Rainwater harvesting is defined as a method for inducing, collecting, storing and conserving local surface runoff (rain or surface water flow that occurs when soil is infiltrated to full capacity) for agriculture in arid and semi-arid regions (Boers and Ben-Asher, 1982). Both small and large-scale structures are used for rainwater harvesting collection and storage including water pans, tanks, reservoirs and dams. Commonly used rainwater harvesting systems are constructed from three principal components:
The catchment area is the area where the rainfall or water runoff is initially captured and is in most cases either the roof-top of a house or building, ground surface or rock surface.
In the roof-top method (Figure 1) water from rainfall is collected in vessels at the edge of the roof or channeled to a storage system via gutters and pipes. Roofs can be constructed with a range of materials including galvanised corrugated iron, aluminum cement sheets, and tiles and slates. Thatch or palm leafed roofs can provide a low-cost alternative but can be difficult to clean and can taint the runoff. Tiled roofs, or roofs sheeted with corrugated mild steel or other materials are preferable, since they are the easiest to construct and give the cleanest water (WaterAid, no date). Health hazards can arise from roofs with asbestos sheeting, metallic paint or other coverings that can contaminate the water (Gould, 1992). Roof-top collection is suitable for household level application and can provide freshwater for domestic purposes and small-scale farming.
In the ground surface method water flowing along the ground during the rains is usually diverted toward a tank below the surface (Figure 2). There is greater possibility of water loss than the roof-top system due to infiltration into the ground. The water is generally of lower quality than that collected directly from rainfall. Techniques available for increasing runoff within ground catchment areas include: i) clearing or altering vegetation cover, ii) increasing the land slope with artificial ground cover, and iii) reducing soil permeability by the soil compaction and application of chemicals (UNEP, 1982). Impermeable membranes can also be used to facilitate run-off. Ground catchment is applicable for low topographic areas and is suitable for large-scale agricultural production as it allows for in-situ storage and usage of fresh water for irrigation.
Box 1: Ground surface collection in Paraguay (source: UNEP, 1997b)
|“In Paraguay, areas of low topography used for rainwater storage are known as tajamares. Tajamares are constructed in areas with clay soils at least 3 m deep. The tajamares are served by distribution canals that convey water from the storage area to the areas of use. The collection and storage areas need to be fenced to avoid contamination by animals. This technology is usually combined with storage tanks built of clay. The water is delivered from the in situ rainfall collection area to the storage tank by means of a pump, usually driven by a windmill. Water stored in tajamares is normally used for livestock watering and may be used for domestic consumption after filtration and/or chlorination. Individual tajamares have also been used as a means of artificially recharging groundwater aquifers. The construction cost of a tajamar in Paraguay has been reported at $4,500. This cost includes not only the cost of soil preparation, but also the cost of ancillary equipment such as the storage tank and windmills.”|
Rock surfaces can also be used as collection catchments. Bedrock surfaces found within rocky top slopes or exposed rock outcrops in lowlands often have natural hollows or valleys which can be turned into water reservoirs by building a dam (Figure 3). Developing a rock catchment area typically involves clearing and cleaning the site from vegetation and marking out the catchment area to be enclosed with gutters. Rock surfaces should not be fractured or cracked, as this may cause the water to leak away to deeper zones or underneath the dam. As with ground catchments, water is generally of lower quality than direct rainfall collection. Water quality can be improved if access to the area (for example, by animals and children) is limited (WaterAid, no date).
Several types of conveyance systems exist for transporting water from the catchment to the storage device, including gutters, pipes, glides, and surface drains or channels. Larger-scale conveyance systems may require pumps to transfer water over larger distances. These should be constructed from chemically inert materials such as wood, bamboo, plastic, stainless steel, aluminium, or fibreglass, in order to avoid negatively affecting on water quality (UNEP, 1997). In the case of rock catchments, gutters can be constructed from a stone wall built with rough stones/hardcore and joined with mortar (Pace Project, no date). For household-level rainwater harvesting, gutters, down pipes, funnels and filters are required to transfer and clean collected water before it enters the storage device.
Storage devices are used to store the water that is collected from the catchment areas and are classified as (i) above-ground storage tanks and (ii) cisterns or underground storage vessels. These facilities can vary in size from one cubic metre to up to hundreds of cubic metres for large projects. Common vessels used for small-scale water storage are plastic bowls, buckets, jerry cans, clay or ceramic jars, cement jars, and old oil drums. Devices can be made cheaply with locally available materials such as bamboo and steel and coated with a sand and cement mix (WaterAid, no date). Increasingly popular are ferro-cement tanks in which mortar is applied to a cylindrical wire frame which helps to control cracking. These tanks are feasible up to a size of 100 m³. For storing larger quantities of water the system will usually require a bigger tank or cistern with sufficient strength and durability. Typically these tanks can be constructed out of bricks coated with cement. For water captured from a rock catchment, a dam is the more common form of storage device.
Maintenance is required for the cleaning of the tank and inspection of the gutters, pipes and taps and typically consists of the removal of dirt, leaves and other accumulated materials. Such cleaning should take place annually before the start of the major rainfall season with regular inspections. In regions with unpredictable rainfall, more regular maintenance and cleaning will be required to ensure that the equipment is maintained in good working order. Cracks in the storage tanks can create major problems and should be repaired immediately to avoid water loss. In the case of ground and rock catchments, additional care is required to avoid damage and contamination by people and animals and to keep the catchment area free from vegetation.
Climate change is disrupting global rainfall patterns meaning some parts of the world are suffering from a drastic drop in precipitation leading to a fall in water levels in many reservoirs and rivers. In Sub-Saharan Africa where two-thirds of the region is desert and dryland, the need for improving water management in the agriculture sector is particularly critical. Rainwater harvesting represents an adaptation strategy for people living with high rainfall variability, both for domestic supply and to enhance crop, livestock and other forms of agriculture (UNEP and SEI, 2009).
Generally, the amount of water made available through rainwater harvesting is limited and should be used prudently to alleviate water stress during critical stages of crop growth. Supplemental irrigation is a key strategy and can help increase yields by more than 100 per cent. A small investment providing between 50 and 200 mm of extra water per hectare per season for supplemental irrigation, in combination with improved agronomic management, can more than double water productivity and yields in small-scale rain-fed agriculture (UNEP and SEI, 2009).
Advantages of the technology
Rainwater harvesting technologies are simple to install and operate. Local people can be easily trained to implement such technologies, and construction materials are usually readily available.Rainwater harvesting is convenient because it provides water at the point of use and farmers have full control of their own systems. Use of rainwater harvesting technology promotes self-sufficiency and has minimal environmental impact. Running costs are reasonably low. Construction, operation and maintenance are not labour-intensive. Water collected is of acceptable quality for agricultural purposes. Other benefits include increasing soil moisture levels and increasing the groundwater table via artificial recharge. Rainwater harvesting and its application to achieving higher crop yields can encourage farmers to diversify their enterprises, such as increasing production, upgrading their choice of crop, purchasing larger livestock animals or investing in crop improvement inputs such as irrigation infrastructure, fertilisers and pest management (UNEP and SEI, 2009).
Disadvantages of the technology
The main disadvantage of rainwater harvesting technology is the limited supply and uncertainty of rainfall. Rainwater is not a reliable water source in dry periods or in time of prolonged drought. Low storage capacity will limit rainwater harvesting potential, whereas increasing storage capacity will add to construction and operating costs making the technology less economically viable. The effectiveness of storage can be limited by the evaporation that occurs between rains. In water basins with limited surplus supplies, rainwater harvesting in the upstream areas may have a damaging impact downstream and can cause serious community conflict. Also, when runoff is generated from a large area and concentrated in small storage structures, there is a potential danger of water quality degradation, through introduction of agro-chemicals and other impurities (UNEP and SEI, 2009).
Financial requirements and costs
The cost of rainwater harvesting systems will depend on the type of catchment, conveyance and storage tank materials used but in general the costs of rainwater harvesting systems is considered to be low (UNEP, 1997). The provision of the storage tank is the most costly element, and usually represents about 90 per cent of the total cost (WaterAid, no date). A review of 311 case studies on watershed programmes in India, with rainwater harvesting and rainwater management as important components, found that the mean cost-benefit ratio of such watershed programmes was relatively high at 1:2.14 (Joshi et al, 2005a).
In Bhutan, a three-year rainwater harvesting project aimed at safeguarding farmers from water shortages during dry periods and irregularities in the monsoon rainfall had a total budget of $850,000. Activities included:
- Small scale irrigation development based on rainwater harvesting technologies
- Strengthening farmers involvement and research and extension services
- Vulnerability assessment
- Land survey
- Rural credit
- Project management
- Identification of areas vulnerable to dry spells and erratic monsoon rainfall
- Arial surveys and evaluation of remote sensing images/photographs to determine areas suitable for water harvesting
- Assessment of available and proven rainwater harvesting technologies for adoption
- Technological adaptation to fit the needs and requirements specific to each vulnerable locations
- Research new designs and package improved technologies (studying and modeling runoff behaviour);
- Establish farmers’ capacity to mobilise local resources for technology adoption and actual application
- Demonstration of emerging technologies like supplemental water system, dual purpose system, combined system, modelling
- Training farmers in the maintenance of their investments in RWHTs, and effective utilisation of harvested rainwater
- Economic analysis of rainwater harvesting techniques.
The costs of each activity under the Project are shown in Table 1 below:
Table 1: Budget breakdown rainwater harvesting in Bhutan (source: UNFCC, 2008)
|Activities||Year 1 (US$)||Year 2 (US$)||Year 3 (US$)|
|Small scale irrigation development based on rainwater harvesting technologies||50,000||100,000||200,000|
|Strengthen farmers involvement and research and extension services||100,000||150,000||50,000|
In Burundi, a four-year project to install pilot rainwater harvesting units and train local technicians in their use totalled $100,000, as detailed in Table 2.
Table 2: Activity costs for rainwater harvesting pilot scheme in Burundi (source: UNFCC, 2008)
|Train A1 or A0 technicians by some 3-month training courses abroad (in Africa) for specialisation in rainwater harvesting/storage and hill irrigation techniques||100,000|
|Train A2 technicians locally (two per commune, 12 for Bugesera) in rainwater harvesting/storage and hill irrigation techniques||50,000|
|Set up at least one pilot installation of rainwater harvesting and hill irrigation in each of the six communes of Bugesera||400,000|
|Facilitate similar installations in targeted farmers/stockbreeders||250,000|
|Install one clean water conveyance system by photovoltaic pumping in the area of Bugesera||200,000|
Institutional and organisational requirements
Rainwater harvesting technology is simple to install and operate and does not imply any specific institutional or organisational requirements. However, governments and donors could play a key role in providing subsidies for equipment purchases by making the technology accessible to a larger number of farmers, particularly small-scale farmers, who may have problems raising capital investment funds (UNEP and SEI, 2009).
Key information for rainwater harvesting planning relates to the supply and demand of water (Box 2)
Box 2: Knowledge Requirements for the Selection of Rainwater Harvesting Technology (source: Practical Action, no date)
Barriers to implementation
The cost of rainwater storage systems is often cited as a potential obstacle to wider dissemination of this technology (Gould, 1992). For poorer households some form of financing mechanism, preferable accompanied by a subsidy, will be the only way of acquiring a rainwater harvesting system. A lack of national policy towards rainwater harvesting could also present an obstacle to widespread implementation, access to funding and technical assistance. Communally-owned systems can suffer from lack of protection, care and maintenance (Haitbu and Mahoo, 1999).
Opportunities for implementation
Low and variable productivity in rainfed agricultural areas is the major cause of poverty of 70 per cent of the world’s poor (UNEP and SEI, 2009). Using rainwater harvesting technology therefore offers a real opportunity to increase productivity in regions with low and irregular rainfall. In those regions, development and use of rainwater harvesting can provide a first entry point for success of development programmes from farm to regional level.