Rainwater harvesting is the technique of gathering or accumulating and storing of rainwater. Rainwater harvesting provides drinking water, domestic water, water for livestock, water for small irrigation and a way to increase groundwater levels. rainwater Harvesting has been practised for a long time now. In ancient times it was one of the main activity for storing water. The source of all water is rain. The water in lakes, ponds and wells is all sourced by the rain.
Rainfall Pattern and Rain Potential play a decisive role in storing or recharging water in a particular region. The sub-surface geology also plays an important role in making this decision. Wherever surface is impermeable recharging will not be feasible. Hence, it would be ideal to opt for storage. In places where the groundwater is saline or not of potable standards, the alternate system could be that of storing rainwater.
Major Sources for Rainwater Harvesting
- Rooftops: Buildings with impervious roofs provide effective free of charge water for various uses.
- Paved and Unpaved Areas: Landscapes, open fields, parks, stormwater drains, roads and pavements and other open areas can be effectively used to harvest the runoff. The main advantage in using the ground as collecting surface is that water can be collected from a larger area. This is particularly advantageous in areas of low rainfall.
- Waterbodies: Lakes, tanks and ponds are precious stores of rainwater. The harvested rainwater can not only be used to meet the water requirements of the city, but it also recharges groundwater aquifers.
- Stormwater Drains: Those regions which have pra oper network of stormwater drains provide a simple and cost-effective means for harvesting rainwater.
Methods of Rainwater Harvesting
There are Traditional and Modern Techniques used for Rainwater Harvesting
Traditional Techniques of Rainwater Harvesting
- Kunds or Kundis: In the sandy villages of the Thar Desert, an ingenious system of rainwater harvesting is known as Kunds or Kundis. Kund is the local name given to a covered underground tank, was developed primarily for tackling drinking water problems. It is usually constructed with local materials or cement. These Kunds were more prevalent in the western arid regions of Rajasthan, and in areas where the limited groundwater available is moderate to highly saline. Under such conditions, Kunds provided convenient, clean and sweet water for drinking.
Kunds were owned by communities or privately, with the rich having one or more Kunds of their own. Community Kunds were built through village cooperation or by a rich man for the entire community. The Kund consists of a saucer-shaped catchment area with a gentle slope towards the centre where a tank is situated. Openings or inlets for water to go into the tank are usually guarded by a wire mesh to prevent the entry of floating debris, birds and reptiles. The top is usually covered with a lid from where water can be drawn out with a bucket. Kunds are by and large circular in shape, with little variation between the depth and diameter which ranges from 3-4.5 m. Lime plaster or cement is typically used for the construction of the tank since stone as a building material is not always available and is relatively more expensive.
- Bamboo Drip Irrigation: In Meghalaya (a northeastern state in India), another system of tapping of stream and spring water is prevalent in which bamboo pipes are used to irrigate plantations. It is so perfect that about 18-20 litres of water entering the bamboo pipe system per minute gets transported over several hundred metres and finally gets reduced to 20-80 drops per minute at the site of the plant. The tribal farmers of Khasi and Jaintia hills use the 200-year-old system. The bamboo drip irrigation system is normally used to irrigate the betel leaf or black pepper crops planted in Arecanut orchards or in mixed orchards.
Bamboo pipes are used to divert perennial springs on the hilltops to the lower reaches by gravity. The channel sections, made of bamboo, divert and convey water to the plot site where it is distributed without leakage into branches, again made and laid out with different forms of bamboo pipes. Manipulating the intake pipe positions also controls the flow of water into the lateral pipes. Reduced channel sections and diversion units are used at the last stage of water application. The last channel section enables the water to be dropped near the roots of the plant. Bamboos of varying diameters are used for laying the channels. About a third of the outer casing in length and internodes of bamboo pieces have to be removed while fabricating the system. Later, the bamboo channel is smoothened by using a Dao, a type of local axe which is around chisel fitted with a long handle. Other components are small pipes and channels of varying sizes used for diversion and distribution of water from the main channel. About four to five stages of distribution are involved from the point of the water diversion to the application point.
- Kul Irrigation of the Trans-Himalaya: Agriculture is the mainstay of Spiti (name of Himachal region in India) in spite of the fact that it is a Col Desert. devised centuries ago to tap distant glaciers for water. Rainfall is negligible in Spiti because it is a rainshadow area. The soil is dry and lacks organic matter. But, despite these handicaps, the Spiti valley has been made habitable and productive by human ingenuity.
Kul irrigation Utilises Kuls, the local name for Diversion Channels to carry water from glacier to village. The Kuls often span long distances, running down precipitous mountain slopes and across crags and crevices. Some Kuls are 10 km long and have existed for centuries. The crucial portion of a Kul is its head at the glacier, which is to be tapped. The head must be kept free of debris, and so the Kul is lined with stones to prevent clogging and seepage. In the village, the Kul leads to a circular tank from which the flow of water can be regulated. For example, when there is a need to irrigate, water is let out of the tank in a trickle. Water from the "Kul" is collected through the night and released into the exit channel in the morning. By evening, the tank is practically empty, and the exit is closed. This cycle is repeated daily. The Kul system succeeds because Spiti residents mutually cooperate and share. The culture also is instrumental in maintaining the carrying capacity of the surrounding cultivable land. However, this system, carefully nurtured through the centuries, now runs the risk of being upset through government intervention.
Modern Techniques of Rainwater Harvesting
- Groundwater Dams: The basic principle of the groundwater dam is that instead of storing the water in surface reservoirs, water is stored underground. The main advantages of water storage in groundwater dams are that evaporation losses are much less for water stored underground. Further, the risk of contamination of the stored water from the surface is reduced because as parasites cannot breed in underground water. The problem of submergence of land which is normally associated with surface dams is not present with sub-surface dams.
- A Sub-surface Dam: A sub-surface dam intercepts or obstructs the flow of an aquifer and reduces the variation of the level of the groundwater table upstream of the dam. It is built entirely under the ground
- Sand Storage Dam: It is constructed above ground. Sand and soil particles transported during periods of high flow are allowed to deposit behind the dam, and water is stored in these soil deposits. The sand storage dam is constructed in layers to allow sand to be deposited and finer material is washed downstream A groundwater dam can also be a combination of these two types. When constructing a sub-surface dam in a river bed, one can increase the storage volume by letting the wall of the Dam rise over the surface, thus causing additional accumulation of sediments. Similarly, when a sand-storage dam is constructed it is necessary to excavate a trench in the sand bed in order to reach bedrock, which can be used to create a sub-surface dam too.
- Ferrocement Tanks: Ferrocement tanks are containers which are much cheaper than masonry, RCC or plastic tanks. Tanks of 1000-2000 litre capacity can be constructed with ease, These are easy to repair, and can be easily transported because of their sturdy nature. Such containers have been used on a wide scale since about the past 25 years in Thailand, Malaysia and some African countries. Ferrocement containers with capacity as much as 5000 litres have been constructed in Thailand. The process of building a Ferrocement container is very simple which users can do themselves, with some training. For a typical circular pot-shaped container, the only materials required are hessian cloth, chaff (waste from agricultural produce), GI wire mesh, MS bars, cement and sand.
To prepare the mould, the hessian cloth, stitched into a sack resembling the shape of the container, is filled with chaff that is compacted in layers. Dry leaves or dry grass can be used in place of chaff. Once the sack is filled with the filler material, it is beaten into the required shape by a wooden bat. A GI wire mesh (22-26 gauge) is tied around the mould leaving sockets at suitable locations for inlet, overflow and cleaning pipes. Tying 6 mm diameter mild steel (MS) bars at wide intervals both horizontally and vertically strengthens the reinforcement cage. Ferrocement consists of a thin sheet of cement mortar which is reinforced with a cage made of wire mesh and steel bars. Because Ferrocement is structurally more efficient than masonry, the thickness of the walls of the container are as low as 10 to 15 mm. Ferrocement components can be cast in any shape using suitable moulds. The technology is extremely simple to implement, and even semi-skilled workpersons can learn it with ease. Ferrocement requires only a few easily available materials - cement, sand, galvanized iron (GI) wire mesh, and mild steel (MS) bars - in small amounts compared to masonry and RCC
Importance and Advantages of Rainwater Harvesting
- Solution to various water problems in areas where water resources are scarce
- The rise in the groundwater level
- Minimises the effects of drought and achieves drought proofing
- Reduces the runoff which chokes the stormwater drains
- Flooding of roads is reduced
- Improvement in the quality of water
- Reduction of Soil erosion
- Saving of energy per well for the lifting of groundwater
- Prevention of sea water ingress
- Reduces wastage of water
World over Practice of Rainwater Harvesting
Harvesting rain has become an ideal way to deal with water scarcity, in many parts of the world. Rainwater harvesting is being accepted as a solution for water-related problems as a result of the rising cost of water as well as ecological concerns. The growth of the city along with an increase in population has resulted in rapid depletion of groundwater around the world. Rainwater harvesting can meet potable and non-potable water demands and also control flooding. Thus it has become necessary for the water authorities across different countries to adopt rainwater harvesting and conservation techniques.
- Rooftop rainwater harvesting is being practised for providing drinking water, domestic water, water for livestock, water for small irrigation and a way to replenish groundwater levels in China and Brazil. Gansu province in China and semi-arid northeast Brazil have the largest rooftop rainwater harvesting projects ongoing.
- In Bermuda, the law requires all new construction to include rainwater harvesting adequate for the residents.
- The U.S. Virgin Islands have a similar law.
- In Senegal/Guinea-Bissau, the houses of the Diola-people are frequently equipped with homebrew rainwater harvesters made from local, organic material.
- In the United Kingdom, water butts are often found in domestic gardens to collect rainwater which is then used to water the garden.
