In rooftop rainwater harvesting, the rainwater is collected from roof of the buildings and stored in ground water reservoir for beneficial use in future. To meet ever increasing demand for water in urban areas. To reduce the runoff which is choking the storm drains. To avoid the flooding of roads. To augment the ground water storage and control decline of water levels. To reduce the ground water pollution. To improve the quality of ground water. To reduce the soil erosion. This is an ideal solution of water problem where there is inadequate ground water supply, or surface resources are either lacking or insignificant. To utilise the rainfall runoff, which is going to sewer or storm drains. Rainwater is bacteriologically pure, free from organic matter and soft in nature. It will help in reducing the flood hazard. To improve the quality of existing ground water through dilution. To remove bacteriological and other impurities from sewage and waste water so that the water is suitable for re-use. Rainwater may be harnessed at place of need and may be utilised at time of need. The structures required for harvesting the rainwater are simple, economical and eco-friendly.   Roof top rainwater is recharge of ground water reservoir through Abandoned dug well Abandoned/running hand pump Recharge pit Recharge trench Gravity head recharge well Recharge shaft Abandoned Dug Well Abandoned / Running Hand Pump Recharge Pit Recharge Trench Gravity Head Recharge Well Recharge Shaft   1. Abandoned Dug Well A dry/unused dug well can be used as a recharge structure. The recharge water is guided through a pipe to the bottom of well or below the water level to avoid scouring of bottom and entrapment of air bubbles in the aquifer. Before using the dug well as recharge structure, its bottom should be cleaned and all the fine deposits should be removed. Recharge water should be silt free. It should be cleaned regularly. It is suitable for large building having the roof area more than 500 sq.m. Periodic chlorination should be done for controlling the bacteriological contaminations This design is suitable for areas with shallow (5 to 15 m) water level in hard rock as well as soft rock terrain for individual houses, group housing societies, schools and small industrial sheds. 2. Abandoned / Running Hand Pump An abandoned / running hand pump can be used for recharge. The structures are suitable for the small building having the roof area upto 150 sq.m. Water is diverted from rooftop to the hand pump through pipe of 50 to 100 mm dia. For running hand pump a closing valve is fitted in conveyance system near hand pump to avoid entry of air in suction pipe. Recharge water should be silt free. During recharging period, the water extracted from hand pump should be utilised after proper chlorination. This design is suitable for areas with moderate to deep water level in both hard rock as well as soft areas for individual houses, group housing societies, schools and small industrial sheds.   3. Recharge Pit Recharge pits are constructed for recharging the shallow aquifer. These are constructed generally 1 to 2 m wide and 2 to 3 m deep. After excavation, the pits are refilled with pebbles and boulders. Water to be recharged, should be silt free. Cleaning of the pit should be done periodically. It is suitable for small buildings having the roof top area upto 100 sq.m. Recharge pit may be of any shape i.e. circular, square or rectangular. If the pit is of trapezoidal shape, the side slopes should be steep enough to avoid silt deposition. This design is suitable for areas with shallow water level in hard rock as well as soft rock areas for individual houses, group housing societies, schools and small industrial sheds. 4. Recharge Trench It is constructed when permeable strata of adequate thickness is available at shallow depth. It is a trench of shallow depth filled with pebbles and boulders. These are constructed across the land slope. The trench may be 0.5 to 1 m wide, 1 to 1.5 m deep and 10 to 20 m long depending upon the availability of land and roof top area. It is suitable for the buildings having the roof area of 200 to 300 sq.m. Cleaning of trench should be done periodically. This design is suitable for areas with shallow water level in hard rock as well as soft rock areas for large group housing societies, schools and small industrial sheds.   5. Gravity Head Recharge Well Bore wells / tubewells can be used as recharge structure. This technique is suitable where Land availability is limited. When aquifer is deep and overlain by impermeable strata (clay) The roof top rainwater is channelised to the well and recharges under gravity flow condition. Recharge water should be silt free. The well can also be used for pumping. Most suitable for the areas where ground water levels are deep. The number of recharging structures can be determined in limited area around the buildings depending upon roof top area and aquifer characteristics. This design is suitable for areas having deep water level in soft rock areas with multiple aquifer system, suitable for large group housing societies, schools and industrial sheds.   6. Recharge Shaft A recharge shaft is dug manually or drilled by the reverse/direct rotary method. Diameter of recharge shaft varies from 0.5 to 3 m depending upon the availability of water to be recharged. It is constructed where the shallow aquifer is located below clayey surface. Recharge shaft is back filled with boulders, gravels and coarse sand. It should end in more permeable strata (sand). Depth of recharge shaft varies from 10-15 m below ground level. Recharge shaft should be constructed 10 to 15 m away from building for the safety of the building. It should be cleaned regularly by scraping the top layer of sand and refilling it periodically. This design is suitable for areas having shallow water level in hard rock and soft rock areas with shallow water level, suitable for group housing societies, schools and small industrial sheds.   Ground water recharge from house of 100 sqm of rooftop will be 66,000 liters in one year, sufficient for a period of four months for a normal family size of five members. For the Ahmedabad city as a whole, the additional recharge to the groundwater will be around 22,000 million liters per annum which is 13% of annual requirement. Estimation of Potential Runoff available from Roof Top AVERAGE ANNUAL RAINFALL = 732.13 mm ROOF AREA (m2) TOTAL RAINFALL VOLUME VOLUME AVAILABLE FROM RAINFALL (90%) (cum) TYPE OF STRUCTURE RECOMMENDED FOR RECHARGE       Alluvial area Hard Rock area 50 37.61 32.95 Recharge pit/ hand pump Recharge pit/ hand pump 100 73.21 65.89 " " 150 109.82 94.84 " Trench/hand pump 200 146.43 131.78 Trench Trench 250 183.03 164.73 " " 300 219.64 197.68 Gravity head recharge well Gravity head recharge well 350 256.25 230.62 " " 400 292.85 263.57 " " 450 329.46 296.51 " " 500 366.07 329.46 " " 1000 732.13 658.92 " Recharging shaft/dug well 2000 1464.26 1317.83 Recharging shaft/dug well " 2500 1830.33 1647.29 " " 3000 2196.39 1976.75 " " 4000 2928.52 2635.67 " " 5000 3660.65 3294.59 " "