Pacific Atolls

For most atolls of the Pacific, fresh water is a seriously limited resource. Generally, water catchment areas are small and groundwater storage is in the form of a shallow fresh water lens. This essential resource is coming under increasing pressure as populations grow and rates of development increase. These things and the realisation of the possible impact of climate change and variability have highlighted the sensitivity of island communities to the availability of water. In many places there is now great concern for sustainability of groundwater reserves and the adequacy of supply of fresh water accumulated from surface catchments for drinking, irrigation and domestic, commercial or industrial use. The site specific effects of geohydrolic and soil characteristics of the land affect the availability of groundwater on particular islands. But for any atoll, the primary constraint on the fresh water resource is climate. It determines the size and temporal distribution the resource base crucial to management and planning of a wide range of activities. To assess this resource base, its sensitivity to climate change and thus the extent which adaptive measures might have to be relied on, a regional water balance model is used with average monthly climate data and uniformly applied to the Pacific Island Region. The Thornthwaite and Mather bookkeeping procedure (Thornthwaite 1948; Thornthwaite and Mather 1955) for assessing the water budget is used to calculate actual evapotranspiration (Ae) and the 'water balance' (DS), given as:

where P is precipitation and ASm is change in soil moisture. When Ds > 0 it is defined as water available for recharge of groundwater storage. A zero or negative value (mm) indicates a deficit. A soil moisture storage capacity (Sm) of 80 mm (Smith 1983) is used to provide a baseline for estimating AE. To calculate AE, the Thornthwaite bookkeeping procedure requires data for potential evapotranspiration (PE) where

The Priestly-Taylor method (Priestley and Taylor 1972) is particularly well suited to regional assessments of PE, and has been used in the humid tropics regions (de Bruin 1983; Nullet 1987). Calculations are based on a 5° latitude-longitude grid of air temperature, rainfall, cloud amount, solar radiation and net allwave radiation.

To assess regional sensitivity to climate change, the per cent change in the current mean rainfall required to bring about a zero water balance, given as (DS/P)100, is calculated. It provides a measure of the sensitivity of the region to change in rainfall that might be brought about by an enhanced greenhouse effect. The results show that areas sensitive to a 20% change in mean annual precipitation occur in a long, narrow zone at about 12°N latitude and a broad belt extending from Fiji through Tonga, the Southern Cook Islands to Pitcairn Island (Fig. 9.3). This zone identifies those atolls that will have to rely heavily on adaptive measures should climate change result in declining rainfall. More importantly, this is the zone most vulnerable to drought should rainfall decrease in the future. Most other parts of the Pacific Islands Region will require increases or decreases in rainfall well in excess of 20% of the mean annual figure for the change to have any significant effect on the water resource base.

E 150' 1701" 180!! 170" 160!! 15QI! 140!l 130" W

E 150' 1701" 180!! 170" 160!! 15QI! 140!l 130" W

Fig. 9.3 Percentage change (±20%) in rainfall required to bring about a zero water surplus or deficit based on the annual mean. The shaded area bordered by +20% and the -20% isolines is the zone of high sensitivity to change in rainfall. In the future, should rainfall decline, atolls in this zone would suffer the most from soil moisture deficits and, under worse case conditions, they would experience an increase in the frequency of droughts. On the other hand, should rainfall increase, atolls in this zone would stand to benefit the most

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