Island Water & Watersheds
Photo: High Island [photo credit: http://tropicalparadise.net].
Photo: Low Island (Ifira Island).
When we understand the importance of water and the watersheds which catch, store, filter & deliver our fresh water to us, we gain a deeper understanding of where we fit in to the ecology of a place.
Permaculture Publications managed to record, transcribe, and publish to the public domain a series of lectures from a PDC given by Bill Mollison in 1981 (New Hapshire, USA). The treatise he gives on High Islands & Low Islands are excellent (and highly entertaining, if you can manage to read them in a lazy aussie drawl to fully capture the dry ironic undertone in his delivery).
There are both High & Low Islands on Vanuatu, so it is important to understand the way each is formed if we are to understand the differences in managing & protecting their watersheds.
Permaculture_on_Islands.pdf Download this file
Download: 'Permaculture on Islands', by Bill Mollison (1981)
For a detailed description of these differences, download the above Mollison transcript (well worth the read), for the cliff notes, see below:
High Islands:
• Air is funneled up the mountains of the windward side. • Clouds form as condensation occurs at upper levels. • Most rain is dropped on windward side. • Rain shadow is created on leeward side. • Upland forests seeds clouds with transpiration, which increases precipitation. • Over time, forests can descend from the upper slopes of the leeward side. • Clouds follow the forest into rain shadow. • Watersheds filter water through sedimentation, solarization, oxygenation & filtration. • Trees protect the watershed.
Low Islands:
• Bird droppings (phosphates) & sand (calcium) form basis of soil. • Pioneer plan species must break through 'platen' layer of phosphate + calcareous sands. • Once platen layer is penetrated by pioneers, groundwater table can form. • Organic matter from vegetation creates humus for succession species. • Groundwater table only 3-5 feet deep. • If groundwater is overdrawn, salt water can get in. • This groundwater is shallow & must be protected! • Surface water catchment strategies help protect groundwater. • Low islands must be protected w/ trees or wind & waves will quickly erode. • Trees protect the island & watershed.