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Freshwater shortage is among the top concerns today. One of the ways to tackle the problem is through rainwater harvesting. Town planners and civic authorities in India are introducing laws making it compulsory in all new structures. No water or sewerage connection would be given if a new building did not have provisions for rainwater harvesting. The Central Ground Water Board (CGWB) is also taking steps to encourage it for recharging groundwater in New Delhi and elsewhere. A number of government buildings have been asked to go in for water harvesting in Delhi and other cities of India.
Rainwater Harvesting
In urban areas, rainwater available from roof-tops of buildings goes waste. This water can be recharged to aquifer and can be utilised gainfully at the time of need. The rainwater harvesting system for urban area needs to be designed in a way that it does not occupy large space for collection and recharge system.
In rural areas, rainwater harvesting is taken up considering watershed as a unit. Surface spreading techniques are common since space for such systems is available aplenty.
In general, water harvesting is the activity of direct collection of rainwater. The rainwater collected can be stored for direct use or can be recharged into the groundwater. Rain is the first form of water that we know in the hydrological cycle, hence is a primary source of water for us. Rivers, lakes and groundwater are all secondary sources of water. In present times, we depend entirely on such secondary sources of water. In the process, it is forgotten that rain is the ultimate source that feeds all these secondary sources and remain ignorant of its value. Water harvesting means to understand the value of rain, and to make optimum use of the rainwater at the place where it falls.
It can be undertaken through a variety of ways, like, capturing runoff from rooftops, capturing runoff from local catchments, capturing seasonal floodwaters from local streams or conserving water through watershed management. These techniques can serve the purposes of providing water for drinking, irrigation, increasing groundwater recharge, reducing storm water discharges, urban floods and overloading of sewage treatment plants and reducing seawater ingress in coastal areas.
Present Scenario
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 equals rainfall (mm) multiplied by 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 estimating the collection efficiency.
Community based rainwater harvesting in rural areas of India - the paradigm of the past - has in it as much strength today as it ever did before. It is, in fact, only with this rudimentary technology that people are able to survive in water scarce areas. Recognising this fact, our ancestors had learnt to harvest water in a number of ways. They harvested the raindrops directly, from rooftops they collected water and stored it in tanks built in their courtyards, from open community lands, they collected the rain and stored it in artificial wells. They harvested monsoon runoff by capturing water from swollen streams during the monsoon season and stored it in various forms of water bodies.
Today there is a lot of thrust on urban rainwater harvesting a few techniques of roof top rainwater harvesting are harvesting through recharge pit, recharge trench, tubewells and trench with recharge wells.
In areas where the surface soil is impervious and large quantities of roofwater or surface runoff is available within a very short period of heavy rainfall, the use of trench/pits is made to store the water in a filter media and subsequently recharged to groundwater through specially constructed rechargewells.
The Centre’s efforts in this regard through the Ministry of Urban Affairs and Poverty Alleviation has made rainwater harvesting mandatory in all new buildings with a roof area of more than 100 sq m and in all plots with an area of more than 1000 sq m, that are being developed. Similarly rainwater harvesting has been made mandatory in buildings with an area of 250 sq m or more in Indore (Madhya Pradesh); with an area of 1000 sq m or more in Kanpur (Uttar Pradesh); with an area of 300 sq m or more Hyderabad (Andhra Pradesh) and by the governments of Tamil Nadu, Rajasthan, Gujarat and Mumbai. In the notified areas in Gurgaon town and the adjoining industrial areas too all the institutions and residential colonies have been asked to adopt water harvesting by the Central Ground Water Authority (CGWA). The CGWA has also banned drilling of tubewells.
West Bengal : A Case Study
Though West Bengal generally receives a fair amount of rainfall during the monsoons, it is found that no measures are usually taken for utilisation of this resource. Most of the rainwater finds its way to drains and nullahs. The water supplied mostly by extraction of ground water and to some extent, by surface water through treatment is used to meet the demand in West Bengal. Thus if rainwater is collected and stored for use could contribute to water conservation.
The rapid urbanisation and industrialisation have also meant rapid withdrawal of ground water without paying much heed to the hydro geological characteristics of the area. Also, ground water is being almost continually withdrawn for various purposes, but recharge or replenishment of the same is not compensated. As a result, the water level in ground is depleting day by day in Bengal.
Legislation plays a very important role in achieving compliance. In some states legislation on rainwater harvesting is in place by promulgating ordinances .
With these views in mind, the West Bengal Pollution Control Board (WBPCB) organised a workshop some time back to create awareness on water conservation and rainwater harvesting, and also to disseminate information on the low cost technological solutions that are available for the same.
The State government has taken several initiates to popularise rainwater harvesting in West Bengal also of late. The State Water Investigation Directorate (SWID) has taken up various pilot schemes for artificial recharge of ground water. These projects have been taken up in the districts of Purulia and Bankura. The Directorate is also engaged in periodical monitoring of ground water level throughout West Bengal. The Central Ground Water Board has taken up a pilot project for artificial recharge of ground water at Baishnabghata–Patuli Township. The West Bengal Pollution Control Board have imposed an additional condition to the effect that all new housing projects (of more than 100 dwelling units and a super-built area of 60,000 sq.ft.) should build rainwater-harvesting structures.
*Special Correspondent, IANS
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