0198566123.pdf

36 ISLAND ENVIRONMENTS

Sea level

Lower

vadose

zone

Upper

vadose

zone

Key

Secondary fractures

Ground water compartment

Isolated ground water

compartment

Spring

Water level

Flow path

Flow direction

Salt water

Fresh and

brackish water

Interface

Figure 2.18Scheme of the vertical distribution and flow of groundwater through a volcanic island. The vadose zones contain groundwater
compartments and chains, interspersed with dry zones. The Ghyben–Herzberg lens of fresh water under an oceanic island rests on the denser
salt/brackish water that permeates the base of the island. (Modified from Ecker 1976, Fig. 4, with permission from Elsevier.)

southern sector, in the rain shadow, receives far less
precipitation. As a consequence of the differences in
precipitation and temperature, the lower and upper
limits of forest growth are higher on the southern
side. Indeed, the southern sector lacks a dense forest
zone at mid-altitude and is much more xerophytic. A
further consequence of the trade-wind inversion is
the existence of well-defined vegetation belts on the
windward slope, whereas on the leeward slopes,
beyond the influence of the orographic cloud layer,
the vegetational landscape seems to be closer to a
coenocline, i.e. a vegetation continuum (Fernández-
Palacios and Nicolás 1995).

Water resources

Availability of water shapes the ecology and human

use of islands (Whitehead and Jones 1969; Ecker

1976; Menard 1986). Most oceanic islands, whether

they are high volcanoes or atolls, contain large

reservoirs of fresh water. Fresh lava flows are

highly permeable, but over time the permeability

and porosity of the rock decreases as a result of

weathering and subsurface depositional processes.

The residence time of groundwater in the fractured

aquifers (Fig. 2.18) of large volcanic islands may be

from decades to centuries. We can divide these

Teide

Sub-alpine zone

SOUTH NORTH

Cloud layer

North East

trade

winds

Dry southern

current Xerophytic

zone

Forest

zone

Figure 2.17Climatic/vegetation zones of
Tenerife. (Redrawn from Bramwell and Bramwell
1974.)