Broadly rainwater can be harvested for two purposes
- Storing rainwater for ready use in containers above or
- Charged into the soil for withdrawal later (groundwater
From where to harvest rain
Rainwater harvesting can be harvested from the following surfaces
Rooftops: If buildings
with impervious roofs are already in place, the catchment
area is effectively available free of charge and they provide
a supply at the point of consumption.
Paved and unpaved areas
i.e., landscapes, open fields, parks, stormwater drains, roads
and pavements and other open areas can be effectively used
to harvest the runoff. The main advantage in using ground
as collecting surface is that water can be collected from
a larger area. This is particularly advantageous in areas
of low rainfall.
Waterbodies: The potential
of lakes, tanks and ponds to store rainwater is immense. The
harvested rainwater can not only be used to meet water requirements
of the city, it also recharges groundwater aquifers.
Stormwater drains: Most
of the residential colonies have proper network of stormwater
drains. If maintained neatly, these offer a simple and cost
effective means for harvesting rainwater.
Whether to store rainwater or use it
The decision whether to store or recharge water depends on
the rainfall pattern and the potential to do so, in a particular
region. The sub-surface geology also plays an important role
in making this decision.
For example, Delhi, Rajasthan and Gujarat where the total
annual rainfall occurs during 3 or 4 months, are examples
of places where groundwater recharge is usually practiced.
In places like Kerala, Mizoram, Tamil Nadu and Bangalore where
rain falls throughout the year barring a few dry periods,
one can depend on a small sized tank for storing rainwater,
since the period between two spells of rain is short. Wherever
sub-strata is impermeable recharging will not be feasible.
Hence, it would be ideal to opt for storage.
In places where the groundwater is saline or not of potable
standards, the alternate system could be that of storing rainwater.
Beyond generalisations, it is the requirement that governs
the choice of water harvesting technique. For example, in
Ahemadabad, which has limited number of rainy days as that
of Delhi, traditional rainwater harvesting tanks, known as
tankas, are used to store rainwater even today in residential
areas, temples and hotels.