Raindrops on the Roof Top by V. K. Joshi (Bijji) SignUp
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Environment Share This Page
Raindrops on the Roof Top
by V. K. Joshi (Bijji) Bookmark and Share

Groundwater resource is depleting faster than the recharge. The problem has been compounded by a poor management especially in the urban centers. The State makes provision for drinking water supply yet in many parts of a town/metro people punch their own boreholes to enjoy, free, round the clock water supply. As has been oft repeated through these articles, groundwater reservoir is like a bank account. If several persons withdraw money from the same account without depositing anything they will be in the red soon.

 

Roof Top Rainwater Harvesting

Instead of only withdrawal if the joint account holders keep depositing small amounts regularly, their account will rather grow. Likewise Roof Top Rain Water Harvesting is like a small recurring deposit scheme. Every single drop of rainwater is carefully diverted to storage devices/groundwater reservoirs. Under normal circumstances rain water from the roof tops flows through the down-pipes to the ground and lost as a run-off. Somehow if this water is collected through a drain pipe along the perimeter of the roof and diverted to the structure where it has to be stored/utilized significantly large quantities of rainwater can be harvested.

How much water can be harvested this way can not be comprehended by a layman. The Central Groundwater Board (CGWB) has computed a table which depicts the volume of water available through rooftop harvesting:

Rainwater Available from the Roof
Rainfall (mm) 100 200 300 400 500 600 800 1000
Area of the Roof (Sq meter) Rainwater available for storage/recharge (cu m)
20 1.6 3.2 4.8 6.4 8 9.6 12.8 16
30 2.4 4.8 7.2 9.6 12 14.4 19.2 24
40 3.2 6.4 9.6 12.8 16 19.2 25.6 32
50 4 8 12 16 20 24 32 40
60 4.8 9.6 14.4 19.2 24 28.8 38.4 48
70 5.6 11.2 16.8 22.4 28 33.6 44.8 56
80 6.4 12.8 19.2 25.6 32 38.4 51.2 64
90 7.2 14.4 21.6 28.8 36 43.2 57.6 72
100 8 16 24 32 40 48 64 80
150 12 24 36 48 60 72 96 120
200 16 32 48 64 80 96 128 160
250 20 40 60 80 100 120 160 200
300 24 48 72 96 120 144 192 240
400 32 64 96 128 160 192 256 320
500 40 80 120 160 200 240 320 400
1000 80 160 240 320 400 480 640 800
2000 160 320 480 640 800 960 1280 1600
3000 240 480 720 960 1200 1440 1920 2400

Statistically the figures appear quite amazing, but a lot depends upon the rainfall and geological, geomorphological and geohydrological conditions of the ground. The techniques have to be varied accordingly. Some of the techniques include:

Recharge Pit

In the areas where thickness of alluvium or weathered rock is few meters only, underlain by a rocky substratum a recharge pit (see figure) is suitable. The pit is filled with gravel to act as a porous filter for water. This method is suitable for rooftops up to 100 sq m.

Continued

Image of Umbrellas under license with Gettyimages.com
Images by courtesy of CGWB

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23-Jul-2006
More by :  V. K. Joshi (Bijji)
 
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