Open Access Te Herenga Waka-Victoria University of Wellington
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Water Supply for Tongatapu; Past, Present and Future.

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posted on 2021-12-09, 06:58 authored by Tufui, Lopeti

This thesis presents an investigation of the sustainability of the freshwater aquifer (groundwater) at Tongatapu, the main island of Tonga. Water balance modelling is applied to meteorological data to estimate freshwater recharge at a daily resolution for the period 1980-2018. These results demonstrate a very close coupling between recharge and precipitation but also the critical role played by the ENSO cycle in modulating the supply of freshwater on Tongatapu. They also show that previous water balance modelling for the island, conducted at a monthly resolution, has tended to underestimate the rate of recharge by ~8%.   Historical groundwater extraction rates for Tongatapu are also calculated by compiling monitoring data from operational pumping stations across the island. This shows that extraction rates have increased progressively over the past 50 years and approximately doubled in the last 10 years, as a consequence of increased demand from agriculture, tourism and population growth. Although the freshwater resource appears to be sustainable overall at current rates of supply and demand, there have been sustained periods of zero recharge, notably during strong El Nino events in winter (the dry season).   Climate model projections of future rainfall show that Tonga is situated in a region of great uncertainty, due to shortcomings in our knowledge of how the inability of the models to capture the ENSO cycle will respond to anthropogenic warming, and but moreover, climate models are currently unable to simulate the precise correct positioning of the South Pacific Convergence Zone which strongly influences the amount and seasonal distribution of regional rainfall. Nevertheless, this study also conducted predictive water balance modelling for Tongatapu for the end of the 21st century using the current CMIP5 climate projections for the region under a medium (scenarios RCP4.5) and high (RCP8.5) emissions scenario, in both cases showing substantial reductions in freshwater recharge rates compared to the present. These results raise serious concerns for the future sustainability of Tonga’s freshwater resource, especially if extraction rates continue to increase and salination of the aquifer increases as is highly likely due to sea level rise.   Although Tonga can do little to influence the global climate change mitigation effort, this research highlights the importance of addressing currently resolvable infrastructural problems in water supply and reticulation.


Copyright Date


Date of Award



Te Herenga Waka—Victoria University of Wellington

Rights License

Author Retains Copyright

Degree Discipline

Physical Geography

Degree Grantor

Te Herenga Waka—Victoria University of Wellington

Degree Level


Degree Name

Master of Science

Victoria University of Wellington Unit

Institute of Geography

ANZSRC Type Of Activity code


Victoria University of Wellington Item Type

Awarded Research Masters Thesis



Alternative Language


Victoria University of Wellington School

School of Geography, Environment and Earth Sciences


Newnham, Rewi; Clem, Kyle