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“Water, water everywhere, and how the boards did shrink. Water water everywhere and not a drop to drink.” Samuel Taylor Coleride In one of the world’s wettest climates, water supply may be the last thing that a home builder may view as a potential concern. Look no further than Cape Matapalo, however, and you will see that reliable water supply may not be taken for granted. Historically, Osa Peninsula residents have depended on springs, shallow wells, and streams for domestic water sources. As the population density has increased, however, dependence on surface water has deprived the regional tropical environment of a portion of its most critical element of sustainability: water itself. Ground water withdrawals are less impacting on the environment, though in a realm of increasing population density, ground water withdrawals will cause depression in the water table and increase the potential for salt-water encroachment along the coast. Also, shallow ground water wells are universally reliable as domestic water sources only in the coastal alluvial plane stretching from Rincón to Carbonera on the eastern coast and from the Piro River to just north of Rio Oro on the western coast and not in hard-rock terranes, such as Cape Matapalo, and the mountainous regions. Residents have not historically looked to the skies for their water supply simply due to the ready availability of springs, streams, and ground water. And while the population density was sparse, the environment was arguably capable of sustaining such withdrawals. Rain water catchment is one of the oldest forms of water supply known to man, however, and one that is used as a principal water supply in regions with limited surface water sources but abundant rains, such as low-lying “desert isles”, such as those that comprise The Bahamas. The amount of water that falls on the Osa Peninsula, using the peninsular area of 135,700 square kilometers and a rough average annual precipitation of 4,000 mm corresponds to 5.4 billion cubic meters of water per year. If one one thousandth of this water were captured for domestic water supply, this volume of water would be sufficient to provide 39,340 people with a quantity of 380 liters of water per day, the amount used for water-supply design purposes in the water-wasteful United States. In other words, capture of 1/1000 of the peninsular rainfall would provide enough potable water for twice the existing population of the peninsula. Under actual water consumption rates, it is probably enough to supply 4-5 times the existing population. For a home builder considering rain catchment for water supply, there are only four design variables, three that are “known” and the fourth calculated. These are the following: 1) average monthly rainfall distribution; 2) roof catchment area; 3) number of persons to be served by the water supply; and 4) the volume required for the storage tank (cistern). Unfortunately, the average monthly rainfall distribution is not a known quantity as there are no rainfall measurement stations on the Osa. This is complicated by the fact that substantial variations in precipitation occur on very small geographic scales on the Osa. In an effort to address the shortage in basic data, Osa Water Works, S.A. has proposed a network of precipitation and river-flow measurement stations distributed across the eleven micro-climates identified in the Osa and the circum-Golfo Dulce drainages, as shown in the figure above. The other concern that homeowners should have in their consideration of rain water catchment systems for domestic water supply is the maintenance of potable water quality during periods of storage. In order to address these concerns, OWW has proposed a model demonstration project in Matapalo that tracks water quality across the months of the year and evaluates the use of environmentally friendly methods for control of biologic pathogens including ultraviolet sterilization and advanced oxidation methods employing either hydrogen peroxide and/or ozone. For additional information, or to obtain your own copy of the research proposal entitled: Osa Peninsula Water Supply Solutions: When it rains they pour,” please stop in at CafeNet El Sol, and ask.
Small Numbers: proposed rain-gaging stations. Large Numbers: proposed river-gaging stations.
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