About Rainwater Harvesting
RWH systems can be very simple
Water is our most vital natural resource, supporting life and life support processes. While there is as much as 1,400 million km3 of water on earth, only one-hundredth of 1% of this amount is easily available for human use. The amount of water available for each person will continue to decrease as the world's population expands. Unfortunately our present and future water supplies in many parts of the world are being degraded by pollution from domestic waste water, solid waste, industrial effluent and agricultural drainage to name a few. As our natural waters become more polluted, less water is available for our needs and the needs of the natural environment. Every year, approximately 25 million people die, by either drinking polluted water or because they do not have enough water to meet their daily needs.
A single person needs at least half a litre (0.11 gallons) per day to meet basic survival needs and two litres (0.44 gallons) per day to avoid thirst. Some 27 to 200 litres (6 to 44 gallons) are needed per person per day for drinking, sanitation, bathing and cooking. Household water needs vary depending on the type of dwelling, number of residents and type of plumbing fixtures.
Household rainwater harvesting (RWH) systems can be very simple as the water is collected and used at the same point, thus reducing the need for lengthy channeling and transmitting infrastructure. This is particularly useful in areas with dispersed housing patterns, where it may not be cost effective to lay pipeline to remote scattered houses. Generally, rooftops serve as catchments for household systems and rainwater is conveyed via roof guttering to the storage unit (usually a tank made out of plastic, concrete or metal). Communal systems require larger catchments e.g. pavements and an extensive conveyance system. Whether simple or complex, it is important that the catchment, conveyance and storage devices are made of inert materials to avoid water contamination. Inert materials include galvanized roofing sheets and PVC gutters. RWH systems usually include a first flush system, which shunts the first set of collected rainwater away from the storage unit, taking away contaminants like dust, debris and organic material which may have accumulated on the catchment between rain showers. Water contamination can also be reduced by using various filtration systems e.g. a fine mesh or a sand and charcoal filter system. For drinking water, UV disinfection and/or chlorination may be needed to reduce microbial threats. RWH systems require some maintenance and care, as tanks, gutters, pipes and catchment areas need periodic inspection and cleaning to minimize water contamination and ensure maximum collection volume.
Rainwater Harvesting in the Caribbean
Rainwater harvesting has been used in the Caribbean for at least 3 centuries. Sweeney (1995) suggests that the technology is appropriate for all areas in the Caribbean but cautions that water quality may be affected by atmospheric pollutants and material deposited on the catchment surface. Currently it is estimated that approximately 500 000 people in the region at least partially depend on RWH. The islands that still heavily use rainwater include Antigua and Barbuda, the Bahamas, the US and British Virgin Islands (USVI and BVI respectively), the Turks and Caicos and the Grenadines (of both St. Vincent and Grenada). Rainwater dependent islands are generally characterized by low annual rainfall, limited land area and intermittent streams. For example Carriacou, in the Grenadines, which relies almost exclusively on rainwater, receives only around 1,200 mm of rainfall per annum. It is only 34km2 and has no perennial streams (Peters 2006). In contrast, larger Caribbean islands generally have more available surface water due to larger surface areas and more perennial rivers and lakes. Higher elevation islands also tend to have greater rainfall levels due in part to orographic rainfall however very steep mountainous islands are subject to high levels of surface runoff and thus less surface water storage.
While rainfall regimes vary across the Caribbean, as a whole, the region exhibits strong seasonality with more than 80% of annual rainfall occurring within the rainy season usually from May to December. Thus, there may be temporal variation in rainwater usage, with peaks during the dry season when rainwater is used to supplement other water sources. Rainwater usage can also vary based on community characteristics. Rural areas tend to have more RWH systems because of the distance from public water supplies. In Jamaica for example, it is suggested that more than 100,000 people especially in the rural areas of that country, rely to some extent on rainwater. Sweeney (1995) suggests that in St. Lucia RWH is rarely used in urban areas, is moderately used in rural areas but is heavily used in areas not serviced by the public water system. Within individual communities, older households are more likely to have RWH systems as exemplified by New Providence in the Bahamas. On islands like Carriacou rainwater is the only water supply so it is used for everything, from drinking to irrigation. In other parts of the Caribbean rainwater may be used for gardening or laundry with the public water supply reserved for drinking. In Jamaica, a few hotels use RWH as a supplementary water supply which reduces water utility costs up to 35%. In the Bahamas rainwater is also used as a supplementary supply for large apartment buildings, hotels and restaurants.