Physiography

As noted, small islands are often classified according to topography as either high or low. This classification attempts to distinguish between those with surface water resources in the form of streams and rivers, due, in part, to their ability to influence precipitation patterns, and those with no significant surface runoff. Volcanic islands are typically high islands, and coral atolls are typically low islands. Exceptions include raised coral, limestone islands which are topographically high yet generally have no surface water.

On high volcanic islands, surface water resources are mainly in the form of ephemeral and "flashy" creeks, although volcanic and bedrock islands with low permeability, rocky soils may have small perennial streams (except during periods of extended drought).

Figure 5. Effect of ENSO on annual rainfall, Christmas Island, Kiribati (UNESCO, 1991)

On these islands, surface runoff can be an important component of the water balance.

However, on most high islands, surface runoff occurs rapidly after rainfall, recedes to little or no flow within hours, and forms a minor component of the water balance.

On low islands, surface water, if it occurs at all, is likely to be in the form of shallow, and often brackish, lakes. Low islands are also directly affected by their height above sea level and the width of their fringing reefs, which, together with sea level movements due to tides, pressure changes and longer term influences, determine their risk of overtopping by storm surges.

Similarly, larger and wider islands are more likely to have significant, perennial surface water resources than smaller, narrower islands (provided that all other conditions such as climate and hydrogeology are similar). The width of a small island is particularly important due to its influence on the occurrence of basal aquifers where favourable hydrogeological conditions for freshwater lens formation exist.