Rain Water Harvesting is the technique of collection and storage of rain water at surface or in sub-surface aquifer,before it is lost as surface run off.The augmented resource can be harvested in the ground water reservoi is augmented at a rate exceeding that under natural conditions of replenishment.

ROOF TOP RAIN WATER HARVESTING THROUGH RECHARGE PIT

1. In alluvial areas where permeable rocks are exposed on land sueface or at very shallow depth,roof top rain water harvesting can be done through recharge pits.
   
   
2. The technique is sutiable for buldings having a roof area of 100sq.m.and are constructed for recharging the shallow aquifers.
   
   
3. Recharge Pits may be of any shape and size and are generally constructed 1 to 2m.wide and 2 to 3m.deep which are back filled with boulders (5-20cm),gravels(5-10mm) and coarse sand(1.5-2mm) in graded form-Boulders at the bottom,gravels in brtween and corase sand at the top of the slit content that comes with runoff will be desposited on the top of the corase sand layer and can easily be removed.For smaller roof area, pit may be filled with broken bricks/cobbles.
   
   
4. A mesh should be provided at the roof so that leaves or any other solid waste/debris is prevented from entering the pit and a desilting/collection chamber may alsobe provided at the ground to arrest the flow of finer paeticles to the recharge pit.
   
   
5. The top layer of sand should be cleaned periodically tp maintain the recharge rate.
   
   
6. By-pass arrangement be provided before the collection chamber to reject the first showers.
   

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1. Recharge trenches are sutiable for bulidings having roof area of 200-300sq.m.and where permeable strata is avalible at shallow depts.
   
   
2. Trench may be 0.5 to 1.0m.deep and 10 to 20m. long depending upon avalibility of water to be recharged.
   
   
3. These are back filled with boulders (5-20cm) gravels (5-10mm) and coarse sand (1.5-2mm) in graded form-boulders at the bottom,gravel in between and coarse sand at the top of that the slit content that will come with runoff will be deposited on the top of the sand layer and can easily be removed.
   
   
4. A mesh should be provided at the roof so that it leavesa or any other soild waste/debris is prevented from entering the trench and a desilting/collection chamber may also be provided on ground to arrest the flow of finner particles to trench.
   
   
5. By-pass arrangement be provided before the collection chamber to reject the first showers.
   
   
6. The top layer of sand should be cleaned periodically to maintain the recharge rate.
   

   
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ROOF TOP RAIN WATER HARVESTING THROUGH EXISTING TUBEWELLS

1. In areas where the shallow aquifiers have dried up and existing tubewells are tapping deeper aquifer,roof top rain water harvesting through existing tubewell can be adopted to recharge the deeper aquifers.
   
   
2. PVC pipes of 10cm dia are connected to roof drains to collect rain water.The first roof runoff is let off through the bottom of drain pipe.It's diameter should vry depending on the area of roof,15cm if roof area is less than 150sq.m.and 20cm.if the roof area ois more.The filter is provided with a reducer of 6.25. on both the sides.Filter is divided into three chambers by PVC screens so that filter material is not mixed up.The first chambers is filled up with gravel(6-10mm),middle chamber with pebbles (12-20mm) and last with gravel (6-10mm),middle chamber with pebbles (12-20mm) and last chamber with bigger pebbles (20-40mm).
   
   
3. If the roof area is more,a filter pit may be provided.Rain water from roof is taken to collection/desilting chambers located on ground.These collection chambers are interconnected as well as connected to the filter pit through pipes having a slope of 1.5.The filter pit through pipes having a slope of 1:15.The filter pit may vary in shape and size depending upon avalible run off and are back-filled with varying thickness (0.30-0.50cm) and may be separated by screen.The pit is divided into two chambers,filter material in one chamber and other chamber is kept empty to accommodate exess filtered water and to monitor the quality of filterred water.A connecting pipe with recharge well is provided at the bottom of the pit for recharging of filtered water through well.
   
   

ROOF TOP RAIN WATER HARVESTING THROUGH TRENCH WITH RECHARGEWELL.

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1. In areas where surface soil is imprevious and large quantities of roof water or surface runoff is avalible within a very short period of heavy rainfall,the use of trench/pits is made to store the water in a filter media and subquently recharge to ground water through specially constructed recharge wells.
   
   
2. This techniques is ideally suited for area where preable horizon is within 3m below ground level.
   
   
3. Recharge well of 100-300mm diameter is constructed to a depth of at least 3 to 5m below the water level.Based on the lithology of the area,well assembly is designed with slotted pipe against the shallow and deeper aquifer.
   
   
4. A lateral trench of 1.5 to 3m. width and 10 to 30m.lenght,depending upon the avalibility of water is constructed with the recharge well in the center.
   
   
5. The number of recharging wells in the trench canbe decided on the basis of water avalibility and local vertical permeability of rocks.
   
   
6. The trench is backfilled with boulders,gravel and coarse sand to act as a filter media for the recharge wells.
   
   
7. If the aquifer is avalible at greater depth say more than 20m,.,a shallow shaft of 2 to 5m.diameter and 3-5meters deep may be constructed depending upon avalibility runoff.Inside the shaft a recharge well of 100-300mm dia is constructed for recharging the avaliable water to deeper aquifers.At the bottom of the shaft a filtter media is provided to avoid choking of recharge well.

MONTHLY RAINFALL IN KARNATAKA-DISTRICT WISE DETAIL