|Sourcebook of Alternative Technologies for Freshwater Augmentation in Small Island Developing States (UNEP-IETC, 1998, 230 p.)|
|Part A - Introduction|
|5. Methodology for the identification and classification of small islands|
The climate of a small island is one of the major influences on the availability of naturally-occurring, freshwater resources (UNESCO, 1991; Hay, 1991). Many SIDS are located within the tropical areas of the Caribbean, Pacific and Indian Oceans. These regions are normally influenced by warm, moist, northeasterly and southeasterly trade winds. The "doldrums" occur at the Equator, and are characterised by low pressure, strong vertical movements (convection) and atmospheric instabilities, high solar radiation and temperatures, and heavy precipitation. However, there are anomalous dry areas within this area. The Inter-Tropical Convergence Zone (ITCZ), where the northeasterly and southeasterly tradewinds meet, occurs near the Equator in both the northern and the southern hemispheres. These features are the controlling mechanisms of tropical climate, but occur at differing time scales, ranging from diurnal convection, easterly waves, tropical cyclones, thirty-sixty-day oscillations, monsoons, quasi-biennial oscillations, semi-decadal El Niouthern Oscillations (ENSO), and long-term climatic changes including global warming and the "Greenhouse Effect". At the time of preparation of this Source Book, one of the strongest ENSO's on record was forming in the Pacific Ocean and beginning to exert meteorological influences on the world weather pattern.
On small islands in tropical regions, precipitation occurs predominantly as rainfall (Figure 1). Other forms of precipitation, particularly dew condensation and fog interception, may occur in highland areas of small, high tropical islands, but these are relatively minor forms of precipitation in comparison with rainfall. For this reason, emphasis is placed on rainfall in this book. The important characteristics of rainfall on a particular island, from the water resources viewpoint, are its spatial and temporal distribution.
Spatial variation of rainfall: Rainfall varies considerably between small islands and can vary within a given island. The spatial rainfall variation on high islands is often very significant owing to orographic effects. Under the influence of moist winds, rainfall on the windward side is considerably higher than on the leeward side, as the moist air currents are forced to rise over elevated terrain. On the leeward side, dry and possibly arid conditions can prevail due to the presence of a "rain shadow" (or F effect. Rainfall gradients on small, high islands are often very steep. On low, flat islands, in contrast, orographic effects (Figure 2) are negligible and there are no significant long term spatial rainfall variations, although minor variations, typically 10% to 20%, in mean annual rainfall have been observed on some low islands over short distances.
Temporal variation of rainfall: Inter-annual variability of rainfall is often high on small islands. For example, the maximum, mean and minimum annual rainfalls on Tarawa, Kiribati, for the period 1948-1991 are, respectively, 3 843 mm (in 1987), 2 029 mm, and 398 mm (in 1950). A more extreme example is Christmas Island, Kiribati, located in the dry equatorial part of the Pacific, where the maximum, mean and annual minimum rainfalls for the period 1939-1991 are, respectively, 3 373 mm (in 1987), 903 mm, and 177 mm (in 1954). The coefficients of variation (= standard deviation / mean) of the annual rainfall are 0.45 and 0.70 for Tarawa and Christmas Island, respectively, and are considerably higher than on many other tropical islands, where the coefficients of variation normally range between about 0.2 and 0.4.
El Niouthern Oscillation (ENSO) influences: ENSO and anti-ENSO (also referred to as La Nina) events can produce very wet and very dry cycles. On many islands in the Southeast Asian region and western parts of the Pacific Ocean, ENSO episodes are often associated with lower than average rainfall. On some islands, periods of up to 6 months may elapse before significant rainfall occurs. This was particularly evident during the major 1982-1983 ENSO event. The influence of ENSO episodes is known to extend across the Pacific Ocean, through Indonesia, to the Indian Ocean, where low rainfalls have been experienced during the more extreme ENSO episodes in the Cocos (Keeling) Islands and Christmas Island in the northeastern portion of the Indian Ocean. In contrast, the opposite influence of ENSO episodes on rainfall volumes has been experienced on islands in the central and eastern part of the Pacific Ocean. The rainfall on Christmas Island, Kiribati, (Figure 4) was well above average during the major 1982-1983 ENSO event. For the 12 month period, August 1982 through July 1983, the rainfall on Christmas Island was 4 312 mm, or about 4.8 times the mean annual rainfall.
ENSO events not only affect precipitation, they cause sea-level rise due to elevated sea-surface temperatures and consequent thermal expansion of the oceans. This can exacerbate the problems of small, low islands. Sea-level rise caused by the 1987 ENSO event resulted in damage to crops on many atolls in the Federated States of Micronesia, adding to the problems caused by the drought conditions induced by the same ENSO event.
In contrast to rainfall, evapotranspiration also is a vital part of the hydrological cycle of tropical small islands (Figure 3), and can account for the loss of more than half of the rainfall on an annual basis. In fact, evapotranspiration often exceeds the rainfall for individual months or consecutive months during dry seasons or drought periods. Generally, and especially in tropical regions, the variability of evapotranspiration is much lower than that of rainfall. Typical annual values of potential evapotranspiration in the tropics are between 1 600 mm and 1 800 mm.
The climates of many islands are also influenced by random cyclonic events. Cyclones are a major natural hazard for small island communities, often resulting in major wind damage. On high islands cyclones can cause floods, hillside erosion with consequent downstream damage and sedimentation, and storm surges that inundate low-lying areas (and even whole islands as in the case of unconsolidated, sand islands, such as those in the Bay of Bengal). Partial flooding as a result of storms has occurred on a number of Pacific Ocean atolls in the Marshall Islands, Tuvalu and Tokelau. Freshwater lenses suffer when these islands are inundated due to the considerable input of seawater through the aquifer recharge zones on the land surface. Many months may be required to naturally flush the saltwater from freshwater lenses and restore them to a potable condition.
Figure 4. Orographic effects of islands of different altitudes on rainfall.
(A) Poor rainfall distribution. Most of the island is dry except on the windward side below 2 000 m;
(B) Ideal rainfall distribution. Rainfall decreases rapidly from maximum near crest. Rainfall in coastal areas depends on distance from rainfall maximum;
(C) Island mostly dry. These examples are based on the Hawaiian islands and the same conditions may not apply to all islands with similar topography (UNESCO, 1991).