0198566123.pdf

systems into two zones: the vadose zoneand the
saturated or basal water zone. The vadose zone
contains groundwater compartments, often linked
into chains, interspersed with dry zones. The basal
zone is characterized by many closely placed
groundwater compartments and by a high percent-
age of saturated secondary fractures. Both fresh
water and seawater occur in this zone, with tides
influencing the water level up to 4–5 km from the
coast (Ecker 1976). In the absence of rain, the sea-
water within islands would be at sea level; how-
ever, rainwater percolating through an island floats
on the denser salt or brackish water that permeates
the base of the island, in what is termed a
Ghyben–Herzberg lens(Menard 1986).
As indicated above, the average annual rainfall
varies greatly within the island of Tenerife, from
over 800 mm in the highest parts of the Anaga
peninsular (a sum that might be doubled if fog
drip were considered), to less than 100 mm in the
extreme south of the island. The rain shadow thus
has profound effects both on the ecology and on
the potential human use of the island. The steep
north side, which benefits from the majority of the
precipitation, is intensively cultivated, whereas

much of the flatter east and west coastal zones is

essentially useless for cultivation without irriga-

tion. Thus, even on this large, high island, ground-

water (which increasingly has been tapped from

aquifers deep in the volcano) is the most

important source of water for the human inhabi-

tants, and water constitutes a key limiting

resource for development (Fernández-Palacios

et al. 2004a). Even low-lying atolls maintain a

freshwater lens but, as noted in the introduction to

this chapter, very small islets (below the order of

about 10 ha) can lack a permanent lens. Such habi-

tats are liable to be hostile to plants other than

those of strandline habitats, thus limiting the vari-

ety of plant species that can survive on them

(Whitehead and Jones 1969).

Tracks in the ocean

One of the intriguing features of the island biogeo-

graphical literature is that isolation is, in general,

rendered merely as distance to mainland. This

ignores the geography of the oceans. Figure 2.19 dis-

plays the pattern of surface drifts and ocean currents

in January. This pattern is, of course, variable on an

THE PHYSICAL ENVIRONMENT OF ISLANDS 37

Oya Shio

current

North Pacific current

N. equatorial c.

Eq. counter-current

S. equatorial current

West wind drift

Peru

or

Humboldt

current

Falkland

Brazil S. eq. c

Eq. c. c

N. eq. c.

California

current Florida

Gulf

stream c.

Labrador

current

North Atlantic

drift

Warm currents

Cool currents

Canaries

current

Guinea c.

Benguela

current

Agulhas

Eq. c. c.

S. eq. c.

West wind drift

N. eq. c.

c.

c.

c.

c

Kuroshio

current

Alaska

current

Figure 2.19Surface drifts and ocean currents in January. (Redrawn from Nunn 1994, Fig. 4.)