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It has been 13 years since it last rained in Arica, Chile, the driest point on Earth. In Cherrapunji, India, which at 11.4 meters per year of precipitation is the planet’s wettest point, it is surely raining right now.
The Osa Peninsula, at an average of 4 meters per year of rain, is—in the overall scheme of things—one of the wettest climates on Earth. Our landscape is ribboned with streams and rivers, its rolling topography punctuated with seeps and springs, its aquifers brimming with water. For a new property owner here, domestic water supply would seem like a no-brainer. Yet, it can prove to be a dilemma, particularly if the new land-owner is somewhat conscientiousness about the environmental integrity of the miraculous ecosystem in which he or she has taken residence. There are two issues afoot, one practical: Is there a year-round stream or spring that I can get my water from or can I dig a well?, the other a bit more of a soul-searcher: If I do get water from the little creek that only has a tiny bit of water in the summertime, will I adversely impact my environment?
If we spaced our rainfall evenly out across the months, there would be enough water for us, the plants and animals, and a probably a good portion of Los Angeles as well. But there is practically no rain here at all for four straight months: January, February, March, and April. During these summertime months, many creeks and springs dry. Ground water levels drop. Water grows scarce in the forest. Meanwhile, properties are being subdivided into ever decreasing parcel sizes, causing a proliferation of home-spun private water works tapping small springs and streams to remove increasingly more water from the surface environment, especially during the summer, the height of the tourist season. Of course this carries an environmental impact.
There is no question but that the Osa is going to continue to develop. If it is to do so in an environmentally sustainable manner, then alternative water supplies are necessary to allow the wilderness its quota during the dry summer months when it desperately needs its water.
Rainfall capture is an alternative water supply that is ideally suited to the climactic circumstances of this region and one that has been implemented in four recently built structures in the Matapalo area. The viability of a rainfall-capture system for domestic water supply depends on two factors within the home-builder’s control: 1) the area of the rainfall-capture surface (the roof of the house being planned); and 2) the size of the storage tank, typically built into the foundation of the house. A conceptual diagram of a fully-functional rainfall capture system is shown above. All that’s required to make it work is that the roof and storage tank be large enough to provide four months of usage during the four summer months.
As an example of how to use this information to design a house, the storage volume necessary for a four-person family living under a 2000 square foot roof, assuming a daily water usage rate of 50 gallons per person is found to be 25,000 gallon. At the mean peninsular daily precipitation of 0.43 inches, it would take 47 days to fill such a tank. If everyone is consuming their 50 gallon daily quota while the tank is filling, then the tank would fill in 74 days. Clearly, so long as your roof is big enough, there is not much danger in this climate of consuming more water than you can capture.
The example illustrates that 500 square feet of roof and 6,250 gallons of storage per person are sufficient to sustain a household’s water demands. However, that may be a relatively conservative design parameter, particularly when the effects of water-conservation practices and fixtures are factored into the equation. When variable occupancy patterns particular to this region are considered, it is likely that even considerably more modest rainfall capture systems can provide for basic domestic requirements, that is, excluding irrigation and pool water
demands. People need to be aware of this and consider rainfall capture as a legitimate alternative water supply that is available here on the Osa.Osa Water Works, in conjunction with regional land-owners, schools, the Ministry of Environment and Energy (MINAE), and The Evergreen Foundation has begun a demonstration program focusing on viability of rainfall capture as a domestic water supply in this region. As part of this program, active rainfall collection water supplies in the Matapalo area are being monitored to determine key technical information, like the efficiency of capture for varying roof configurations and materials of construction. The effects of water conservation fixtures will also be evaluated to determine the lower limits per full-time occupant of tank size and rain-capture area. Finally, a regional network of rain gages is being set up for monitoring of seasonal rainfall characteristics of each of the different microclimates of the region. From this information, a pragmatic model for rainfall-capture home design will be advanced so this water supply alternative can be more practically considered by prospective new homeowners wishing to sustain a low-impact relationship with the environment. For more information about the program, or how you can participate or contribute, see www.tropicalrain.org.
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