In low coral atolls, groundwater exists as freshwater lenses floating over saline transition zones, grading into seawater. To lessen the risk of seawater intrusion, groundwater is pumped from long, horizontal, infiltration galleries. Population growth, limited land, human activities including agriculture and frequent droughts place groundwater under stress.
Kiribati and Tonga rely on agriculture for valuable export earnings and subsistence. Both countries have major water resource limitations. Surface water is constrained by limited land area, permeable soils and high evaporation rates. Groundwater is therefore of critical importance, especially during drought. However, groundwater is vulnerable to seawater intrusion and mixing as well as contamination from inputs of agriculture, and demand is increasing. There are also issues to do with land ownership and water-use regulation.
Project objectives were: to understand the impact of agriculture on groundwater resources and the impact of groundwater extraction on agriculture; to explore management options for mitigating droughts; and to combine these outcomes with information on hydrology and on the economics and social aspects of island communities to produce a system to lessen conflicts over water allocation and use.
Researchers used standard geohydrologic methods to describe the physical and chemical characteristics of groundwater at the study sites and developed techniques to sample soil water beneath crops and determine their irrigation requirements. They undertook standard climate measurements to estimate evapotranspiration, and heat pulse technology to measure transpiration losses from permanent crops. They conducted surveys to identify perceived demands for water, record socioeconomic and cultural constraints and determine the needs and aspirations of stakeholders.
The research team also developed methods to assess the effects of severity and frequency of drought on groundwater systems in atolls and small islands, and explored risk minimisation strategies.
The team deployed a 'multi-agent system' (MAS) - a simulation tool using bottom-up decision-making to study the interactions between natural and social systems in ecosystem and renewable resource management - to integrate hydrologic, socioeconomic and cultural factors.
Tarawa atoll in Kiribati has two regions - the heavily populated, urbanised South Tarawa and sparsely populated rural North Tarawa. In the low islands studied in Tarawa, Bonriki and Buota in South Tarawa, currently used for groundwater extraction, and the undeveloped Abatao and Tabiteua in North Tarawa, the spatial extents of their groundwater lenses have now been measured. These measurements permitted water balance estimates, using the highly variable climate record, of their sustainable pumping yields of fresh groundwater and identified potential sites for additional galleries.
A critical concern in small island communities with land overlying fresh groundwater stores is the impact of groundwater pumping for reticulated water supplies on traditional crops such as swamp taro and coconuts. On Bonriki, the community attributes a general decline in productivity of coconuts to groundwater pumping. An extensive study of shallow groundwater drawdown and salinity by pumping from all infiltration galleries on Bonriki and Buota found the mean drawdown due to pumping was close to the theoretically predicted drawdown. This was an order of magnitude less than diurnal tidal fluctuations of groundwater that coconut tree roots are exposed to. Pumping also had a negligible influence on groundwater salinity. A theoretical study of the width of the saline transition zone beneath the freshwater lens estimated pumping increased its width by 37 per cent but the frequent El Nio droughts increase the transition zone width by 90 per cent. Examination of the sparse coconut trees at Bonriki and a review of their physiology indicated that declines in productivity were due to tree senescence and lack of crop management.
Because of the scarcity of land on small islands, agricultural activities, such as market gardens and swamp taro production, often encroach over shallow groundwater reserves. The use of animal manures and inorganic fertilisers, construction of open wells for watering and increased cropping density of coconut trees have raised concerns over impacts on groundwater. Extensive sampling of groundwater on Bonriki revealed E. coli bacteria and elevated nitrate and dissolved organic carbon levels in areas with market gardens and abandoned swamp taro pits. Large concentrations of hydrogen sulfide were also found due to reduction of sulfate in groundwater.
A water balance model was used to examine impacts of coconut tree density on groundwater recharge. Tree density had little influence during major wet periods. During droughts, however, it has a major impact causing significant periods of net groundwater loss. Predictions are consistent with measured groundwater salinity records. Analyses of the drought impact on the thickness and salinity of the of the freshwater lens predicted watertable falls of up to 400 mm during prolonged droughts, close to the measured decrease of 440 mm. Predicted increases in salinity of the freshwater lens are consistent with observed increases and measured saltwater intrusion. The nonparametric method of identifying droughts can provide a lead time of about three months warning of droughts.
Local and expert knowledge on groundwater and water supply has been collected and this has been incorporated with the hydrology and salinity dynamics into a Multi-Agent System for groundwater use and management, AtollScape. The system includes all the principal factors in water extraction and use, down to households. A role-playing game, AtollGame to explore different scenarios through use of AtollScape and reduce conflicts has been developed. A two-day trial of the game with relevant representatives from the islands of Abatao and Tabiteua and key government stakeholders produced a flow chart of financial, technical and social solutions and a proposal for a sequential refining of the process to arrive at equitable management options.
Modelling and monitoring of the major fresh groundwater resources demonstrated that a 30 per cent increase in sustainable freshwater extraction was possible for the capital, South Tarawa. In a country with less than 30 L/capita/day of reticulated freshwater this is a significant increase. But analyses of demand and the impact of frequent severe El Nio-related droughts has demonstrated that additional groundwater sources will have to be found for South Tarawa by 2010. The government has decided to initiate investigations for these additional sources.
Links:
[1] http://www.aciar.gov.au/country/Kiribati
[2] http://www.aciar.gov.au/country/Tonga
[3] http://cres.anu.edu.au/
[4] http://www.aciar.gov.au/programarea/Land and Water Resources