CONDITIONS REQUIRED FOR RAIN WATER CONSERVATION :

	Wherever hydraulic gradient is very high i.e. in hilly area only rain water conservation can be done in PVC tanks, Cemented tanks and also in cemented lakes/tanks.
	In any area where hydraulic gradient is not very high but hydrogeological condition is not feasible for artificial recharge to ground water there rain water conservation may be done in PVC tanks, Cemented tanks and in ponds with clayey base.
	In any area where ground water is polluted or saline there also rain water conservation techniques are may be adopted.

TECHNIQUES OF ARTIFICIAL RECHARGE TO GROUND WATER BY RAINWATER :

A wide spectrum of techniques is in vogue for recharging the groundwater by rainwater and these techniques will depend mainly on local hydrogeological and topographical conditions. The artificial recharge techniques can broadly be categorized as Direct Surface Techniques, Direct Sub Surface Techniques, Combination of Surface and Sub Surface Techniques and Indirect Techniques.

Different techniques under Direct Surface Techniques are Flooding, Ditch and Furrow Systems, Gully Plug, Check Dam, Nala Bund, Percolation Tank, Gabion Structure and Over Irrigation and under Direct Sub Surface Techniques these are Dug Wells, Recharge Shafts with or without injection well.

Indirect Techniques include Induced Recharge from Surface Water Sources, eg – pumping from the aquifer hydraulically connected with surface water will induce recharge to ground water because cone of depression developed due to pumping intercepts surface water recharge boundary and a hydraulic connection gets established with the surface water which starts providing part of pumpage yield.

CONDITIONS GOVERNING THE TECHNIQUES OF ARTIFICIAL RECHARGE TO GROUNDWATER BY RAINWATER

If artificial recharge to groundwater in an area is necessary, then the techniques to be adopted are to be based on the knowledge of the HYDROGEOLOGICAL and the TOPOGRAPHICAL conditions of the concerned area.

Hydrogeology : Aquifer conditions (unconfined / semi-confined/ confined), aquifer potentiality and geometry, ground water flow directions, depth to water level, chemical quality of ground water etc. are to be under stood.

Topography : Terrain conditions (hilly/undulating/flat), natural drainage density, slope of the stream bed etc. are to be understood.
One can adopt the techniques of artificial recharge to ground water in a particular area if the information desired above for that area are known.

EFFECT ON ENVIRONMENT

The following are the effects on environment during artificial recharge to groundwater by rainwater harvesting.

· Schemes of artificial recharge to groundwater executed only after considering the pre monsoon water levels declining trend and not post monsoon water levels trend (if it does not show declining trend) may have negative effect on the environment.
In certain parts like Sunderban delta in West Bengal both pre and post monsoon groundwater level show declining trend but the groundwater level is very shallow. In this situation artificial recharge to groundwater is not feasible. Conservation of rainwater is the best solution, as it will reduce the withdrawal of groundwater and help in arresting the declining trend of groundwater level.

· In many parts of our country ponds are common and generally rainwater recharges these ponds. Many of these ponds over flow during normal monsoon period and this surplus water may be utilized for recharging the ground water (wherever necessary).

· Check dams and Nalah Bunds should be constructed on a streambed that has got gentle slope and by this the submergence area and the quantum of water stored will be more.

· Any work related to artificial recharge to groundwater is to be supported by monitoring of water level and to have better effect on environment attempt has to be made so that by artificial recharge the water level should not rise above 3m below ground level (post monsoon period).

· In coastal area or in the area having inland salinity artificial recharge to groundwater may reduce the salinity of groundwater by diluting the salt concentration of the groundwater or may forms a lens of fresh water over the saline water due to gravitation. In areas where groundwater is polluted by the presence of some chemical constituents like arsenic, fluoride etc above permissible limit or by industrial effluents artificial recharge is a very effective tool to improve the quality.

· In undulating terrain if percolation tanks are located near to any stream then there is possibility that water from percolation tank, after percolating may reach stream without giving any benefit of recharge. In such situation depending on the availability of water, a few check dams / nalah bunds are to be constructed on the streambed located near to the percolation tank. These techniques are suitable for Purulia and Bankura districts of West Bengal.

· While deciding the height of percolation tank/check dam/nalah bund water demand of the people of the down slope/down stream side is to be kept in mind and certain amount of water is to be allowed to flow as over flow for these people.

· If it is decided that the groundwater of the confined aquifer is to be recharged and which can be done by the tube well only then the design of the recharge well should be such that the shallow aquifer is separated out from the confined aquifer by proper clay/cement sealing (e.g. Kolkata metropolitan area).

· Wherever planning is to be made for artificial recharge to groundwater by rainwater through recharge well/shaft, the rainwater needs to be silt and sand free.

· In hilly terrain artificial recharge to groundwater by rain water is to be avoided