Monday 20 February 2012

Rainwater Harvesting in Rajasthan.
Influencing factors

Among the several factors that influence the rainwater harvesting potential of a site, eco-climatic conditions and the catchment characteristics are considered to be the most important.

a. Rainfall

i)Quantity: Rainfall is the most unpredictable variable in the calculation and hence, to determine the potential rainwater supply for a given catchment, reliable rainfall data are required, preferably for a period of at least10 years. Also, it would be far better to use rainfall data from the nearest station with comparable conditions.

ii) Pattern: The number of annual rainy days also influences the need and design for rainwater harvesting. The fewer the annual rainy days or longer the dry period, the more the need for rainwater collection in a region. However, if the dry period is too long, big storage tanks would be needed to store rainwater. Hence in such regions, it is more feasible to use rainwater to recharge groundwater aquifers rather than for storage.

b. Catchment area characteristics

Runoff depends upon the area and type of the catchment over which it falls as well as surface features.

All calculations relating to the performance of rainwater catchment systems involve the use of runoff coefficient to account for losses due to spillage, leakage, infiltration, catchment surface wetting and evaporation, which will all contribute to reducing the amount of runoff. (Runoff coefficient for any catchment is the ratio of the volume of water that runs off a surface to the volume of rainfall that falls on the surface).

Runoff coefficients for various catchment surfaces


Type of Catchment
Coefficients
Roof Catchments
 - Tiles
 - Corrugated metal sheets

0.8- 0.9
0.7- 0.9
Ground surface coverings
 - Concrete
 - Brick pavement

0.6- 0.8
0.5- 0.6
Untreated ground catchments
 - Soil on slopes less than 10 per cent
 - Rocky natural catchments

0.0 - 0.3
0.2 - 0.5
Untreated ground catchments
 - Soil on slopes less than 10 per cent
 - Rocky natural catchments

1.0 - 0.3
0.2 - 0.5

Source : Pacey, Arnold and Cullis, Adrian 1989, Rainwater Harvesting: The collection of rainfall and runoff in rural areas, Intermediate Technology Publications, London

Based on the above factors the water harvesting potential of a site could be estimated using the formula given below.



Water harvesting potential
      = Rainfall (mm) x Area of catchment x Runoff coefficient
The Potential
The total amount of water that is received in the form of rainfall over an area is called the rainwater endowment of that area. Out of this, the amount that can be effectively harvested is called the water harvesting potential.

Friday 17 February 2012


Maintenance


  • Before collecting water the roof, gutters and tank should be cleaned
  • Let the first 2-3 rains flow out through the first flush system
  • Remember to clean the tank once in a year
  • Replace the filtering agents every year
  • Keep the tank and surroundings clean and hygienic
  • Apply white cement on the tank every year
  • Make sure that sunlight does not pass through the manhole to prevent algae growth
  • Remember to preserve water and use it judiciously

Components


A Roof water harvesting system should have the following components:

Down pipe and first flush arrangement


This is an arrangement to prevent the dust and other unwanted materials that may be seen on the roof, from reaching the storage tank. By fitting a down pipe with an end cap or valve can ensure that the washed water from the roof does not reach the storage tank. It is always safe to ensure that the first flush arrangement remains open during non- rainy days and should be closed after first rain up to the satisfaction of the end user.

Filter unit


The rainwater collected from the roof should be allowed to reach the storage tank only through a filtering mechanism. Rubble, sand and charcoal, as used in the traditional three-pot filtration, can be adopted here. 10 cm thick 20mm rubbles, 10 cm thick charcoal/coconut shell, 15 cm thick coarse sand, 5 cm thick 6mm rubbles etc. may be arranged from bottom to top in the filter unit. The water that is passed through this filter should remain safe for a long period of storage. Charcoal/coconut shell is added to eliminate gaseous pollutants.

Storage tank


Mainly three types of storage tanks are constructed for roof water harvesting. They are aboveground, underground or sub surface tanks.

Provision for drawing water and spill over


For drawing water from the storage tanks any method may be adopted provided it shall be drawn only when needed. The easier the method adopted for drawing water, the more will be the chances for its misuse. Three types of drawing water are usually adopted. They are by using a tap (small capacity tanks with basement and over ground tanks), hand pump (under ground tanks) or electric pump sets (large capacity tanks).
It is important that the system is sized to meet the water demand throughout the dry season. In general, the size of the storage tank should be big enough to meet the daily water requirement throughout the dry season. In addition, the size of the catchment area or roof should be large enough to fill the tank.

Ancient period


Rainwater harvesting has been used since biblical times. It was done in ancient Palestine, Greece and Rome. Around 3rd Century BC., farming communities in Baluchistan and Kutch used it for irrigation.[5]In Ancient Tamil Nadu, India, Rainwater harvesting were done by Chola kings.[6]Rainwater from Brihadeeswarar Temple was collected in Sivaganga tank.[7]In the Indus Valley Civilization, Elephanta Caves and Kanheri Caves in Mumbai rainwater harvesting alone has been used to supply in their water requirements.

Now


  • Currently in China and Brazil, rooftop rainwater harvesting is being practiced for providing drinking water, domestic water, water for livestock, water for small irrigation and a way to replenish ground water levels. Gansu province in China and semi-arid north east 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 and Guinea-Bissau, the houses of the Diola-people are frequently equipped with homebrew rainwater harvesters made from local, organic materials.
  • In the United Kingdom water butts are often found in domestic gardens to collect rainwater, which is then used to water the garden. However, the British government's Code For Sustainable Homes encourages fitting large underground tanks to new-build homes to collect rainwater for flushing toilets, washing clothes, watering the garden, and washing cars. This reduces by 50% the amount of mains water used by the home.
  • In the Irrawaddy Delta of Myanmar, the groundwater is saline and communities rely on mud-lined rainwater ponds to meet their drinking water needs throughout the dry season. Some of these ponds are centuries old and are treated with great reverence and respect.
  • Until 2009 in Colorado, water rights laws almost completely restricted rainwater harvesting; a property owner who captured rainwater was deemed to be stealing it from those who have rights to take water from the watershed. Now, residential well owners that meet certain criteria may obtain a permit to install a rooftop precipitation collection system (SB 09-080).[8] Up to 10 large scale pilot studies may also be permitted (HB 09-1129).[9] The main factor in persuading the Colorado Legislature to change the law was a 2007 study that found that in an average year, 97% of the precipitation that fell in Douglas County, in the southern suburbs of Denver, never reached a stream—it was used by plants or evaporated on the ground. In colorado you cannot even drill a water well unless you have at least 35 acres. In New Mexico, rainwater catchment is mandatory for new dwellings in Santa Fe.[10]
  • In Beijing, some housing societies are now adding rain water in their main water sources after proper treatment.

Professor Micheal McGinley established a project to design a rain water harvesting prototype in the Biosystems design Challenge Module in University College Dublin.

  • In Australia rainwater harvesting is typically used to supplement the reticulated mains supply. In south east Queensland, households that harvested rainwater doubled each year from 2005 to 2008, reaching 40% penetration at that time (White, 2009 (PhD)).

In India


  • In Tamil Nadu, India rainwater harvesting was made compulsory for every building to avoid ground water depletion. It proved excellent results within five years and every other state took it as role model. Since the implementation, Chennai saw 50 per cent rise in water level in five years and the water quality significantly improved.[11] [12]
  • In Rajasthan, India rainwater harvesting has traditionally been practiced by the people of the Thar Desert. There are many ancient water harvesting systems in Rajasthan, which have now been revived [13]
  • Kerala, India,
As rainwater may be contaminated due to pollutants like microscopic germs etc., it is often not considered suitable for drinking without treatment. However, there are many examples of rainwater being used for all purposes — including drinking — following suitable treatment.

Rainwater harvested from roofs can contain human, animal and bird faeces, mosses and lichens, windblown dust, particulates from urban pollution, pesticides, and inorganic ions from the sea (Ca, Mg, Na, K, Cl, SO4), and dissolved gases (CO2, NOx, SOx). High levels of pesticide have been found in rainwater in Europe with the highest concentrations occurring in the first rain immediately after a dry spell;[3] the concentration of these and other contaminants are reduced significantly by diverting the initial flow of water to waste as described above. The water may need to be analysed properly, and used in a way appropriate to its safety. In the Gansu province for example, harvested rainwater is boiled in parabolic solar cookers before being used for drinking.[4] In Brazil alum and chlorine is added to disinfect water before consumption.[citation needed] So-called "appropriate technology" methods, such as solar water disinfection, provide low-cost disinfection options for treatment of stored rainwater for drinking.
Rainwater harvesting can ensure an independent water supply during water restrictions, though somewhat dependent on end-use and maintenance, usually of acceptable quality for household needs and renewable at acceptable volumes, despite forecasted climate change (CSIRO, 2003). It produces beneficial externalities by reducing peak storm water runoff and processing costs. In municipalities with combined sewer systems, reducing storm runoff is especially important, because excess runoff during heavy storms leads to the discharge of raw sewage from outfalls when treatment plant capacity cannot handle the combined flow. Rainwater harvesting systems are simple to install and operate. Running costs are negligible, and they provide water at the point of consumption. Rainwater harvesting in urban communities has been made possible by various companies. Their tanks provide an attractive yet effective solution to rainwater catchment.

Rain water harvesting law Some U.S. states have water law in favor of land owners, and some states own all water rights, leaving the owner only water rights the states are willing to grant through permits. For example, in Colorado you may not catch, collect or harvest rainwater from your roof unless you first buy a permit.

In Australia In Australia it is common to have a rain water tank with rainwater running off the house roof to fill tank, it is common to flush toilets with this water and have a back up valve like an Acquasaver that gives the home owner mains(town water) as a back up in case rainwater supply is exhausted or power failure. These types of valves automatically switch from rain water to mains water supply when available, so the homeowner never needs manually operate this valve.

In Sri Lanka Urban Development Authority of Sri Lanka has been directed to include rain water harvesting by changes to its act of establishment.
Rainwater may also be used for groundwater recharge, where the runoff on the ground is collected and allowed to be absorbed, adding to the groundwater. In the US, rooftop rainwater is collected and stored in sump.[1]by 6 th std student
A subsurface dike is built in an aquifer to obstruct the natural flow of groundwater, thereby raising the groundwater level and increasing the amount of water stored in the aquifer. The subsurface dike at Krishi Vigyan Kendra Kannur under Kerala Agricultural University with the support of ICAR, has become an effective method for ground water conservation by means of rain water harvesting technologies. The subsurface dike has been demonstrated to be a feasible method for conserving and exploiting the groundwater resources of the Kerala state of India. The dike is now the largest rainwater harvesting system in that region.
Rainwater harvesting is the accumulating and storing of rainwater for reuse before it reaches the aquifer. It has been used to provide drinking water, water for livestock, water for irrigation, as well as other typical uses. Rainwater collected from the roofs of houses and local institutions can make an important contribution to the availability of drinking water. It can supplement the subsoil water level and increase urban greenery. Water collected from the ground, sometimes from areas that are especially prepared for this purpose, is called Stormwater harvesting. In some cases, rainwater may be the only available, or economical, water source. Rainwater harvesting systems can be simple to construct from inexpensive local materials, and are potentially successful in most habitable locations. Roof rainwater may not be potable and may require treatment before consumption. As rainwater rushes from your roof it may carry pollutants, such as mercury from coal burning buildings, or bird faeces. Although some rooftop materials may produce rainwater that would be harmful to human health as drinking water, it can be useful in flushing toilets, washing clothes, watering the garden, and washing cars; these uses alone halve the amount of water used by a typical home. Household rainfall catchment systems are appropriate in areas with an average rainfall greater than 200 mm (7.9 in) per year, and no other accessible water sources (Skinner and Cotton, 1992). Overflow from rainwater harvesting tank systems can be used to refill aquifers in a process called groundwater recharge; though this is a related process, it must not be confused with rainwater harvesting.

There are several types of systems to harvest rainwater, ranging from very simple home systems to complex industrial systems. The rate at which water can be collected from either system is dependent on the plan area of the system, its efficiency, and the intensity of rainfall (i.e., annual precipitation (mm per annum) x square meter of catchment area = litres per annum yield) ... a 200 square meter roof catchment catching 1,000mm PA yields 200 kLPA.

ld be large enough to carry peak flows. Storage tanks should be covered to prevent mosquito breeding and to reduce evaporation losses, contamination and algal growth.