What is Rainwater Harvesting


Rainwater harvesting is the accumulation and deposition of rainwater for reuse on-site, rather than allowing it to run off. Its uses include water for garden, water for livestock, water for irrigation, water for domestic use with proper treatment, and indoor heating for houses etc.

Rainwater Harvesting and Conservation is the activity of direct collection of rain water. The conservation of rain water so collected can be stored for direct use or can be re-charged into the ground water. The main goal is to minimize flow of rain water through drains / nallahs to the rivers without making any use of the same.

It is a known fact that ground water level is depleting and going down and down in the last decades. Thus Rainwater Harvesting and Conservation aims at optimum utilization of the natural resource that is rain water, which is the first form of water we know in the hydrological cycle and hence is a primary source of water for us. The rivers, lakes and ground water are secondary sources of water. In present time, in absence of Rainwater Harvesting and Conservation, we depend entirely on such secondary sources of water and in the process it is forgotten that rain is the ultimate source that feeds to these secondary sources. The value of this important primary source of water must not be lost. Rainwater Harvesting and Conservation means to understand the value of rain and to make optimum use of rain water at the place where it falls.

Why to harvest rain?

  • In areas where there is inadequate groundwater supply or surface resources are either lacking or insufficient, rainwater harvesting offers an ideal solution.
  • Helps in utilising the primary source of water and prevent the runoff from going into sewer or storm drains, thereby reducing the load on treatment plants
  • Reduces urban flooding
  • Recharging water into the aquifers help in improving the quality of existing groundwater through dilution.

The artificial recharge to ground water is a process by which the ground water reservoir is augmented at a rate exceeding that obtained under natural conditions of replenishment.

The basic purpose of artificial recharge of ground water is to restore supplies from aquifers depleted due to excessive ground water development. There are two main techniques of rainwater harvesting:

  • Storage of rainwater on surface for future use
  • Recharge ground water

Rainwater Harvesting from roofs consists of collecting, storing, and putting to use rooftop rainwater from houses or any construction is Rooftop Rainwater Harvesting.

Rainwater Harvesting can also be collecting, filtering and recharging ground water through percolation pits, open wells or bore wells.


Benefits of Rainwater Harvesting


Geologically and geomorphologically, Kolkata belongs to the lower deltaic plain of the Ganga-Padma river system.. The surface material is clay and clay loam. This clay extends up to a depth of 10 to 25 m bgl in most of the area. Below this clay bed a fine sand bed is found which extends up to a depth of 30-to 35-metre bgl. Below this level another clay, dark brown to grayish brown in colour occur up to a depth of 60 to 100-metre bgl. From this depth another sand zone occur which comprises of fine, medium and coarse sand and extends up to a depth of 120 to 180 metre bgl. Below this sand zone gravel bed occurs. Tertiary black and sticky clay occurs at the bottom of the sand and gravel zone.

Due to unique geological setting rainfall in Kolkata does not percolate downwards to recharge the groundwater regime. Rainwater is totally wasted and poses a serious problem like water logging and sewage overload.

The demand of water for domestic purpose in Kolkata is increasing day by day. The municipal water supply to some extent and private domestic water supply in some areas also depend on ground water. Since Kolkata is not within the groundwater recharge zone and the withdrawal of groundwater is very high in comparison with the groundwater recharge from remote area groundwater level has already depleted to a great extent. Recent studies have revealed that groundwater level of Kolkata has already gone down to 10 to 13 metre below sea level.

To combat all the problems related to the water supply and demand, and also for the problems developed due to over withdrawal of groundwater scientists are advising to utilize rainwater in urban areas.

It will never be possible to harness all available rainwater through any type of harvesting system. Moreover the water requirement of Kolkata is much higher than the amount of rainfall falling directly overhead. So Kolkata will remain dependent on the transported water from upstream regions. But rainwater harvesting in Kolkata has three visible benefits.

  1. If a large amount of rainwater is used for groundwater recharge it will supplement the natural recharge to some extent.
  2. If about 10% of rainwater is harvested properly through some well managed system in specific locations it will significantly reduce water logging during storm seasons.
  3. If rainwater is harvested by individuals or by community housing societies the consumption of piped water will be reduced to a great extent.

Design of storage tanks

The volume of the storage tank can be determined by the following factors:

  • Number of persons in the household: The greater the number of persons, the greater the storage capacity required to achieve the same efficiency of fewer people under the same roof area.
  • Per capita water requirement: This varies from household to household based on habits and also from season to season. Consumption rate has an impact on the storage systems design as well as the duration to which stored rainwater can last.
  • Period of water scarcity: Apart from the total rainfall, the pattern of rainfall -whether evenly distributed through the year or concentrated in certain periods will determine the storage requirement. The more distributed the pattern, the lesser the size.
  • Type and size of the catchment: Type of roofing material determines the selection of the runoff coefficient for designs. Size could be assessed by measuring the area covered by the catchment i.e., the length and horizontal width. Larger the catchment, larger the size of the required cistern (tank).

Can rainwater harvested at ones house?

Yes one can. Structures to harvest rain require little space. A dried borewell, a row of soak pits or a tank--concealed below the ground- are all that one needs. The open spaces -- rooftops and ground - can be used as catchment (surface to catch rain).

How much will it cost?

It varies, depending on the area of the roof and other structures that will be used to harvest rain. But rainwater harvesting does not require major construction work, so the expenses suit one’s pockets.

Who will build it and how long will it take?

One need someone who understands rainwater harvesting. It is simple but it still needs someone who has experience in the principles of rainwater harvesting. Then a skilled mason or a plumber can do the job for you within 10 days.

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