van der Velde, Marijn
[UCL]
(eng)
Small island developing states that wish to sustain their freshwater resources face two main challenges: intensification of agriculture and climate change and variability. These topics were studied on the main island of the Kingdom of Tonga, Tongatapu (272 km2 , 175°12'W, 21°08'S). In 1987, a squash (Cucurbita maxima L.) industry emerged on Tongatapu that solely exports to the Japanese market during an off-season niche period in November. At a national level, the squash industry has accounted for about 40% of the total export revenue. With the export of the squash, a ten-fold increase in the importation and use of agri-chemicals has occurred. At the same time an increased population (currently 100.000) puts pressure on the fresh groundwater resources of the island (there is no surface water on Tongatapu). These pressures include both pollution and an increase in pumping that leads to salt water intrusion. Coupled with expected sea level rise, and an increase in the frequency of El Niño episodes, this puts the water resources and the environment under pressure.
Research was done on two scales, the so-called field scale and the island scale. At the field scale measurements and modelling was carried out of plant transpiration using heat-pulse and meteorological measurements made in this tropical maritime climate, water movement, and nitrate and ammonium transport using a hydraulic characterisation of the soil. The leaching of nitrates and ammonium from the field was quantified using fluxmeters and alternative agricultural strategies were proposed.
The hydraulic transfer properties of the limestone aquifer were deduced from the time lag between the variations in rainfall and salinity measurements of the freshwater lens. Using an unique 10-year dataset of monitored salinity of pumped water and a model, it was shown that the rainfall variations related to the El Niño-Southern Oscillation determined the quality of the freshwater in the aquifer. It was also possible to predict the relative salinity of the freshwater 10 months in advance using the Southern Oscillation Index.
The pressures on the water resources that result from agricultural and climatic impacts need to be resolved through a balanced management strategy. This management strategy should at one hand account for the benefits and impacts of agriculture to the long-term economical development of the coral atoll, and on the other hand use climatic information and predictability to harness the islands water resources against impacts of climate variability and change. Finally, the case-study of Tongatapu was used to sketch a framework for the sustainable development of a small island developing state.


Bibliographic reference |
van der Velde, Marijn. Agricultural and climatic impacts on the groundwater resources of asmall island : measuring and modelling water and solute transport insoil and groundwater on Tongatapu. Prom. : Vanclooster, Marnik ; Clothier, Brent |
Permanent URL |
http://hdl.handle.net/2078.1/22799 |