33 Rain water harvesting
Rashmi Paliwal
Objectives:
The students will able to:
1. Understand the rainwater harvesting system
2. Understand different practices of rainwater harvesting
3. Explain the water quality in harvesting system
4. Understand the water quality maintenance
Introduction
Water plays a crucial role in maintaining all the life forms on earth. It may seem abundant as 71 % of earth’s surface is covered with water but a very small fraction i.e., 0.014 % is available for human use and this tiny fraction is shared by world’s competing users. The global growing population is demanding for more clean water which will continuously increase with the development of livelihood status. This may future increase the stress on freshwater resources leading to water scarcity in many places. This again results in declining of per capita water availability worldwide. In India, the per capita water availability is reduced from 5177 m3 in the year 1951 to 1608.26 m3 in the year 2010 (CWC, 2005 and 2013). Water conservation thus can be considered as to meet the challenge of freshwater availability for population demand. In the region where access to clean surface and groundwater is limited, another appropriate source of water must be sought. Rainwater can provide naturally occurring cleanest source of water in such areas. Collection of rainwater in tanks, wells, and ponds also known as “Rainwater Harvesting” has always been a common practice performed in ancient India. As the country’s agriculture system primarily depends upon the rainfall which changed almost every year and as flood and drought occurred regularly since ancient time. There is still a need to understand the traditional water harvesting techniques for the successful development and implementation of modern water conservation engineering. Rainwater harvesting is the activity of optimum utilization of natural resource i.e., rainwater by collecting and storing rainwater for direct use and can be used to recharged the groundwater (CPWD, 2002). Rainwater harvesting systems fall under two broad categories i.e., roof-top rainwater harvesting and the surface rainwater harvesting (Critchley, et. al 1991). Therefore, the roof-top system includes collecting of runoff from roof of a building/house through pipes/gutter and stored in storage tanks, whereas the surface rainwater harvesting systems include collecting the runoff from ground surfaces and intermittent or ephemeral watercourses in a storage reservoir or constructed catchment system. The surface rainwater harvesting systems are the natural and traditional techniques that can also be used to recharge the groundwater (Patel and Shah, 2015).
Traditional Techniques for Rainwater Harvesting
Water has considered as sacred by ancient Indian civilization. Rainwater harvesting is a traditional practice that is gaining the attention of modern water conservationists. India’s traditional rainwater harvesting system includes a wide array of structures used to store water in different parts the country such as tankas in Bikaner (Rajasthan), khadin in Rajasthan, zabo in Nagaland, pokhariyan in Bundelkhand etc. Tanka (small tank) of Rajasthan’s was once used in every house of Bikaner to store rainwater. Tankas are the underground small tanks constructed by digging holes in the ground, lined with polished lime. Tanka collected water from the rooftop or artificially prepared catchment flows and the water stored was used for drinking purpose. Kundis/Kunds of western Rajasthan, Gujrat and in some parts of Uttar Pradesh also, are the underground covered tanks used to harvest rainwater for drinking purpose. In hyper arid parts of Rajasthan, the khadin system involved the harvesting of rainwater on farmland by building an earthen embankment of 100 – 300 m across the lower hill slopes. The water stored in khadin is used for crop production. It also recharged the groundwater naturally. Nadis are the village ponds used to store water from adjoining catchments during the rainy season. Kuis / Beris used in western Rajasthan, these are 10-12 m deep pits dug near tanks to collect the seepage. Kuis can also be used to harvest rainwater in areas with meager rainfall. The mouth of the pit is usually made very narrow. This prevents the collected water from evaporating. As the pit burrows deep into the ground it gets wider. This provides the large surface area for water to seep into. The openings of these entirely kuchcha (earthen) structures are generally covered with planks of wood, or put under lock and key. The water is used sparingly, as a last resource in crisis situations. Water soak pits are one of the oldest water harvesting systems used to conserve and recharge groundwater. They are called with different names in different parts of the country such as madakas in Karnataka, pemghara in Odisha and johads in Rajasthan. Johads are simple barrier built by mud and stones across the contour slope to store the rainwater. A johad contains high embankments on three sides to arrest the rainwater while leaving one side open to allow the rain water to enter. Johads collect rain water, which slowly percolates into the groundwater, recharging the aquifer, maintain soil moisture and improve the earth’s water balance. The water from johads also used to meet the water needs of people. Zabo of Nagaland is a common practice used extensively in Kikurma village. The region receives adequate rainfall however it has no river resource. Thus, the people created the pond-like structures called “zabo” on the terraced hills which received the rainwater falls on the forested hilltops through channels. The water from zabo goes into the paddy fields which are also used to rear fishes. The ponds are constructed in a manner that the bunds on the sides used to grow herbs and medicinal plants (Agarwal and Narain, 1997). Bamboo drip irrigation system being practice for more than 200 years by tribal farmers of Khasi and Jaintia hills of the Northeast region of India. It is an indigenous system developed by farmers to irrigate the betel leaf or black pepper crops that need little water. The technique involves the diversion of water from perennial springs to terrace fields using bamboo pipes of diverse shapes and size (Agarwal and Narain, 1997).
Modern techniques for Rainwater Harvesting
A modern rainwater harvesting system comprises combinations of various components and processes, such as a catchment surface, conveyance system, pre-storage filtration, storage container, pump, post-storage filtration/treatment and post-storage distribution system. However, the system is not limited to these components only. The requirement of different components varies depending upon the harvested rainwater utilization. Hence, modern rainwater harvesting techniques can be broadly classified for two purposes.
1. Collection and storage of rainwater for direct use
2. Groundwater recharging.
However, these two can also practice in combination, where the collected rainwater is stored for direct use and the excess of it is directed to groundwater recharge system.
Surface Water Storage: Storage of surface water is used in a majority of rainwater harvesting systems and considered since ancient period, for example, rooftop rainwater harvesting system (Fig. 1). The basic components of rooftop rainwater harvesting system include a collection area for water, conveyance system (gutter and conduits) to deliver the collected water to the storage tank by a network of pipes and the storage facilities.
1. Catchment: The catchment or collection area is an area which directly receives the rainfall and provides water to the system. The roof of a house or a building can be used to collect the water as these areas are generally safe and cleaner compared the paved ground surfaces. The effective roof area and the material used in constructing the roof influence the efficiency of collection and the water quality.
2. Coarse Mesh: Provide at the roof to prevent entry of any debris.
3. Gutters: These are the channels installed all around the edges of the sloped roof to collect the water and deliver to the storing system via conduits. These are semi-circular or rectangular shaped drains can be made by folding galvanized iron (GI) sheets or by cutting Poly Vinyl Chloride (PVC) pipes and bamboo or betel trunk vertically into the half.
4. Conduits: Conduits usually consists of pipes that deliver rainwater falling on the rooftop to cisterns or other storage vessels. To avoid adverse effects on water quality, the chemically inert materials such as wood, plastic, aluminum, or fiberglass should be used in construction of drainpipes and roof surfaces.
5. First Flushing: It is a valve devised to check the entry of runoff from the first spell of rain to the system. As the first spell carries the heavy load of pollutants collected from the atmosphere and catchment during first few minutes of rainfall.
6. Filters: A filter is a chamber filled with fibre, coarse sand, gravels and charcoal etc. to remove any debris or dirt particles from the harvested water before it enters the storage tank. These filters can efficiently remove any color, silt, clay, and microorganisms from the water.
7. Storage tank: The water ultimately is stored in a storage tank or cistern, which should also be constructed of an inert material such as, reinforced concrete, fiberglass, wood, aluminum or stainless steel. The tank can be built near the building as part of it or can be constructed separately at some distance away from the building. Storage tank can be of two types: 1) underground, and 2) above ground. The underground tank can be constructed of masonry or reinforced cement concrete (RCC) structure and suitably lined with waterproofing materials. Whereas, the surface tank can be of GI sheet, RCC, Plastic or ferrocement tank and usually placed on a raised platform. The choice and size of the tank depends on the factors such as daily demand, duration of the dry spell, catchment area and rainfall, local availability of material and space.
The tank is provided with the following arrangements:
- A manhole of 0.50 m × 0.50 m size with cover
- Vent pipe/ over flow pipe (with screen) of 100 mm dia.
- Drain pipe (100 mm dia.) at bottom
For underground construction, at least 30 cm of the tank should remain above ground and the water withdrawal can be made by installing the hand pump on it.
8. Overflow Pipe: Installed on the top of a storage tank to remove excess of water from the tank during heavy rainfall. The size of pipe should be same as of the inlet pipe and installed with the wire mesh at the end to avoid entry of any insect, rat, squirrel to the tank.
Groundwater Recharging: Groundwater recharging is the indirect method of rainwater harvesting and is comparatively a new concept. The basic principle of groundwater recharge is to restore the supplies from an aquifer. Detailed knowledge of hydrological and geological features of the area is necessary for selecting the suitable site of recharge units. Groundwater recharge system involves the installation and construction of a variety of structures (CPWD, 2002).
Recharge Pits: Pits also called as recharge pits constructed for recharging the shallow aquifer. These are constructed with the dimensions of 1–2 m wide and 3 m deep backfilled with boulders, gravels, coarse sand. An aquifer is an underground formation of water-saturated layers of soil, sand, gravel or bedrock that can yield water.
Recharge Trenches: These are constructed when the permeable strata are available at shallow depth. 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 water (Fig 2). These are backfilled with filter materials.
Dug wells: Existing dry/unused dug wells can be utilized as recharge structure. The water should pass through filter media before putting into a dug well (Fig. 3).
Hand pumps: The abandoned/running hand pumps can be used for recharging the shallow/deep aquifers if the availability of water is limited (Fig. 4). Water can be diverted from rooftop to hand pump through a pipe (50–100 mm dia). Water should pass through filter media to avoid choking of recharge wells (CPWD, 2002).
Estimating the rainwater harvested quantity
In a rainwater harvesting system, the received rainfall over a surface catchment is collected and directed to the storage tank and consumed as non-potable water. The surplus water is allowed to flow through surface drainage system or by wastewater network. The size of a catchment area and the storage tank should be enough to fulfill the water requirements of users during the dry period. The capacity of the storage system can be decided by the available roof area and the rainfall. The total amount of rainwater received over an area is called the rainwater endowment of that area and the amount of rainwater that can effectively be harvested is called as the rainwater harvesting potential (CPWD, 2002). The rainwater harvesting potential can be calculated as:
Rainwater Harvesting Potential = Rainfall (mm) × Area of the catchment × Runoff Coefficient Rainwater endowment of an area = Area plot (sqm) × Rainfall height (m)
All the calculations related to determine the rainwater harvesting potential of a catchment involve the use of runoff coefficient. As runoff coefficient is important to count the runoff losses due to spillage, leakage, infiltration, catchment surface wetting and evaporation. Runoff is the water that flows away from the catchment after rainfall and depends upon the surface features, area and type of the catchment over which it falls. Therefore, runoff coefficient for any catchment is the value that represents the ratio of runoff to rainfall. Hence, depend on certain parameters such as material use in roof or catchment construction, slope, soil type, land use, degree of imperviousness, surface roughness and duration and intensity of rainfall. Runoff coefficient of various surfaces is given in Table 1.
Example 1: Data given for annual rainfall of an city is 600 mm (Height of rain fall 0.6 m), area of a city or place is 100 Sqm, Roof catchment with tile finishing (runoff coefficient for tile finishing roof is 0.85 from table 1), coefficient for evaporation, spillage and first flush wastage can be considered as constant 0.80 (for all situation). Calculate the effective harvested quantity.
Rainwater harvesting potential of given catchment = 0.6 (m) × 100 (Sqm) × 0.85 × 0.80 = 40800 litres.
The volume of the tank can be calculated as per the given formula:
V = (t × n × q) + et
Where,
V = Volume of tank (litres)
t = Length of the dry season (days)
n = Number of people using the tank
q = Consumption per capita per day (litres)
et = Evaporation loss during the dry period
Since evaporation from a closed storage tank is negligible, the evaporation loss (et) can be considered as zero.
Example 2: If consumption per capita per day in litres (lpd) is 40 and a dry period of 80 days (t) is normally not exceeded, a storage volume required for a family of 5 members (n) would be:
V = 80 (t) × 5 (n) × 40 (q) = 16,000 litres or 16 m3
Quality of Harvested Water
Rainwater is generally considered as the purest form of water and can be used directly for various domestic purposes. However, when rainwater comes in contact with different components of atmosphere and rainwater harvesting systems, it gets contaminated with the substantial amount of heavy metal, nutrients, dirt, sediments and microbiological contaminants such as bacteria, viruses, and protozoa, etc. Therefore, knowledge of potential contaminants associated with water from rainwater harvesting system is very important.
Types of contaminants of a rainwater harvesting system
Quality of harvested rainwater depends firstly upon the environmental conditions (such as topography and weather) in which the rainwater harvesting unit is installed and the materials used in the construction of the system (Lee et al. 2010; Abbasi and Abbasi 2011; DeBusk and Hunt, 2014). Different contaminants of a rainwater harvesting system broadly fall into three categories, i.e., physical (dirt, dust, ash, debris, plant material etc.) chemical (reactive chemical species emitted from automobile and industrial activities) and microbiological (bacteria, protozoa, and viruses). These contaminants can come from various sources such as the atmospheric pollutants (i.e. wet deposition), atmospheric deposition accumulated on the roof surface (i.e. dry deposition) and materials used in the construction of roof (Abbasi and Abbasi 2011; DeBusk and Hunt, 2014).
Wet Deposition
As rain droplets reach the earth atmosphere, it collects the gases, aerosols, dust, and ash from the atmosphere. Therefore, the composition of rain influenced by the following:
1. Proximity and strength of emission sources: Closer the proximity to the industrial area, agricultural fields aerially sprayed with pesticides, heavily trafficked roads etc. higher the chances of rainwater to pick the sulfate, nitrite, nitrate, carbon dioxide and pesticides from the atmosphere.
2. Reactions of rainwater with the chemical species present in the atmosphere and moving air masses. The most common phenomenon known to be the formation of acid rain that generally begins with the scavenging of sulfur oxides (SOx) and nitrogen oxides (NOx) by rainwater. These atmospheric particles released into the air from anthropogenic activities such as combustion of fossil fuels (coal and oil), vehicular emission etc. and some natural causes also such as forest fire (natural), volcanic eruption etc. Generally, the pH of rainwater remains slightly acidic but when it reacts with SOx and NOx, the pH further decreases than the normal and becomes acidic rain (Hamdan 2009; Lee et al. 2010; Abbasi and Abbasi 2011; DeBusk and Hunt, 2014).
Dry Deposition
When the atmospheric pollutants generated from various anthropogenic and natural processes settled down on the surface is known as dry deposition or atmospheric deposition. Constituents of dry deposition include dust (contribute turbidity and suspended solids in water), nitrates from fertilizers, nitrites, heavy metals (Pb, Cu, Zn, Al, Fe, etc.), calcium etc. The dust and other particulates when accumulated on the surface (rooftop) which is used as catchment to collect rainwater can contaminate the rainwater with sediments, nutrients, and heavy metals. The concentration of dry deposits is usually extremely high in first few minutes of the precipitation event that is due to the phenomenon called “first flush”.
Roofing Materials
Roofing material plays a crucial role in deciding the quality of roof runoff as the material used in roofing contribute significant contaminants in collected water. The roofing materials are known to pollute the roof runoff with dissolved and particulate matter due to weathering processes and the reactions (chemical and physical) occurring between the rainwater and the roofing constituents. Iron-zinc, aluminum, galvanized iron and zinc in roofing material have been reported to reduce the rainwater pH (Adeniyi and Olabanji 2005; Mendez et al. 2011; DeBusk and Hunt, 2014). This increase in acidity of rainwater further causes the leaching of chemicals and metals (Chang et al. 2004). Roofing material such as, concrete, gravel, asphalt shingles, clay, or pantile are alkaline in nature and are known to increase pH of rainwater that further facilitates the precipitation of heavy metals (Mendez et al. 2011; DeBusk and Hunt, 2014).
Conveyance (Distribution Pipes)
The distribution piping material and plumbing fixtures in a rainwater harvesting system can significantly contribute the elemental loads in harvested rainwater. The elemental loads include nickel, iron, copper, zinc, lead, arsenic, strontium, molybdenum etc. Nickel is potentially used in plating taps and plumbing fittings.
Storage tank
Storage tank material also significantly influences the harvested water quality. Concrete or plaster tanks can facilitate the precipitation and removal of heavy metal which cannot be achieved in a plastic or metal storage tank. Concrete tanks increase the stored water pH while the tank with plastic material lowers the water pH. Metal tanks can potentially contribute the metal load in water and the plastic tanks may contribute the organic compounds if not comply the specific manufactured standards (DeBusk and Hunt, 2014).
Microbiological contaminants in a rainwater harvesting system
Microbiological contaminants in a rainwater harvesting system come from the soil, plant material decomposed and accumulated on the roof and drainage, dead insects and other organisms on catchment area and storage tank, fecal deposition of birds, mice, lizards, rodents etc., air born microbes blown by winds etc. The microbial contaminants include enterococci, fecal coliform, fecal streptococci, E. coli, Salmonella, Giardia, Cryptosporidium, Campylobacter etc and viruses also.
Maintenance of Water Quality in Rainwater Harvesting System
The improper maintenance of rainwater harvesting unit can affect the water quality and pollute the water with different contaminants such as physical, chemicals and biological (microorganisms). A rooftop rainwater harvesting system can contaminate the stored water by transferring the pollutants (such as dust, organic matter, bird and animal droppings and pollutants from human activities) accumulated on the rooftop catchment and also can cause sedimentation in the storage tank. The harvested water quality can be maintained by regular cleaning and monitoring the proper working conditions of the system including the catchments, gutters, conveyance networks, filters and storage tanks (Lee et al. 2010). Proper functioning of rainwater harvesting system and the maintenance of harvested water quality can be achieved by following ways:
- Fabricating or coating of catchment area should be done with non-toxic materials.
- The roofing surface should be smooth as the rough surfaces trap more particles and pollutants.
- Material containing the constituents that are prone to leach such as zinc, copper etc. should be avoided in roofing.
- All the inlets should be covered with nylon or wire mesh to prevent the entry of any insect, debris, birds dropping etc into the storage tank.
- The entry of leaves and debris into the system can be prevented by installation of mesh filters at the mouth of drainpipes.
- The tank must be covered avoiding the exposure of stored water to sunlight. As the sunlight promote the algal growth in water.
- To facilitate cleaning of the tank, an outlet pipe should fit and fixed in the tank at the bottom level.
- The quality of collected rainwater can be improved significantly by diverting the first flush. This can be achieved by installing a first flush device in the conduit before it connects to the storage container. A properly maintained first-flush device can efficiently improve the collected rainwater quality to a great extent.
- Proper treatment and maintenance of hygienic conditions should be done to avoid development of any pathogen in stored water. The stored water treatments include chlorination, boiling, filtration and sunlight disinfection discussed as under:
o Chlorination is preferred if the rainwater contains color and/or any odor. Calcium hypochlorite (CaOCl2) a mixture of chlorine and lime also known as stabilized bleaching powder is used for disinfection as it kills bacteria and makes the water suitable for drinking. The preferred dose of calcium hypochlorite is one gm/200 lt. of water for chlorination. Chlorine tablets are also used to disinfect the water. One tablet (0.5 gm) is sufficient to treat a bucketful (20 lt.) of water.
o Boiling is an effective and simple method to kill pathogens present in water. Usually, 10 to 20 minutes of boiling is preferred and enough to eliminate any biological contaminants.
o Direct sunlight exposure of stored water before consumption effective to remove many pathogens. This can be achieved by exposing the water filled in clear glass or plastic bottles for several hours (6 hrs.). Solar disinfection (SODIS) method can be preferred to disinfect the water. SODIS uses the UV (ultra-violet) radiation of the sun and has been found that the combined effect of radiation and thermal treatment significantly kills the microbial contamination in water. The efficiency of SODIS method depends upon several factors such as temperature, weather conditions, bottle material, water quality (turbidity and oxygen content) and bottle shape.
o If the weather conditions and the temperature are not optimal the disinfection can be achieved by using the half blackened plastic bottle for SODIS or simply by placing the water bottle on a black surface.
o The water used to disinfect in SODIS system should free from turbidity i.e., usually less than 30 NTU.
o Transparent or plastic bottles of Poly Ethylene Terephthalate (PET) are preferred over PVC bottles as PET bottles contain fewer UV stabilizers than PVC.
o The maximum efficiency of SODIS can be achieved by using the bottles with more surface area.
o SODIS method is efficient for cleaning water with high oxygen content as the sunlight produces oxygen free radicals and hydrogen peroxides (the highly reactive species of oxygen) that kill microbes (Jain, 2007).
The Government of India is promoting the conservation of water to secure the water resource. National Water Policy and the State Water Policy have also recognized the role of rainwater harvesting in maintaining the national water security. As a result, the state government and urban development authorities of different states have made the development of rainwater harvesting system as mandatory in their regions. For example, Tamil Nadu under the rainwater harvesting scheme in 2001 has made the constructions of rainwater harvesting structures mandatory in all the new buildings and rejuvenation of all the existing RWH structures to avoid the depletion of groundwater. Ahmedabad Urban Development Authority (AUDA) in Gujarat state had made rainwater harvesting mandatory for all buildings covering an area of over 1,500 square metres (sq m) and Ministry of Urban Affairs and Poverty Alleviation has made rainwater harvesting mandatory in all new buildings with a roof area of more than 100 sq m and in all plots with an area of more than 1000 sq m (Eslamian, 2015). Various Governmental and Non – governmental organization are also working in developing the manual for rainwater harvesting system.
Summary
In this lecture we learnt about:
• Importance of rainwater harvesting
• Different types of rainwater harvesting system
• Maintenance of water quality in rainwater harvesting system
you can view video on Rain water harvesting |
References
- Abbasi, T. and Abbasi. S. A. (2011). Sources of pollution in rooftop rainwater harvesting systems and their control. Crit. Rev. Env. Sci. Tec. 41: 2097-2167.
- Agarwal, A. and Narain, S. (1997). Dying wisdom. Rise, fall and potential of India’s traditional water harvesting systems. State of India’s environment: a citizens’ report. Centre for Science and Environment. Thomson Press (India) Ltd., Faridabad.
- Anant D. Patel, A.D. and Shah, P.K. (2015). Rainwater Harvesting- A Case Study of Amba Township, Gandhinagar. Conference Paper, National Conference on “Transportation and Water resources Engineeirng” NCTWE, Ahmedabad.
- Bahadir E. B. and Meric, S. (2015). Rainwater Use in Urban Ares: Rainwater Quality, Harvesting, Toxicity, Treatment, and Reuse. In: Urban Water Reuse Handbook. Eslamian, S. (Ed.). CRC Press, USA. ISBN: 978-1-4822-2914-1, Pp: 771-786.
- CPWD (2002). Rain Water Harvesting and Conservation – Manual. Consultancy Services Organisation, Central Public Work Department, Govt. of India, New Delhi.
- Critchley, W., Siegert, K and Chapman, C. (1991). Water Harvesting (AGL/MISC/17/91), A Manual for the Design and Construction of Water Harvesting Schemes for Plant Production. Food and Agriculture Organization of the United Nations, Rome. http://www.fao.org/docrep/U3160E/u3160e00.HTM
- CWC (2005). Water Sector at a Glance, Water Data Complete Book. http://www.cwc.nic.in/main/downloads/Water_Data_Complete_Book_2005.pdf
- CWC (2013). Water and Related Statistics. Water Resources Information System Directorate Information System Organisation Water Planning & Project Wing, Central Water Commission, New Delhi.
- DeBusk, K and William Hunt, W. (2014). Rainwater Harvesting: A Comprehensive Review of Literature, Report No. 425, WRRI Project No. 11-12-W, Water Resources Research Institute, North Carolina.
- Hamdan, S. M. (2009). A literature based study of stormwater harvesting as a new water resource. Wat. Sci. Tech. 60: 1327-1339.
- Huston, R., Chan, Y. C., Gardner, T., Shaw, G. and Chapman, H. (2009). Characterisation of atmospheric deposition as a source of contaminants in urban rainwater tanks. Water Res. 43: 1630-1640.
- Jain, A.K. (2007). Rain Water Harvesting. In: Water – A Mannual for Engineers, Architects, Planners and Managers. Daya Publishing House, Delhi. ISBN: 81-7035-373-4. Pp. 248-302.
- Kus, B., Kandasamy, J., Vigneswaran, S and Shon, H. K (2010). Analysis of first flush to improve the water quality in rainwater tanks. Wat. Sci. Tech. 61: 421-428.
- Lee, J. Y., Yang, J.-S., Han, M. and Choi, J. (2010). Comparison of the microbiological and chemical characterization of harvested rainwater and reservoir water as alternative water resources. Sci. Total Environ. 408: 896-905.
- UNFPA (2001). State of World Population 2001. Footprints and Milestones: Population and Environmental Change. United Nations Population Fund, New York. ISBN: 0-89714-609-3.