What is Rain Water harvesting?
Rain Water Harvesting is an act of accumulating and storing rainwater for reuse. As we know rainwater is the purest form of water, we can utilize this water for various purposes after necessary purification process as per required. This system can be a boon in regions where there is shortage of water. This problem is common in today’s big cities. Hence rainwater harvesting can be very effective in these areas.
Broadly rainwater can be harvested for two purposes:
- Storing rainwater for ready use in containers above or below ground.
- Charged into the soil for withdrawal later (groundwater recharging).
These techniques can serve the following purposes:
- Provide drinking water
- Provide irrigation water
- Groundwater recharge
- Reduce storm water discharges, urban floods and overloading of sewage treatment plants
- Reduce seawater ingress in coastal areas.
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, storm water 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.
- Water-bodies: 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.
- Storm-water drains: Most of the residential colonies have proper network of storm water drains. If maintained neatly, these offer a simple and cost effective means for harvesting rainwater.
Whether to store rainwater or use it for recharge?
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.
How much rainwater can be harvested?
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.