Rainwater Harvesting

Water of Class 6

RAINWATER HARVESTING

One way of increasing the availability of water to overcome its shortage is rainwater harvesting. The activity of collecting rainwater directly and store it in big tanks for later use, or making the rainwater percolate into ground more efficiently to recharge the groundwater, is called rainwater harvesting. Thus, there are two main techniques of rainwater harvesting :

(i) Collection and storage of rainwater in tanks for future use when there is scarcity of water, and

(ii) To make rainwater percolate into the ground more efficiently by constructing percolation pits and recharge wells so as to recharge (or replenish) groundwater.

The groundwater which we take out for our use by digging wells, tube-wells and installing hand- pumps, comes from the rainwater which seeps into the ground naturally. Now, when too much water is drawn through wells and tube-wells for use in homes, agriculture and industry, the water level in the wells and tube-wells goes down drastically. Some of the wells and tube-wells may even dry up completely. This leads to shortage of groundwater.

In rural areas (village areas), most of the ground has open soil due to which rainwater can seep into the ground naturally to make up for the loss in groundwater due to excessive use. In urban areas (city areas), however, most of the ground is covered with buildings, concrete pavements and metalled roads due to which only very little rainwater seeps into the ground naturally. Most of the rainwater which falls in cities flows into dirty water drains and goes away. So, rainwater harvesting is necessary in city areas.

Rainwater harvesting by making more water percolate into the ground is usually done in those areas of a city where tube-wells for supplying water are located. This is to make sure that the tube-wells will never go dry. Rainwater harvesting can be done in two ways :

  1. Rooftop rainwater harvesting, and
  2. Rainwater harvesting from open spaces around buildings.

We will now describe both these methods of rainwater harvesting in somewhat detail, one by one

Rooftop Rainwater Harvesting

Rooftop rainwater harvesting can be done for two purposes : collecting rainwater falling on roof and storing it in big tanks for later use or collecting rainwater falling on roof and making it percolate into ground to recharge groundwater.

In one method of rooftop rainwater harvesting, the rainwater collecting on the roof of a house (or building) is brought down through rainwater pipe and collected in big underground tank made of concrete. This water may contain soil particles from the roof and needs to be filtered before use. The stored rainwater can be used in times of water scarcity.

In the second method of rooftop rainwater harvesting, the pipe bringing down rainwater collecting on the roof of a house (or building) goes directly into a percolation pit made in the ground. The percolation pit has a layer of broken bricks at the bottom; a layer of gravel in the middle and a layer of coarse sand at the top. It is covered by a concrete slab. A narrow bore (or hole) about 3 metres deep is dug into the ground. The outlet pipe com- ing out of percolation pit is put in this bore. This rainwater harvesting system works as follows.

The rainwater which collects on the roof of a house is brought down by rainwater pipe into the percolation pit. This rainwater gets filtered by the sand, gravel and broken bricks in the percolation pit.

The filtered rainwater comes out of the percolation pit through the outlet pipe. The outlet pipe takes the rainwater deep into ground where it can easily seep into the soil. Please note that the digging of a deep bore in the ground to put the water pipe in it is necessary to absorb rainwater in city areas because the depth of clay soil (through which water can seep easily) is more in such areas. So, whenever the depth of clay soil is more, rooftop rainwater harvesting to recharge groundwater is done through percolation pits with deep bore.

Rooftop rainwater harvesting collects only a small amount of rainwater at a time which can seep into the soil quickly through a percolation pit with a narrow bore (described above). If, however, rainwater harvesting is done from large, open spaces around buildings, then a very large amount of rainwater collects in a short period of time which cannot be absorbed into soil by a percolation pit with a narrow bore. In that case, a percolation pit with a wide recharge well is used. This is discussed on the next page.

Rainwater Harvesting From Open Spaces Around Buildings. The rainwater harvesting from open spaces around the buildings in a city is done by constructing percolation pits covered with concrete slabs having holes in them, and connected to a recharge well through a pipe. The recharge well is about 1 metre in diameter and 3 metres deep. The rain water falling in the open spaces around buildings goes into the percolation pit through the holes in its concrete slab cover. After filtration in percolation pit, rainwater enters the recharge well through the outlet pipe and gradually seeps into the soil. Please note that the purpose of recharge well is to collect the vast amount of water falling on the ground quickly when it rains and then make it seep into soil gradually. The main purpose of rainwater harvesting is not to hold rainwater on the surface of earth but to make rainwater percolate under the ground more efficiently so as to recharge (or replenish) groundwate. This groundwater can then be taken out through tube-wells as and when required. The advantage of rainwater harvesting is that it increases the availability of groundwater and helps in overcoming water shortage. We are now in a position to answer the following questions:

 

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