Every raindrop matters at this time when large parts of the world are staring at water scarcity. Here are the answers to some FAQs on urban water harvesting
In urban areas, rainwater can be collected from the roof, paved and unpaved areas of a house, a block of flats, a colony, a park, a playground, parking areas, schools, office complexes lakes and tanks.
There are two ways of using the harvested rainwater:
There are instances where the citizens have come together to revive waterbodies, which act as groundwater recharge structures in urban centres.
How to calculate?
Thumb rule: 10 millimetre of rainfall over 100 square metres of roof area will fetch 1,000 litres (volume = rainfall x area).
Accounting for some loss due to evaporation or absorption by catchment surfaces, the actual volume can be ascertained by determining the run-off coefficient of the catchment — it indicates the proportion of rainwater that can be harvested from the total rainfall.
Let’s take a roof, for example:
Elements of rainwater harvesting
A few basic elements are common to all rainwater harvesting (RWH) systems
A simple RWH system. Illustration: CSE
The catchments
The catchment is a structure or land area that is used to collect rainwater and drain run-off. Clean catchments are necessary for maximising harvest. They can be either paved (roofs, courtyards, roads, etc) or unpaved (lawns, playgrounds, open spaces, etc).
Types of catchments. Illustration: CSE
Charecteristics of catchments
Collection efficiency: Not all the rain that falls on the catchments is available for rainwater harvesting. Some of the water that falls on catchments may be lost because of evaporation, seepage into the ground, absorption by roof materials, improper fittings, leakages from pipes and gutters and clogging in various parts of the system. These factors and the effective catchment area largely influence the collection efficiency.
Run-off coefficient: This is the ratio of the volume of water that runs off a surface to the volume of rainfall that falls on the surface. More water runs off smooth and impervious surfaces such as roofs or paved areas than soils and unpaved areas. Different catchment materials absorb water to differing extents.
Conveyance systems
Conveyance, or conduit systems, direct water flow from the catchment area to the storage area. A carefully designed and constructed conveyance system can divert more than 90 per cent of all the water that falls on to the roof.
A roof outlet. Illustration: CSE
Harvesting rain from a sloping roof. Illustration: CSE
Filter systems
Four types of filtration processes can be used in an RWH system:
Harvesting for storage
Storage tanks for harvested rainwater can be built both underground and overground. In the cities of Gujarat and Rajasthan, where rooftop harvesting was practised traditionally, rainwater from the roof was collected in underground tanks in the courtyard or within the buildings. Called tankas, these tanks would supply drinking water throughout the year. Some are still in use today.
The number of tanks will depend on site conditions like:
Siting of tanks
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