Tips and Technique of Rooftop Rainwater Harvesting

Sanjay Kumar Ray
ACTO, ICAR Nagaland Centre

 

1. Introduction: Water is the most precious natural resource, is essential for all life and for sustaining the ecosystems. Water is a finite and vulnerable resource and should be used efficiently, equitably and in an ecologically sound manner for the present and for the future generations. Freshwater is a becoming a scarce commodity and it is considered as liquid gold in most part of the country. The demand of water is increasing day by day not only for agriculture but also for household and industries. Out of the total water resources, fresh water constitutes only about 2.5%. Nearly 30% of total fresh water are in liquid form and 80% of it is not readily available. That means only less than 0.01% of the total water is available for human development (Das et al., 2012). Now we are increasingly becoming aware of the importance of water for our survival and its limitation for both in terms of quantity and quality. The peoples of the NE part of India usually face water scarcity problem during the month of November to March. However, the region receives an average annual rainfall of about 2000-2500 mm and almost all the rainwater is lost as surface runoff by reason of hilly terrain.

 

Rooftop rainwater harvesting is the technique through which rain water is captured from the roof catchments and stored in reservoirs. The rooftop rainwater harvesting system is an ideal practice in NE regions where groundwater and surface sources are lacking or insufficient. This rainwater harvesting system has special importance for the hilly farmers. The rainwater is pure and free from bacteriological contamination, organic matter and soft in nature. The quality of the rooftop rainwater harvesting system is being good and could also be used for drinking purposes in addition to agriculture and domestic use.

 

2. Components of Rooftop Rainwater Harvesting System: A typical rooftop rainwater harvesting system comprises of following components likes; Roof catchment, Gutters, Down pipe and first flush pipe, Filter unit, Storage tank, Collection pit and Recharge well

 

2.1. Roof Catchment: Roof of the house serves as the catchment area for harvesting rainwater. Catchment area or simply catchment has defined the area which contributes its runoff to a single point. Roof may be of galvanized iron or asbestos sheet, clay tiles or concrete. The amount of rain falling on the roof is channelized through guttering and pipe system to a storage tank or recharge pit. In case of rooftop water harvesting the amount of runoff is taken as 80-90% of the rainfall amount. It is considered that 10-20% of rainwater is lost through splash and leakage. Information about the annual rainfall pattern of any particular area helps to calculate amount of water could be harvested in a particular catchment area by measuring its length and breadth. Formula for calculating the quantity of rainwater to be collected:

 

Total quantity of water to be collected (cubic meter) = Roof Top Area (Sq.m.) x Average Monsoon Rainfall (meter) x 0.8

Note: 1 cubic meter = 1000 litres of water

 

For example: Annual rainfall of any particular area is 1000 mm and roof area is 100 (20m x5m) square meters. Therefore, rainwater falling on the roof is 100,000 liters. If 80% can be collected then 80,000 liters of rainwater is available for collection annually in that particular area.

 

2.2. Gutters: The PVC gutter collects roof water from the roof and conveys the water to the down pipe. Gutter is a semi circular channel which can be fabricated from plain galvanized sheet. Now-a-days a ready made plastic gutters are also available in the market. If plastic gutters are available and it should be preferred as plastic is corrosion resistant. Gutters are fixed below the roof sheet along its length to receive the runoff. A gentle slope, normally of 0.5% towards the outlet is provided along the gutter (Fig. 1).

 

Fig. 1 Ideal gutter for rooftop rainwater harvesting.

 

Fig. 3 Storage pit

 

Fig. 2 First rain separator.

 

2.3. Down Pipe and First Flush Pipe: Gutters convey the runoff from all sides of roof to a common point. Then from that point runoff is to be conveyed down on the ground and to the storage tanks through pipes called down pipes. A valve or ninety degree elbow is fitted at a suitable location in the down pipe to flush out (Fig. 2) the initial runoff, which may contain impurities in the form of dust, bird droppings or toxic substances. Either plastic or soil pipe should be used to avoid rusting and to reduce the cost.

 

2.4. Filter Unit: Sometimes wind may also accompany rain due to which runoff may contain foreign materials such as leaves and small twigs. Sometimes, rusts from the cast iron pipes may also enter the water tank. These foreign materials may contaminate harvested stored water rendering it unfit for domestic use.

 

2.5. Storage Tanks: The rainwater storage tank collects all the filtered rainwater and keeps it for future use. Storage tanks can be made above the ground or below the ground as per the requirement and convenience. Paint white on the outside to keep the water inside cool and prevent the growth of bacteria.

 

2.6. Collection Pit: The harvested rainwater from rooftop may also be diverted in the collection pit for storage (Fig. 3). The size of pit depends upon the water yield from the roof catchment and water demand.

 

3. Benefits of Rooftop Rainwater Harvesting: Benefits of rainwater harvesting are immense as it offers an ideal solution to problems in areas having adequate water resources. Some specific benefits are;

 

  • Provides self-sufficiency in water supply.
  • The rooftop rainwater harvesting is less expensive, simple and flexible, easy to construct, operate and maintain.
  • It provides an essential reserve in times of breakdown of public water supply systems, particularly during natural disasters.
  • Crop productivity can be increased by two-three folds because of assured irrigation in kitchen garden for vegetables viz. pea, tomato, cabbage, cauliflower, french bean, carrot, coriander etc. even during lean periods. Fishery and animal husbandry may flourish.
  • Improve soil moisture content.

 

4. Conclusion: Rooftop rainwater harvesting for crop and livestock production has a great potential for enhancement of farm income and livelihood development of the farming community. It has a significant role in respect of irrigating crop and vegetables of hill agriculture and drinking purposes of livestock animals like piggery and poultry. Kitchen gardening could also be started with harvested rainwater. The enhancement in productivity, income and employment are due to diversified farming activities by efficient utilization of harvested water and this year round farming activities may generate more employment and higher productivity.