|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|
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.