Indians, over centuries, developed a range of techniques to harvest rainwater,
as a part of their daily lives. In this issue, we will concentrate on the urban rainwater
harvesting techniques, while keeping rural ones for the next issue.
India has more than 250 million city dwellers.The water supply sector is faced
with a number of constraints. Almost all the cities depends on groundwater as a major
source. The result is rapid depletion. In addition, the problem of quality is getting
grim. Problem of increasing salinity, nitrate, hardness, flouride is common.
Broadly, rainwater can be harvested for two purposes: storage for direct usage
and groundwater recharge. If the number of dry days is less, than it can be used for
direct usage like in Kerala and Mizoram . But it will not work in cities like, Delhi,
where the recharge option holds water, as rains are for a short duration.
Suppose, a person has a roof with an area of 100 cubic meters and is getting 100
millimetres of rain - then, his rainfall endowment is 10,000 litres (l) and, the potential
is 8,000 l.
Direct storage is a
well tested technique.The rooftop runoff is taken to storage containers. As per the rain
coefficient calculated above, 8,000 l of water can be used by a family of four members for
200 days. It will fulfil their drinking water needs. To prevent leaves and debris entering
the system, mesh filters are used. In some cases this rooftop harvesting system comprises
of commonly used storage containers like masonry or plastic water tanks. Some maintenance
measures like cleaning and disinfection are needed to ensure the quality of water.
Alternative to storing, rainwater may be charged into the groundwater aquifers.
While some structures promote the percolation of water through soil strata at shallower
depth (like recharge trenches, permeable pavements), others conduct water to greater depth
from where it joins the groundwater (like recharge wells). At many locations, existing
features like wells, pits and tanks can be modified, eliminating the need to construct
afresh - reducing the cost.