MODERN METHODS OF RAINWATER HARVESTING

The numbers of water stressed regions at various part of the world are increasing due to rapid growth of real estate. The urban areas are facing twin challenges of water scarcity and inadequate capacity of wastewater disposal systems. The rapid growth of urban population leads escalation of water demand. Meeting these increasing water demands with a geographically constrained water supply system is often a very difficult task. Large construction projects are also being taken up at areas where municipal water supply is yet to be made available. The developers are frequently exploring and abstracting groundwater to ensure the basic amenities at their housing projects. Conservation of ground water is important because it takes years to be replenished. In areas where ground water is used, care must be taken to minimize the quantity of water withdrawn and bring it on per with quantity of water being replenished.

According to Chatterjee (2008), the modern methods of rainwater harvesting can be broadly categorized under two:

1. Collection and storage of rainwater for direct use, and

2. Groundwater recharging.

The combination of these two are also practiced, where rainwater is collected and stored in containers for direct use while the collected rainwater in excess of the storage capacity is charged into ground (groundwater recharging).

Some of the modern systems that are being used for rain water harvesting are described below:

1. Artificial Recharging

1.1 Absorption pit method

A percolation / absorption pit is a hand bore made in the soil with the help of an augur and filled up with pebbles and river sand on top. The depth of these pits will be anywhere between 4 and 8 meters depending on the nature of the soil. If the soil is clayey, the pit has to be dug to a depth till a reasonably sandy stratum is reached. The diameter of these pits will be 25 cm (10 inches).

A square / circular collection chamber with silt arrester is provided at the top. An absorption pit should be constructed in the sloped area of the house veranda. The size of the absorption pit should be minimum of 5 feet in width and a maximum of 10 feet in depth. The pipe which is installed for collecting the rainwater from the rooftop and the veranda of the house should be linked to this absorption pit. Another pipe should be deployed for allowing the excess water to run off into the storm water drainage.

1.2 Absorption well method

These wells are constructed using cement rings readily available in the market. The diameter of these rings range from 2 ft to 6 ft. The depth to which these wells are dug depends on the nature of the soil and the diameter depends on the number of roof top pipes that are likely to be connected to each one of these wells. These wells are left unfilled and are covered with RCC slabs of suitable thickness to facilitate free pedestrian and vehicular movement on the ground.

Rainwater from the terrace is diverted to the existing open well using PVC pipes through a filter chamber. The minimum size of the filter chamber is 2" x 2" x 2" filled with broken bricks in the bottom and sand on the top. The chamber may be covered with RCC slab.

1.3 Well cum bore method

In areas where the soil is likely to be clayey upto say 15ft. and more, it is advisable to go in for a percolation well upto 10ft. or 15ft. and a hand bore pit within this well upto a depth of 10ft. to 15ft. from its bottom.

1.4 Recharge trench cum injection well

In this technique, 1 to 2 m. wide and 2 to 3 m. deep trench is excavated, the length of which depends on the site availability and volume of water to be handled. An injection well of 100 to 150 mm diameter is constructed, piercing through the layers of impermeable horizons to the potential aquifer reaching about 3 to 5 metres below water levels ( 1 to 10 m.) from the bottom of the trenches. Depending upon the volume of water to be injected, the number of injection wells can be increased to enhance the recharging rate.

1.5 Bore well Recharging

Recharging of bore wells should be done to prevent them from drying up and improve their water table. It is usually done by the following method. First, a pit should be dug in the region surrounding the casting pipe and cement rings should be installed in it. The size of the pit should be one meter in diameter and 10 feet in depth. At the bottom of the pit, filter holes should be made and a casting pipe with steel mesh should be fixed tightly to the bore well pipe. This casting pipe will function as a filter. A second layer of two feet height filled with jelly stones of 40 mm should follow this. The third layer of one foot height should be filled with jelly stones of 20 mm size. The fourth layer should be filled with charcoal. The exercise should be repeated till there in 3 feet of space left from the ground level. A nylon curtain should be spread on the layers and the remaining space of the pit should be filled with sand until one foot from the ground level. A pipe should be fixed to collect the rainwater from the roof and this pipe should be connected to the pit. A pipe should be fixed to prevent the excess water from running away to roadside drainage.

1.6 Ground Water Recharging

Ground water recharging in urban areas is done by collecting the rainwater from the rooftop and the portico of the house and by making it easily absorbed within the veranda space. The method used is absorption pit method.

Harvesting Methods

Water falling on a flat rooftop should be made to run through a pipe connected to the roof and prevented from running off to drainage on the roadside. This water should be filtered and stored in tanks constructed for the purpose of storing rainwater. A good quality filter is needed for purifying the water. This is because, although the rain water is basically pure, the water flowing from the rooftop usually will contain waste material in it.

Harvesting methods can be implemented for the following:

2.1 For Individual Houses

Roof top rainwater can be diverted to the existing Open / bore well. Along with this, rainwater available in the open spaces around the building may be recharged into the ground through the following simple effective methods.

· Percolation pits (Small Houses).

· Recharge trench (Big houses / Apartments).

· Recharge wells (Large buildings / Industries).

Based on the size / area of the building and the underlying litho logical nature of the formation the said methods may be used either individually or in combination. To enhance nature recharge of rainwater avoids pavements since unpaved surfaces have more percolation rate.

2.2 Rainwater Harvesting in Group Houses

Utilize the open well if any, within the complex to divert the rainwater from the terrace into it. If not, construct a well for the purpose. The rainwater falling on the open space around the complex can be collected near the gate by providing a gutter with perforated lid. The collected water can be led through necessary piping arrangements into a recharge well of 1 meter diameter and 5 meter deep.

3. Groundwater Dams

Groundwater dams are structured that intercept or obstruct the natural flow of groundwater and provide storage for water underground. They have been used in several parts of the world, notably India, Africa and Brazil. Their use is in areas where flows of groundwater vary considerably during the course of the year, from very high following rain to negligible flows during the dry season.

The main principal of 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, risk of contamination of the stored water from the surface is reduced because as parasites cannot breed in underground water.

There are two main types of groundwater dam:

3.1 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.

3.2 Sand storage dam

The sand storage dam 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 be deposited and finer material be washed downstream.

A groundwater dam can also be combination of these two types. When constructing a sub-surface dam in a river bed, one can increase the storage volume by letting the dam wall rise over the surface, thus causing additional accumulation of sediments. Similarity, 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.

4. Ferrocement Technology

One of the primary requirements of a water harvesting system is that of containers to store the harvested water in a hygienic condition. This need is more pronounced in high-rainfall areas, where it is more feasible to store water in containers for direct use, rather than for recharging the groundwater. Generally, in small domestic systems, the cost of constructing tanks with conventional materials like masonary or RCC is far more than that of the rainwater collection and piping component. Ferro cement can provide a low-cost and easy-to-build solution to the need for low-cost containers. This technology is particularly relevant for regions like Meghalaya, Arunachal Pradesh and Kerala, which have frequency of rainfall.

TRADITIONAL METHODS OF RAINWATER HARVESTING

Water has been harvested in India since antiquity, with our ancestors perfecting the art of water management. Many water harvesting structures and water conveyance systems specific to eco-regions and culture has been developed. There are evidences that, even during Harappan period, there was very good system of water management as could be seen in the latest excavation at Dholavira in Kachch.

Traditional rainwater harvesting which is still prevalent in rural areas was done in surface storage bodies like lakes, ponds, irrigation tanks, temple tanks etc. In urban areas, due to shrinking of open spaces, railways will have to necessarily be harvested as ground water, hence harvesting in such places will depend very much on the nature of soil viz., clayey, sandy etc.

According to Londonkar (2007) various kinds of traditional rainwater harvesting can be described as follows:

S Paar System

Paar is a common water harvesting practice in the western Rajasthan region. It is a common place where the rainwater flows from the agar (catchments) and in the process percolates into the sandy soil. In order to access the rajani pani (percolated water) kuis or beris are dug in the agar (storage area). Kuis or Beris are normally 5 mts to 12 mts deep. The structure was constructed through traditional masonary technology.

This is the most predominant form of rainwater harvesting in the region. This method is also known as Patali Paani.

S Talab / Bandhis

Talabs are reservoirs. They may be natural, such as the ponds (pokhariyan) at tikamgarh in the bundelkhand region. They can be human made. A reservoir area of less than five bighas is called a talai, a medium sized lake is called a Bandhi or Talab, and bigger lakes are called sagar or samand. The pokhariyan serve irrigation and drinking purposes.

S Saza Kuva

An open well with multiple owners (Saza = partner), Saza Kuva is the most important source of irrigation in the Aravalli hills in Mewar, eastern Rajasthan. The soil dug out to make the well pit is used to construct a huge circular foundation or an elevated platform sloping away from the well.

Saza Kuva construction is generally taken up by a group of farmers with adjacent landholdings.

S Johad

Johads are small earthen check dams that capture and conserve rainwater, improving percolation and groundwater recharge. Starting 1984, the last sixteen years have seen the revival of some 3000 Johads spread across more than 650 villages in Alwar district, Rajasthan. This has resulted in a general rise of the groundwater level by almost 6 metres and a 33 percent increase in the forest cover in the area.

Five rivers that used to go dry immediately following the monsoon have now become perennial, such as the River Arvari, has come alive.

S Pat System

This system was devised according to the peculiarities of the terrain to divert water from swift-flowing hill streams into irrigation channels called pats. The diversion bunds across the stream are made by piling up stones and then lining them with teak leaves and mud to make them leak proof. The pat channel has to negotiate small nullahs that join the stream off and on and also sheer cliffs before reaching the fields.

The villagers irrigate their fields by turns. The channel requires constant maintenance and it is the duty of the family irrigating the fields on a particular day to take care of the pat on that particular day.

S Naada / Bandha

Naada/Bandha is found in the Mewar region of the Thar Desert. It is a stone check dam, constructed across a stream or gully, to capture monsoon runoff on a stretch of land. Submerged in water, the land becomes fertile as silt deposits on it and the soil retains substantial amounts of water.

S Rapat

A Rapat is a percolation tank, with a bund to impound rainwater flowing through a watershed and a waste weir to dispose of the surplus flow. If the height of the structure is small, the bund may be built of masonary, otherwise earth is used. Rapats and percolation tanks do not directly irrigate land, but recharges well within a distance of 3-5 km downstream.

S Chandela tank

These tanks were constructed by stopping the flow of water in rivulets flowing between hills by erecting massive earthen embankments, having width of 60m or more. These hills with long stretches of quartz reefs running underneath them, acted as natural ground water barrier helping to trap water between the ridges. The earthen embankments were supported on both sides with walls of coarse stones, forming a series of stone steps. These tanks are made up of lime and mortar and this is the reason why these tanks survived even after thousand years but the only problem, which these tanks are facing, is siltation of tank beds.

S Bundela Tank

These tanks are bigger in size as compared to Chandela tanks. These tanks had solidly constructed steps leading to water in the tank but these structures had chabootaras, pavillions and royal orchards designed to show off the glory of the king who built them. But these tanks are not as cost effective and simple as Chandela tanks. These tanks were constructed to meet the growing water demands in the area; maintenance of these tanks was done by the person employed by the king but in case of smaller tanks villagers collectively removed silt and repair embankment.

S Kunds / Kundis

A kund or kundi looks like an upturned cup nestling in a saucer. These structures harvest rainwater for drinking, and dot the sandier tracts of the Thar Desert in western Rajasthan and some areas in Gujarat. Essentially a circular underground well, kunds have a saucer-shaped catchment area that gently slopes towards the centre where the well is situated. A wire mesh across water-inlets prevents debris from falling into the well-pit. The sides of the well-pit are covered with (disinfectant) lime and ash. Most pits have a dome-shaped cover, or at least a lid, to protect the water.

They can be owned by only those with money to invest and land to construct it. Thus for the poor, large public Kunds have to be built.

S Baoris / Bers

Baoris or bers are community wells, found in Rajasthan, that are used mainly for drinking. Most of them are very old and were built by banjaras (mobile trading communities) for their drinking water needs. They can hold water for a long time because of almost negligible water evaporation.

S Jhalaras

Jhalaras were human-made tanks, found in Rajasthan and Gujarat, essentially meant for community use and for religious rites. Often rectangular in design, Jhalaras have steps on three or four sides. Jhalars are ground water bodies which are built to ensure easy & regular supply of water to the surrounding areas. The water from these Jhalaras was not used for drinking but for only community bathing and religious rites. Jodhpur city has eight Jhalaras two of which are inside the town & six are found outside the city.

S Nadis

Nadis are village ponds, found near Jodhpur in Rajasthan. They are used for storing water from an adjoining natural catchment during the rainy season. The site was selected by the villagers based on an available natural catchments and its water yield potential. Water availability from nadi would range from two months to a year after the rains.

S Tobas

Tobas is the local name given to a ground depression with a natural catchment area. A hard plot of land with low porosity, consisting of a depression and a natural catchment area was selected for the construction of Tobas.

S Tankas

Tankas (small tank) are underground tanks, found traditionally in most Bikaner houses. They are built in the main house or in the courtyard. They were circular holes made in the ground, lined with fine polished lime, in which rainwater was collected. The water was used only for drinking. If in any year there was less than normal rainfall and the Tankas did not get filled, water from nearby wells and tanks would be obtained to fill the household Tankas.

The tanka system is also to be found in the pilgrim town of Dwarka where it has been in existence for centuries. It continues to be used in residential areas, temples, dharamshalas and hotels.

S Khadin

A khadin, also called as a dhora, is an ingenious construction designed to harvest surface runoff water for agriculture. Its main feature is a very long (100-300 m) earthen embankment built across the lower hill slopes lying below gravelly uplands. Sluices and spillways allow excess water to drain off. The khadin system is based on the principle of harvesting rainwater on farmland and subsequent use of this water-saturated land for crop production.

A similar system is also reported to have been practiced 4,000 years ago in the Negev desert, and in southwestern Colorado 500 years ago.

S Vav / vavdi / Baoli / Bavadi

Traditional step wells are called Vav or vavadi in Gujarat, or baolis or bavadis in Rajasthan and northern India. Built by the nobility usually for strategic and/or philanthropical reasons, they were secular structures from which everyone could draw water. Most of them are defunct today. Step well locations often suggested the way in which they would be used. When a step well was located within or at the edge of a village, it was mainly used for utilitarian purposes and as a cool place for social gatherings. When step wells were located outside the village, on trade routes, they were often frequented as resting places. Many important step wells are located on the major military and trade routes from Patan in the north to the sea coast of Saurashtra.

S Ahar Pynes

An ahar is a catchment basin embanked on three sides, the 'fourth' side being the natural gradient of the land itself. Ahar beds were also used to grow a rabi (winter) crop after draining out the excess water that remained after kharif (summer) cultivation.
Pynes are artificial channels constructed to utilize river water in agricultural fields. Starting out from the river, pynes meander through fields to end up in an ahar. Most pynes flow within 10 km of a river and their length is not more than 20 km.

S Bengal's Inundation Channel

Bengal once had an extraordinary system of inundation canals. Sir William Willcocks, a British irrigation expert who had also worked in Egypt and Iraq, claimed that inundation canals were in vogue in the region till about two centuries ago.

According to Willcocks, the distinguishing features of the irrigation system were:

ü The canals were broad and shallow, carrying the crest waters of the river floods, rich in fine clay and free from coarse sand.

ü The canals were long and continuous and fairly parallel to each other and at the right distance from each other for purposes of irrigation.

ü Irrigation was performed by cuts in the banks of the canals, which were closed when the flood was over.

S Dungs or Jampois

Dungs or Jampois are small irrigation channels linking rice fields to streams in the Jalpaiguri district of West Bengal.

S Kohli Tanks

The Kohlis, a small group of cultivators, built some 43,381 water tanks in the district of Bhandara, Maharashtra, some 250-300 years ago. These tanks constituted the backbone of irrigation in the area until the government took them over in the 1950s. It is still crucial for sugar and rice irrigation. The tanks were of all sizes, often with provisions to bring water literally to the doorstep of villagers.

S Kul

Kuls are water channels found in precipitous mountain areas. These channels carry water from glaciers to villages in the Spiti valley of Himachal Pradesh. Where the terrain is muddy, the kul is lined with rocks to keep it from becoming clogged. In the Jammu region too, similar irrigation systems called kuhls are found.

S Naula

Naula is a surface-water harvesting method typical to the hill areas of Uttaranchal. These are small wells or ponds in which water is collected by making a stone wall across a stream.

S Zings

Zings are water harvesting structures found in Ladakh. They are small tanks, in which collects melted glacier water. Essential to the system is the network of guiding channels that brings the water from the glacier to the tank. As glaciers melt during the day, the channels fill up with a trickle that in the afternoon turns into flowing water. The water collects towards the evening, and is used the next day.

A water official called the churpun ensures that water is equitably distributed.

S Kere

Tanks, called Kere in Kannada, were the predominant traditional method of irrigation in the Central Karnataka Plateau, and were fed either by channels branching off from anicuts (chech dams) built across streams, or by streams in valleys. The outflow of one tank supplied the next all the way down the course of the stream; the tanks were built in a series, usually situated a few kilometers apart.

This ensured:

a) No wastage through overflow.

b) The seepage of a tank higher up in the series would be collected in the next lower one.

Other types of rain water harvesting are:

S Kuhl

S Zabo

S Cheo-ozihi

S Eri

S Ooranis

S Dongs

S Bamboo Drip Irrigation

S Apatani

S Virdas

S Katas / Mundas / Bandhas

S Surangam

S Korambus

S Jackwells