The harvesting of water from the surface of roofs for domestic or agricultural uses is a technology employed in developing countries, particularly in the Central American region and specifically in Honduras, by rural communities and marginal urban areas that do not have access to conventional water supply systems. This technology is described in Part B, Chapter 1, "Freshwater Augmentation Technologies."
· Domestic Use
The system basically involves collecting the water that falls on the zinc, asbestos, or tile roof of a house during rainstorms, and conveying it by an aluminum, PVC, wood, or plastic drain or collector to a nearby covered storage unit or cistern, as shown in Figure 1.
Currently the most common container is a metal drum or barrel with a capacity of some 200 liters (54-gallons), set up at the foot of a fall-pipe. This is enough to supply a family for four to five days with 7 liters per person per day, with the possibility of using more than one storage barrel. Particularly when the resident owns the property, a larger storage unit can be built with sufficient capacity - some 9 cubic meters - to meet the dry-season demand of a family of six or seven people.
· Agricultural Use (Irrigation and Animal Drinking Troughs)
This involves collecting water that falls on the roofs of agricultural installations or on the land (microbasins) and piping it to a nearby covered or open storage or impoundment. Harvesting for specialized livestock facilities (confinement, processing, etc.) is a concept similar to the harvesting-storage system for domestic use.
For large-scale watering or irrigation, where the quality of the water is not a limiting factor, a harvesting area has to be selected where irrigation ditches or piping are built leading to a collection dam in which water is stored when it rains. This dam can include wood, stone, or sand filters to prevent obstructions in the irrigation system or physical pollution of the reservoir.
Extent of Use
The range of the technology could be limited to places with minimum rainfall of 600 mm per year - and no maximum limit - preferably concentrated over a few months.
This technology for domestic use can be found throughout the country but to a moderate extent, except in the zone, particularly in the departments of Valle and Choluteca (Pacific watershed), where for different reasons of a cultural and climatic nature, the population feels more pressure to supply itself with water, thereby resulting in a very high potential for water demand for human consumption.
For agricultural use, this technology is used extensively, preferably in the cattle-raising valleys of the center and south of the country, primarily through the construction of earthen dams over seasonal channels containing rainwater, which serve to provide water to numerous cattle ranches.
The establishment of both uses has increased substantially over the last 10 years primarily as a result of competition for water service in those areas where the land is used intensively.
Operation and Maintenance
Operation and maintenance are very simple and consist particularly in cleaning the harvesting system at the beginning of the winter, filtering foreign matter, and maintaining the storage container.
For domestic purposes, operation and maintenance of the system basically involve performing two main tasks:
· Cleaning the roof at the beginning of each rainy season to remove any type of trash and foreign matter, usually by using the water from the first rains; and
· Cleaning the tank at the end of the rainy season. Both operations require a maximum of three days' work per season.
For livestock and irrigation purposes, additional efforts are required because the collection area and storage volume are larger. When microbasins are used, the collection area and piping systems must be maintained with permanent vegetation to filter sediment and refuse. The reservoirs should be drained with the first rainfall to eliminate summer concentrations and should be kept clean and weeded.
When dams are used for watering animals it is preferable to convey the water through pipes to cement or wooden troughs. If the dams are used directly, the cattle should not be allowed to enter the reservoirs and excrement should be prevented from being dragged in by rainwater. For irrigation reservoirs, filters should be installed for sediments, seeds, or obstructions of the piping or distribution mechanisms.
The reservoirs should be surrounded by windbreaks to reduce evaporation and if they are made of cement they could be roofed or covered with plastic sheets.
Level of Involvement
The systems mentioned are for individual - not communal - use, and sometimes are set up thanks to promotion by a state agency or NGO.
The resources used come from the beneficiaries, particularly those with sufficient funds, and when the system is for communal use, for potable water, it is financed by the members of the community. As far as is known, there is not much participation by the central government or private business, with the exception of the "Water for the People" program, which carried out similar projects in the southern part of the country, using uncovered storage tanks.
Agricultural systems for irrigation are developed by individuals, usually with technical assistance from the Water Resource Administration, some international agencies, and NGOs. Financing comes from development funds for irrigation microprojects or from the individuals themselves.
Impoundments for cattle primarily require agricultural equipment for the construction of earthen dams and are decided upon and financed by the ranchers themselves.
The Development and Adaptation Unit of the Ministry of Natural Resources, in coordination with the CATIE project in Nicaragua and Guatemala, initiated some projects in the central zone.
Analyses conducted by the Pan American Health Organization/Pan American Center for Sanitary Engineering and Environmental Sciences (CEPIS) estimated that the cost of harvesting systems using the roofs of dwellings was US$107 for a 5 m3 tank, which would amount in the case at hand to approximately US$241 for a 9 m3 tank.
For storage systems for agricultural use, 350 to 1.500 m3 of water per quarter hectare need to be stored for 15 to 60 days, which involves reservoirs costing from US$1.500 to US$4.000, i.e., similar to the cost of reservoirs for cattle troughs, which contain almost twice the volume.
Effectiveness of the Technology
Harvesting water from rainwater collection systems for human use is considered the most appropriate technology in areas that do not have aqueducts to supply the community with continuous and reliable service. Properly treated and maintained roofs are the best choice as a collection surface, because their location protects the water from pollution, which is typical in ground-level collection surfaces. With this technology, pollutants can be reduced by 80 to 90%.
For example, with precipitation of 700 mm per year and a collection area of 100 m2, 28 to 30 m3 can be stored over a five-month cycle with 40% efficiency - which can benefit a family of 10 during the dry season at a cost of some $2 per m3 of water.
For agricultural use, a 1.000 m2 collection area and rainfall of 700 mm per year, 420 m3 could be collected with losses of 40%, which would be enough to irrigate a family market garden of some 300 m2 in a production cycle of highly profitable crops.
Because of the homogeneous cultural and productive conditions in Honduras, the use of the technology is recommended for the entire country. It is particularly recommended in areas where groundwater is polluted by seawater intrusion and where run-off from rivers and watercourses is minimal. In general, the lack of precipitation during the dry season, the poor quality of river water, and the distance between consumers makes this the most attractive solution.
In isolated areas with high precipitation, such as Gracias a Dios, Islas de la Bahía, and the Atlantic coast, it should be promoted for the purpose of obtaining higher quality water and avoiding high water transportation costs.
· The water collected is of higher quality and safer than that in rivers and watercourses, and therefore reduces medical costs.
· It is an independent system and therefore very appropriate for isolated and disperse communities or settlements.
· It uses local construction materials and labor.
· Sources of energy are not needed to operate the systems.
· The owner/user can easily maintain the systems.
· The water is convenient and accessible; valuable time and effort are saved in collecting and/or hauling water, particularly for the household's women and young people.
· It provides a supply of water to meet future agricultural needs.
· The high initial cost of building the permanent storage facilities could be a prohibitive obstacle to some families; the use of a barrel is more likely, but the volume of water available is then limited.
· The quantity of water available depends on rainfall and the surface of the roof, and additional sources of water are almost always needed. For long periods of drought it is necessary to store excessively large volumes of water.
· The mineral-free water is tasteless and could cause nutritional deficiencies; people prefer to drink water rich in minerals.
· For livestock use, it is important to operate the reservoir properly to avoid pollution of the water by the animals themselves.
· Open reservoirs are systems that promote proliferation of, and provide refuge to, disease vectors and pests.
Future Development of the Technology
The water harvesting technology for human purposes could be vastly improved with the addition of unit nitration and purification systems and of the conduit to the tank. In this way, at least the quality of water for human consumption would be guaranteed. Since the addition of these unit systems has a direct impact on the family economy, the use of stone filters and earthen pots to keep the water cool is recommended. This technology is widely known and used in the country to increase the supply of safe water free of bacteria and other pathogens.
It is necessary to implement programs to disseminate, demonstrate, and promote the use of the technology. Special funds should be established to finance the programs, and the technology could be included on a large scale in rural or urban housing development programs and in comprehensive rural development programs.
Ernesto Bondy Reyes, Director General de Recursos Hídricos, Ministerio de Recursos Naturales, Tegucigalpa, Honduras.
Eddy Larios, Coordinador de la Infraestructura, Proyecto de Desarrollo de los Recursos Hídricos del Valle de Nacaome, Dirección de Recursos Hídricos, Tegucigalpa, Honduras.
Walter Santos, Centre de Entrenamiento de Desarrollo Agrícola (CEDA), Dirección de Recursos Hídricos, Comayagua, Honduras.
Schiller, E.J., and B.G. Latham. 1995. Rainwater Harvesting Systems. Santiago, Chile, PAHO/CEPIS.
Figure 44: Storage of harvested rainwater in Honduras: in a drum (top) or an impoundment (bottom).
Source: Ernesto Bondy Reyes, Director General of Water Resources, Ministry of Natural Resources, Honduras.