Hydrology
Climate | Cyclones | Fauna | Flora | Marine Environment | Geology | Geomorphology and Typography | Hydrology
The water resources of the Cocos (Keeling) Islands consist essentially of groundwater and rainwater. Where conditions are favourable, fresh groundwater on coral islands occurs in the form of shallow freshwater lenses. Such lenses are found in some of the larger islands within the Cocos (Keeling) Islands. The groundwater from these lenses has been and is currently used as the major source of freshwater for potable and other uses on the Southern Home and West Islands. North Keeling Island is too narrow for a fresh water lens to develop. Due to the generally porous nature of the soils and underlying geology, there is no significant surface run-off. Run-off only occurs in localised areas where the ground is compacted and only for very short periods after heavy rain.
Freshwater lenses occur beneath the surface of some islands. The upper surface of a freshwater lens is the water table and the lower surface is a boundary between freshwater and saline water. The lower boundary is not a sharp interface but rather is in the form of a transition zone. Within the transition zone the water salinity increases from that of freshwater to that of seawater over a number of metres (Falkland 1994). The size and salinity distribution of freshwater lenses, particularly the depth of freshwater and transition zones, are dependent on many factors including rainfall, amount and nature of surface vegetation and the nature and distribution of soils (these factors influence the evapotranspiration) and size of the island, particularly the width from sea to lagoon (Falkland 1994).