Precipitation over the continents is a major source of fresh water. It replenishes surface and groundwater and is used for agriculture, industry and human consumption. In many parts of the world, people are dependent on precipitation and run-off due to the lack of surface water such as rivers and lakes. In fact, 90 percent of Africa‘s agriculture is and will remain mostly rain-fed .
Earth‘s globally exposed land surfaces receive approximately 113,000 km3 of rainfall each year . Of this, approximately 41,000 km3, or 36 percent, is manifested as surface run-off and replenishes rivers, streams and lakes. The remaining amount, 64 percent, of the rainfall, is evaporated through vegetation, from soil and water surfaces within the landscape .
The following map shows the mean annual precipitation worldwide between 1961 and 1990:
Figure 1: Map of the Mean Annual Precipitation in mm/year (1961-1990) 
Comparing this to a map showing water stressed regions worldwide, you can see the strong correlation between precipitation and availability of water resources:
Figure 2: Map of Water Stress Worldwide 
Hence, areas that experience moderate to high amounts of rainfall per year are less likely to be affected by physical water scarcity. However, it is not only the amount of precipitation per year that determines whether or not a country encounters water scarcity. Variability in rainfall, losses due to evaporation and run-off also contribute to the amount of water available. A comparison of the annual precipitation of the world‘s continents shows some parts of Africa to have a level comparable to Europe and North America. However, the higher evaporation losses that occur on the African continent result in a lower percentage of precipitation contributing to renewable water resources, setting it apart from other continents . The high variability of rainfall, especially in Africa and Asia, makes it even more difficult to predict how much precipitation can be used for agricultural and domestic purposes. South Asia, mainly India, experiences monsoon-type rainfall from June to September. Throughout the rest of the year, however, rainfall in India is moderate to low .
Figure 3: Average precipitation in Pune, India in mm 
The same applies to Sub-Saharan Africa. Northern Africa experiences heavy rainfall in the summer months, Southern and equatorial Africa experience monsoon-type rainfall in spring and autumn. This uneven distribution of rainfall often leads to a cycle of floods in the rainy seasons and droughts in the dry season . Water-stressed regions do not necessarily lack precipitation but they often lack the means to harvest run-off. According to the UN, the overall quantity of rainfall in Africa is equivalent to the needs of 9 billion people, 9 times more than the current population in Africa . In order to make use of the full rainwater potential of a country, effective ways of managing precipitation must be implemented. One approach is to capture rainwater using Rainwater Harvesting techniques.
What is Rainwater Harvesting?
Rainwater Harvesting (RWH) employs of a wide range of technologies to collect and store rainwater. These technologies can be divided into in situ and ex situ types depending on the source of the collected water. In situ rainwater harvesting technologies are soil management strategies that enhance rainfall infiltration and reduce surface run-off, such as planting different plant species. Ex situ systems capture water from areas such as rooftops, land surfaces, steep slopes, road surfaces or rock catchments and store it in storage tanks.
Storage methods for this captured water often are basins behind dams, ponds, tanks or cisterns. Depending on the size of the storage, ex situ systems can be divided into Passive and Active Harvesting systems. Passive harvesting systems (e.g. rain barrels) are typically small volume systems (50-100 gallons) that capture rooftop run-off without further treatment. Consequently, the captured water is generally not used for drinking purposes. Due to their size, passive harvesting systems are commonly used in residential applications . Active harvesting systems (e.g. cisterns) are larger volume systems (typically 1,000 – 100,000 gallons) that capture run-off from roofs or other suitable surfaces such as terraces and road surfaces. Active harvesting systems provide water quality treatment and use pumps to supply water to a distribution system . While passive systems are usually used by individual households, active systems can be used on a municipal level, thus benefitting the entire community.
The collected rainwater can then be stored for direct use or can be recharged into the groundwater .
Rainwater harvesting and utilization systems have been an important factor in community development since the beginning of human settlement . However, most of these systems were and are small scale, in gardens, next to houses or on small fields. Hence, small reservoirs and farm ponds have been overlooked as a possible solution for water scarcity in many countries. Instead, the focus has been on large-scale, centralized water supply projects that extract and store water from rivers and groundwater resources. However, because of its many advantages, rainwater harvesting has received increasing attention in the last 10 to 20 years .
The main advantage of rainwater capture is that it provides a more continuous and reliable access to water despite severe seasonal and meteorological fluctuations in precipitation. It eases the stress on groundwater, rivers and basins and is therefore an ideal solution to water problems in areas with dwindling or inadequate water resources. Rainwater can be collected and stored within accessible distances, while traditional sources are often located away from the community. The time that women and children spent collecting water could be used for other work or education .
Unlike large dams, which impound and store water over large areas, small-scale rainwater harvesting projects lose less water to evaporation . Additionally, rainwater harvesting reduces off-site flooding and erosion by holding rainwater on site .
Rainwater harvesting technologies are flexible and can be build to meet almost any requirement: Catchment areas can range from large land surfaces to rooftops and roads, hence rainwater capturing systems can be implemented in large cities as well as in rural areas. Additionally, construction, operation and maintenance of RWH systems are generally not labour-intensive or time-consuming .
Most importantly, the amount of water that could be captured through RWH systems is surprisingly high: For example, a flat terrace of area 100 m2 in a region with an average annual rainfall of 600 mm could accumulate 60,000 liters a year. According to the UN, about one third of the annual rainfall is needed to sustain the wider environment, forests, grasslands and base-level river flows . In this case, harvesting only 60 percent of the rainfall would still yield 36,000 liters. This is about twice the annual drinking water requirement of a 5- member family if each person consumes 10 liters per day . Even in regions with only 200 mm of average annual rainfall, about 12,000 liters of rainwater could be harvested each year on a surface of only 100 m2.
Problems: Water Contamination
Rainwater is is a clean source of water; the quality of rainwater is often better than ground water or water from rivers and basins . Rainwater is salt-free which means that it can easily be used for irrigation purposes without damaging plants and roots. However, atmospheric pollution due to high industrial and agricultural activities or traffic and contamination of the catchment surfaces are major issues as they contaminate the rainwater and make it unsuitable for drinking. Consequently, in many cities around the world, rainwater should not be used for drinking without prior treatment . Additionally, especially in tropical and subtropical areas, storage tanks can provide a habitat for harmful vector diseases, bacteria, microorganisms and algae .
There are a number of water treatment techniques that can be implemented to improve the water quality: The first flush of the harvested rain has a higher concentration of contaminants due to the washing off of particles. Hence, most water harvesting systems have a “first flush diverter”, which avoids that the first flush reaches the tank . Screens and filters are also generally employed as a first step to improve the quality of the harvested water. Sand and gravel filters for example are easy and inexpensive to construct and can be used to filter turbidity, particles like silt and clay and microorganisms out of the water .
Figure 4: Gravel, charcoal and sand filter 
Chemical treatment like chlorination is also an option that is sometimes employed. Chlorination is done with a stabilized bleaching powder (calcium hypochlorite – CaOCl2) that can kill all types of bacteria and make water safe for drinking purposes. Approximately 1/4 of a teaspoon is sufficient to treat 200 liters of water . Ultraviolet water disinfection is a process that kills most microbiological organisms. A pair of sediment filters removes larger particles from the water so that the UV light can do its job disabling the DNA of bacteria in the rainwater. Without these filters, the particles could block the UV light and the bacteria would be able to pass through the water unharmed, thereby getting into the drinking water system. UV light has the distinction of adding no harsh chemicals or flavors to the water. Therefore it is often a preferred method to kill bacteria versus chlorination . However, the sediment filters need to be changed regularly and the UV-lamp needs to be powered by electricity. Hence, UV water disinfection might not necessarily be the best option for people living in rural areas of developing countries without access to electricity.
One of the easiest and yet very effective methods of purification is boiling the water. 10-20 minutes are usually enough to remove all biological contaminants . To boil the water, especially in countries in Sub-Saharan Africa with high solar energy potential, a solar cooker or heater can be used. A solar cooker focuses solar radiation onto a kettle by reflecting the radiation from its curved surface. Hence, the energy of the sun can be used to boil the water instead of burning wood or using electricity .
Figure 5: A solar cooker focusing solar radiation onto a kettle 
Boiling the water removes most of the biological contaminants in the water. Unfortunately, heavy metals and other non-organic substances are still in the water after treatment. Hence, to achieve the best water quality possible, a combination of different methods, for example filters and boiling, should be employed.
Due to its many benefits, rainwater harvesting has received increased attention in the last decades and both governments and NGOs have started promoting rainwater harvesting as a solution to water scarcity and water access . Consequently, a rapid expansion of rainwater catchment systems has occurred, especially in Asian and African countries facing water scarcity . Rainwater harvesting is mostly a local intervention  and a decentralized method of gaining access to clean drinking water. Unlike large-scale water systems such as pipelines or dams which have to be built, financed and managed by governments, rainwater harvesting techniques can be implemented by small communities or even individuals. Passive harvesting systems, such as rain-barrels, can be purchased from a hardware store or online retailer for about $70 . Active harvesting systems are generally more expensive. The total costs for active systems vary widely depending on the size and complexity of the system. For simple systems, which collect water from roof areas without further water treatment, the storage volume is the main driver of the costs. Depending on the size and material used, the costs are typically between $1.50 – $3.00 per gallon of storage, with per gallon costs generally decreasing with increasing tank size . Additional costs for water treatment, filtration, pumps and distribution systems often increase the costs by an additional $2.00 – $5.00 per gallon of harvesting system capacity . Consequently, prices for active harvesting systems may range from $1,500 to tens of thousands of dollars, depending on size and complexity. Additionally, maintenance costs of a few hundred dollars per year also have to taken into account .
Even though rainwater harvesting systems can be expensive – especially for small villages and communities in developing countries – they are still far more economically feasible than large-scale projects and relatively easy to subsidize by governments. Building dams or a sufficient infrastructure to distribute water across countries can cost many millions of dollars, far more than developing countries could afford.
Small-scale projects like rainwater harvesting systems can be funded by multiple sources: governments, communities and non-governmental organizations. In fact, many NGOs like The Water Project or Charity Water fund rainwater harvesting technologies . However, just the willingness to help does not necessarily lead to a successful project. The question that must be addressed is whether residents of towns and villages will accept the water harvesting technologies.
It is very important to promote the advantages of rainwater harvesting: it increases access to clean water for domestic uses. Water harvesting systems can also benefit regional economies if local companies are commissioned to build and maintain the systems. In general, projects should always be carried out by the population: they need to be able to make decisions and implement their plans. The role of NGOs – and Mission 2017 – is to guide and advise. If villagers are not included in the planning and construction process and if their opinions are not recognized, they are much less likely to accept the changes. Additionally, if people are financially or otherwise committed to the water collection systems, they are more likely to maintain them. Thus governments and NGOs should subsidize the projects but make sure that communities contribute money and labor to their implementation.
Keeping all the benefits of rainwater harvesting in mind, Mission 2017 proposes to encourage and facilitate the construction of more Active Harvesting Systems in rural areas in Africa and Asia. The construction of larger tanks and cisterns including water treatment systems would improve water quality as well as water access. For cities, Mission 2017 proposes to encourage and facilitate the construction of more Passive Harvesting Systems to capture as much water as possible from surfaces like roads and rooftops that would otherwise be lost as run-off. In comparison to rural areas, most cities have space limitations. Hence, constructing smaller catchment and storage tanks will be easier than building large centralized ones that take up additional space. Due to higher air pollution, captured water in cities should only be used for non-potable purposes such as gardening and toilet flushing. In return, more water is saved that can now be used as clean drinking water.
The funding for the projects should come from governments, non-governmental organizations and communities. Additionally, governments and non-governmental organizations have to implement educational programs that inform people about the benefits and the problems of rainwater harvesting. But most importantly, villagers have to be taught how to maintain the systems so that they can operate them independently.
If these steps are implemented in the future, Africa and Asia could finally make use of their high rainwater potential, with more people having a stable access to clean drinking water.
1. Michael E. McClain, Balancing Water Resources Development and Environmental Sustainability in Africa: A Review of Recent Research Findings and Applications, (Springer Link, December 2012), Accessed November 2013
2. United Nations Environment Programme, Rainwater harvesting: a lifeline for human well-being, (UNEP and Stockholm Evironment Institute, 2009), http://www.unwater.org/downloads/Rainwater_Harvesting_090310b.pdf, Accessed November 2013
3. Figure 1, Mean Annual Precipitation in mm/year, http://www.whymap.org/whymap/EN/Downloads/Additional_global_maps/precipitation_g.html?nn=1577156, Accessed November 2013
4. Figure 2, Map of Water Stress World-Wide, (BBC News, September 2012), http://www.bbc.co.uk/news/science-environment-11435522, Accessed November 2013
5. United Nations, The Africa Water Vision for 2025: Equitable and Sustainable Use of Water for Socioeconomic Development, http://www.unwater.org/downloads/African_Water_Vision_2025.pdf, Accessed November 2013
6. Figure 3, World Weather and Climate Information, Average precipitation in Jaipur, India in mm, http://www.weather-and-climate.com/average-monthly-Rainfall-Temperature-Sunshine,pune,India, Accessed November 2013
7. United Nations Environment Programme, Rainwater harvesting could end much of Africa‘s water shortage, (UN News, November 2006), http://www.un.org/apps/news/story.asp?NewsID=20581&Cr=unep&Cr1=water#.UpA3fGTknRg, Accessed November 2013
8. United States Environmental Protection Agency, Rainwater Harvesting, Conservation, Credit, Codes and Cost, (EPA, January 2013), http://water.epa.gov/polwaste/nps/upload/rainharvesting.pdf, Accessed November 2013
9. M.T. Amin and A.A. Alazba, Probable Sources of Rainwater Contamination In A Rainwater Harvesting System and Remedial Options, (Australian Journal of Basic and Applied Sciences,2011), Accessed November 2013
10. Dominik Wisser, Steve Frolking, Ellen M. Douglas, Balazs Charles J. Vörösmarty, The significance of local water resources captured in small reservoirs for crop production – A global-scale analysis, (Journal of Hydrology, 2009) http://faculty.umb.edu/ellen.douglas/files/Wisser_etal_JoH2009.pdf, Accessed November 2013
11. UN-Habitat, Book 3: Project Managers and Implementing Agencies, Rainwater Harvesting and Utilisation, Accessed November 2013
12. Patricia H. Waterfall, Rainwater Harvesting for landscape use, (Revised 2006), http://cals.arizona.edu/pubs/water/az1344.pdf, Accessed November 2013
13. Rain Harvest Systems, http://www.rainharvest.com/rain-harvesting-pty-downspout-first-flush-diverter.asp, Accessed November 2013
14. Figure 4, Centre for Science and Environment, Components of Rainwater Harvesting System, http://cseindia.org/node/657, Accessed November 2013
15. Doug Pushard, UV and Carbon Filters, (Harvest H2O), http://www.harvesth2o.com/uv_carbon_filtration.shtml#.Uo_oLmTknRg, Accessed November 2013
16. Zhu Qiang, Li Yuanhong, John Gould, Every Last Drop – Rainwater Harvesting and Sustainable Technologies in rural China, (2012), Accessed November 2013
17. Figure 5, A solar cooker focusing solar radiation onto a kettle, www.qstowell.webspace.virginmedia.com, Accessed November 2013
18. The Water Project, http://thewaterproject.org, Accessed November 2013
19. Charity Water, http://www.charitywater.org, Accessed November 2013