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<Sourcebook of Alternative Technologies for Freshwater Augumentation
in Small Island Developing States>


PART A - INTRODUCTION

5. METHODOLOGY FOR THE IDENTIFICATION AND CLASSIFICATION OF SMALL ISLANDS

The concept of smallness, for the purposes of this study, is largely based on hydrological characteristics. From a hydrological perspective, a small island can be considered to be one on which water resources are very scarce and where special measures often need to be adopted to develop and manage such resources as may be available. These special measures typically go beyond those normally considered as standard practice on large islands or continents.

Various definitions of small islands have been adopted. In 1991, UNESCO adopted 2000 km2 as the areal limit for a "small island". Their definition also included islands where the width did not exceed 10 km. UNESCO also recognised the concept of a "very small island" where the problem of water resource scarcity was even more acute and where surface water resources were normally absent. Very small islands were defined as ones where the area did not exceed 100 km2 or the width was not greater than 3 km. In such islands, groundwater was normally the only naturally occurring source of water. Some very small islands have unfavourable geological conditions for the formation of freshwater lenses (groundwater) and the only option for conventional water resources development may be rainwater harvesting. For the purposes of this study, a small island means all islands less than 2 000 km2 in areal extent, but with special consideration given to the very small island category (defined as islands with areas not greater than 100 km2).

Hydrological Characteristics

Historically, small islands within a particular region have been divided into hydrological zones on the basis of the major influences on their hydrology. Because of the emphasis on their hydrological characteristics in this process of zonation, the term "hydrological characteristics" has been adopted in this study, small islands with different hydrological characteristics are often found adjacent to each other (e.g., coral islands may occur close to volcanic islands). Such geographic realities defeat the use of the term "zone", which suggests a contiguous area in which a group of islands with basically identical characteristics occur.

The major influences on small island hydrological characteristics, apart from their island status, are climate; physiography (including topography and width); geology and hydrogeology; and, soils and vegetation cover. Other factors which are equally, or sometimes more, important are their relative location to large islands or continents and human-induced impacts. Location can influence the optimal solution for water resources development in some cases; for instance, islands close to continents may find it more economic to "import" water via pipeline from the mainland than to develop onsite technologies. Similarly, human-induced impacts such as mining, deforestation, and urbanisation, may yield both advantages and disadvantages for an island. For example, islands that are set up as tourist resorts or military installations often have economic resources and trained personnel that permit them to consider relatively more expensive and technologically sophisticated options such as desalination. While these islands tend to be special cases, they sometimes occur within SIDS, and, indeed, islands that are set up as tourist resorts generate considerable income for the national economy of such island states.

Climate

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 Niño Southern 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 Föhn) 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 Niño Southern Oscillation (ENSO) influences: ENSO and anti-ENSO (also referred to as La Niña) 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 5) 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

Figure 4
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).

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.

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). 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.

Figure 5

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


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.

Geology and Hydrogeology

The geology and hydrogeology of small islands greatly influences the type and distribution of their water resources. This influence is manifested mainly through the spatial distribution of rocks and soils with varying permeability and porosity. Surface water resources occur only on islands with soils of relatively low permeability. Groundwater resources are most abundant on small islands with soils and rocks of moderate to high permeability and porosity. Where the permeability or porosity is low, the availability of exploitable groundwater is generally low - although the converse is not necessarily true. Where permeability is very high, the mixing of freshwater and seawater is likely, resulting in the occurrence of brackish groundwater, which limits the availability of exploitable, fresh groundwater resources.

Small islands can be geologically classified in a number of ways. A convenient classification, outlined by UNESCO (1991) and adopted herein, is volcanic islands; limestone islands; coral atolls; bedrock islands; unconsolidated (or sand) islands; and, islands of mixed geology, although other classifications are equally valid (e.g., Hehanussa, 1993, presented a classification system for Indonesian islands based upon both geological and topographical characteristics). The UNESCO system is elaborated below.

Volcanic Islands: Common in the tropical regions of the Pacific Ocean (e.g., Hawaiian Islands, many islands of the Federated States of Micronesia, the Cook Islands, and French Polynesia), volcanic islands also occur in the Indian Ocean (e.g., Mauritius) and in the archipelagos of the seas between the Indian and Pacific Oceans, as well as in the Caribbean where the West Indies, with the exception of the Bahamas and Cayman Islands, consist of volcanic and related intrusive igneous rock islands. There are at least two sub-types of volcanic islands; namely, the andesitic sub-type which normally forms as island arcs on the continental sides of deep trenches, and the basaltic or oceanic sub-type which rises from the ocean floor in the middle of tectonic plates. Volcanic islands of the andesitic sub-type generally have low permeability and water-bearing properties (Peterson, 1985). Groundwater yields are generally low (less than 1 l/sec). The basaltic sub-type of volcanic islands, where lava rather than pyroclastic rock predominates, varies in permeability and, hence, in exploitable groundwater. Where the lava flows are young, as in the Hawaiian Islands, Western Samoa and French Polynesia, permeability and groundwater potential are high. In older, basaltic islands with a higher degree of pyroclastic material, as in many of the islands of the Federated States of Micronesia, permeability is low and exploitable groundwater limited.

Limestone Islands: Common in the oceans and seas within the humid tropics, examples of limestone islands include raised coral atolls such as Nauru, Niue and many of the islands in Tonga in the Pacific Ocean. Raised atolls are uplifted coral atolls that have undergone subsequent erosion and karstification. Some limestone islands and coral atolls have been subsequently tilted and may be covered by other geologic deposits (e.g., the volcanic ash layers on the limestone islands in Tonga, and the phosphate deposits on Nauru and Christmas Island, Indian Ocean). Limestone islands are generally karstic and weathered as a result of alternate periods of submergence and exposure due to fluctuating sea levels. Caves and solution cavities are often found along the shoreline and within the interior of the islands. The permeability of the limestone is often very high (generally greater than 1 000 m/day) and, consequently, due to saltwater intrusion, freshwater lenses are generally no more than about 10 cm to 20 cm thick even though the islands may be quite wide (e.g., Tongatapu where the island ranges from 3 km to 5 km in width).

Coral Atolls: Common in the Pacific Ocean (e.g., the islands of Kiribati and Tuvalu, and the Marshall Islands) and in the Indian Ocean (e.g., the Maldives, some of the islands in Seychelles, and the Cocos (Keeling) Islands), there are many variants of the coral atoll-type of island but typically they consist of a chain of low coral islands surrounding a shallow lagoon. Coral atolls generally consist of a layer of recent (Holocene) sediments, comprising mainly coral sands and fragments of coral, on top of older limestone similar to that described above. An unconformity separates these two layers at typical depths of 10 m to 20 m below mean sea level. Several, deeper unconformities may exist due to fluctuations in sea level which result in alternate periods of emergence and submergence of the atoll. During periods of emergence, solution and erosion of the reef platform can occur, while further deposition of coral limestone can occur during periods of submergence. The upper sediments are of primary importance from a hydrogeological viewpoint as freshwater lenses occur solely or mainly within this layer (Figure 6). The occurrence of such lenses within this layer is due to its moderate permeability (typically 5 to 10 m/day) compared with the higher permeability of the older limestone (typically 50 to 100 m/day). Permeabilities greater than 1 000 m/day occur in solution cavities within the limestone. These extremely high permeabilities allow almost unrestricted mixing of freshwater and seawater which is less likely to occur in the upper sediments. The upper unconformity, therefore, is one of the main controlling features of the depth of the freshwater lens.

Bedrock Islands: Formed by igneous or metamorphic rocks such as granite, diorite, gneiss and schists, bedrock islands are mainly found on continental shelves or adjacent to large islands of similar geology. Many of the islands of the Seychelles are of this type.

Figure 6

Figure 6. Small island fresh-water lens (exggerated vertical scale).

Unconsolidated Islands: Generally consisting of sand, silt and/or mud, unconsolidated islands are generally found in the deltas of major rivers (e.g., in the Bay of Bengal).

Mixed Geology Islands: Common amongst the oceanic islands are those islands with a mixture of volcanic and limestone rocks. Over long periods of time, the geologic nature of these islands can change. For instance, volcanic islands can subside, forming fringing reefs, and, eventually, erode further, leaving coral limestone as the only rock type visible above sea level.

Soils and Vegetation

Soils play an important role in the hydrological cycle through their influence on evapotranspiration, surface runoff and groundwater recharge. Limestone and coral islands typically have sandy soils of high permeability resulting in little or no surface runoff except where the surface has become compacted. Clay soils, produced by weathering on volcanic islands, have lower permeability than sandy soils, and surface runoff is common.

Soil water retention capacity is an important determinant of evapotranspiration and recharge. Fine-grained soils with high retention capacities favour evapotranspiration, thus reducing recharge. Coarse soils with low water retention capacities allow rainfall to quickly infiltrate below the root zone, decreasing evaporative losses but increasing recharge. During prolonged dry seasons, clay soils may develop cracks and enable rapid infiltration, but, following resumption of the rains, tend to swell, closing the cracks and reducing recharge.

Most islands have only a thin soil covering unless there has been some unusual event such as the deposition of volcanic ash from active volcanoes (e.g., as on the islands of Tonga). Thicker soils have greater water retention capacities than thinner soils. Thick, low-permeability soils can be an effective agent in preventing or minimising groundwater pollution and, hence, in the protection of aquifers. Thin, high-permeability soil layers, such as the sandy soils found on coral atolls, offer very little protection to underlying freshwater lenses.

The natural vegetation on small tropical islands consists of a variety of trees, commonly including coconut trees, and a range of bushes and grasses. Generally, this vegetation is adapted to local climatic conditions and receives adequate moisture for growth from rainfall. It rarely requires irrigation since it has adapted to local climatic conditions. However, extreme droughts and severe weather (e.g., cyclones and storm surges) can damage or destroy even the locally adapted vegetation, although, in most cases, regeneration does occur. Some of the natural vegetation, such as the coconut tree, is remarkably salt tolerant and can grow in brackish water with relatively high salinity levels. Notwithstanding, native vegetation is often partially cleared and food crops have been planted. Such non-native crops may requires irrigation in order to maximise production.

The type and density of vegetation cover has a number of effects on the hydrological cycle and availability of water resources. Vegetation intercepts part of rainfall and causes transpiration to occur. Interception and transpiration tend to decrease recharge and, hence, decrease the available groundwater resources (although, perversely, direct soil evaporation is decreased due to the protective vegetation cover). Depending on the depth to the water table and type of vegetation, direct transpiration losses from a groundwater aquifers can increase. Coconut trees growing on coral atolls, for example, act as phreatophytes which draw water directly from the water table, and can contribute to a reduction in groundwater resources during dry periods. Vegetation may also slow surface runoff and reduce erosion on high islands, increasing infiltration of water into the ground. Reduced erosion losses are desirable for catchment management, protection of river banks, and for extending the useful life of lakes and reservoirs. Thus, vegetation cover on small islands of both the high and low types, while desirable from many viewpoints (food production, forestry, agriculture, soil conservation and other environmental management, aesthetic and social factors), has mixed desirability from the viewpoint of water resources, although, even in cases where net losses of water occur (such as in the case where vegetation transpires water directly from groundwater), the negative effects on water resource potential tend to be outweighed by the positive economic, aesthetic and cultural benefits.

Relative Location

Small islands can be further categorised according to those which can be supplied from continental or large island sources via submarine pipeline or sea transport (tankers or barges), and those which have no practical means of supply other than from the island itself. Generally only those islands which are close to larger land masses (near-shore islands) can be considered in the first category. Most islands fall into the second category (oceanic islands). Some examples of water resources development for nearshore islands in the regions covered in this book include the use of submarine pipelines to supply water to the island of Penang, Malaysia, to resort islands on the Great Barrier Reef off the northeastern coast of Australia, to Manono Island from the main island of Upolu in Western Samoa, and to small offshore islands in the Solomon Islands using small diameter pipes; the use of inter-basin transfers to supply water to Singapore from the Malay Peninsular and the proposed use of such transfers to supply water to Malaysia from Batam and Bintang Islands in Indonesia; the use of ships used to export phosphates to import water as a return cargo to the island of Nauru in the Pacific Ocean; and, the use of barges to supply water to the Bahamas under contract, and to some of the small islands of Fiji and Tonga.

Human-induced Impacts

Human activities influence both the availability of freshwater and the water quality. Humans use, and often pollute, water resources. For example, over-abstraction of water for human use has contributed to the depletion of available water resources, particularly groundwater resources, on a number of small islands (e.g., Male in the Maldives). Increased development, particularly residential development, on even the smallest of islands has led to contamination of underlying or nearby aquifers and surface waters. Contamination from domestic animals, such as pigs and dogs, is also a problem on many islands. In addition, many islands are at risk from chemical pollution from such sources as fuel storage facilities and agricultural activities. In order to maximise food production, native vegetation is often cleared for cash crops and is sometimes accompanied by irrigation, which increases the use of available water resources, and the use of agro-chemicals (e.g., fertilisers, herbicides and insecticides), which are potential contaminants of available water resources. In the case of taro, which is commonly grown in pits dug down to the water table on coral atolls, insecticides are sprayed directly onto the crop. Traces of potentially hazardous organic waste have been found in water supply wells on some islands (e.g., Oahu and Maui in the Hawaiian Islands). In extreme cases, where available water on an island has become fully utilised or polluted to such an extent as to be unusable, off-island sources of water, or more expensive on-island water resource development technologies, must be used.

Region of Interest

This Source Book of Alternative Technologies for Freshwater Augmentation on Small Island Developing States includes technologies that may be potentially applicable to Small Island Developing States in the South Pacific Ocean, Indian Ocean, South China Sea, and Caribbean, as shown in Figure 7. These four regions of the world have been identified as containing all of the Small Island Developing States (SIDS). In the Pacific Ocean, there are nearly 30 000 small islands, over 1 000 of which are populated, and several thousand small islands in the Indian Ocean, including about 1 300 islands in the Maldives alone, of which about 200 are inhabited. Between these oceans lie many thousands of islands, including the island groups of Indonesia, Malaysia and The Philippines, and the islands of Papua New Guinea and Vietnam. In the Caribbean Sea, there are several thousand small islands, of which about 100 are inhabited.

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