Rainwater Harvesting

What is Rainwater Harvesting and why is it Necessary?

Water is our most precious natural resource and something that most of us take for granted. We are now increasingly becoming aware of the importance of water to our survival and its limited supply, especially in dry season.


The harvesting of rainwater simply involves the collection of water from surfaces on which rain falls, and subsequently storing this water for later use. Normally water is collected from the roofs of buildings and stored in rainwater tanks. The water collected can be considered to be precious.


The collection of rainwater from the roofs of buildings can easily take place. All that is necessary to capture this water is to direct the flow of rainwater from roof gutters to a rainwater storage tank. By doing this, water can be and used for various purposes. It is possible to replace all or at least a substantial portion of your collected fresh water requirements by the capture and storage of rainwater from your roof. Being largely self sufficient in water supply is possible for a vast majority of households and buildings.


What are the Benefits in Rainwater Harvesting?

Economic Benefit

By capturing water directly, we can significantly reduce our reliance on municipal water supply. Collecting and using your own water can also significantly reduce your water bills.


Environmental Benefit

While the economic benefits outline the affordability of a rainwater system, there are environmental benefits that justify implementing such a system. By re-using rainwater, the demand is reduced on the regions already scarce water resources.


How much water can be harvested?

The amount of rainfall that can be effectively harvested is called the water harvesting potential. 25.4 mm (1 inch) of rainfall precipitation on one square foot catchment area can yield 2.36 liters of water. However, in practice, this volume can never be achieved since a portion evaporates and a portion may be lost to the drainage system, including the first flush. Therefore, we can only collect about 80 % of the rainfall.


The following is an illustrative theoretical calculation that highlights the enormous potential for rainwater harvesting. The same procedure can be applied to get the potential for any area, using the rainfall data.


Consider a catchment area of 100 sq. m. The average annual rainfall in Kathmandu is approx 1610 mm. In theory the catchment area is assumed to be impermeable and all the rain falling on it without evaporation, then in one year.


Area of plot = 100 sq. m.
Height of rainfall = 1610 mm
Volume of rainfall = Area of plot X Height of rainfall
= 100 sq. m X 1610 mm (1.610 m)
= 161.0 cu. m (161, 000 liters)
Assuming that only 80% of the total rainfall is effectively harvested
Volume of water harvested = 161, 000 liters X 0.8
= 128, 800 liters.

The average daily drinking water requirement per person is 10 liters.