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Friday, January 28, 2011

RAIN WATER HARVESTING AND ARTIFICIAL RECHARGE TO GROUND WATER

WHAT IS RAIN WATER HARVESTING ?

The principle of collecting and using precipitation from a catchment surface. An old technology is gaining popularity in a new way. Rain water harvesting is enjoying a renaissance of sorts in the world, but it traces its history to biblical times. Extensive rain water harvesting apparatus existed 4000 years ago in the Palestine and Greece. In ancient Rome, residences were built with individual cisterns and paved courtyards to capture rain water to augment water from city's aqueducts. As early as the third millennium BC, farming communities in Baluchistan impounded rain water and used it for irrigation dames, build of stone rubble, were found in Baluchistan and Kutch in Gujarat in India.

ARTIFICIAL RECHARGE TO GROUND WATER

Artificial recharge to ground water is a process by which the ground water reservoir is augmented at a rate exceeding that obtaining under natural conditions of replenishment. Any man-made scheme or facility that adds water to an aquifer may be considered to be an artificial recharge system.


WHY RAIN WATER HARVESTING ?

Rain water harvesting is essential because:-
1. Surface water is inadequate to meet our demand and we have to depend on ground water.
2. Due to rapid urbanization, infiltration of rain water into the sub-soil has decreased drastically and recharging  of ground water has diminished.
3. Over - exploitation of ground water resource has resulted in decline in water levels in most part of the country.
4. To enhance availability of ground water at specific place and time.
5. To arrest sea water ingress.
6. To improve the water quality in aquifers.
7. To improve the vegetation cover.
8. To raise the water levels in wells & bore wells that are drying up.
9. To reduce power consumption.

RAIN WATER HARVESTING TECHNIQUES

There are two main techniques of rain water harvesting.
a) Storage of rain water on surface for future use.
b) Recharge to ground water.
The storage of rain water on surface is a traditional technique and structures used were underground tanks, Ponds, check dams, weirs, etc. Recharge to ground water is a new concept of rain water harvesting and the structures generally used are:-
1. Pits:- Recharge pits are constructed for recharging the shallow aquifers. These are constructed 1 to 2 m. wide and 2 to 3 m. deep which are back filled with boulders, gravels & coarse sand.
2. Trenches:- These are constructed when the permeable strata is available at shallow depths. Trench may be 0.5 to 1 m. wide, 1 to 1.5 m. deep and 10 to 20 m. long depending upon availability of water. These are back filled with filter materials.
3. Dug wells:- Existing dug wells may be utilised as recharge structure and water should pass through filter media before putting into dug well.
4. Hand pumps:- The existing hand pumps may be used for recharging the shallow / deep aquifers, if the availability of water is limited. Water should pass through filter media before diverting it into hand pumps.
5. Recharge wells:- Recharge wells of 100 to 300 mm. diameter are generally constructed for recharging the deeper aquifers and water is passed through filter media to avoid choking of recharge wells.
6. Recharge Shafts:- For recharging the shallow aquifers which are located below clayey surface, recharge shafts of 0.5 to 3 m. diameter and 10 to 15 m. deep are constructed and back filled with boulders, gravels & coarse sand.
7. Lateral shafts with bore wells:- For recharging the upper as well as deeper aquifers lateral shafts of 1.5 to 2 m. wide & 10 to 30 m. long depending upon availability of water with one or two bore wells are constructed. The lateral shafts is back filled with boulders, gravels & coarse sand. 8. Spreading techniques:- When Permeable strata starts from top then this technique is used. Spread the water in streams / Nalas by making check dams, nala bunds, cement plugs, gabion structures or a percolation pond may be constructed.


BENEFITS
1. An ideal solution to water problems in areas having inadequate water resources.
2. The ground water level will rise.
3. Mitigates the effects of drought & achieves drought proofing.
4. Reduces the runoff which chokes the storm water drains.
5. Flooding of roads is reduced.
6. Quality of water improves.
7. Soil erosion will be reduced.
8. Saving of energy per well for lifting of ground water – a one meter rise in water level saves about 0.40 KWH of electricity.


Source : UNESCO, 2000. Rain Water Harvesting and Artificial Recharge to Groundwater. New Delhi : Central Groundwater Board Ministry of Water Resources.
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