 m (600 mm or 24 inches)
- Volume of rainfall over the plot = Area of plot x height of rainfall
- Assuming that only 60 per cent of the total rainfall is effectively harvested
- Volume of water harvested = 36,000 litres (60,000 litres x 0.6)
This volume is about twice the annual drinking water requirement of a 5-member family. The average daily drinking water requirement per person is 10 liters.