Fresh-water lenses and practical limitations of their three-dimensional simulation |
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Authors: | F Ghassemi K Alam K Howard |
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Institution: | (1) Centre for Resource and Environmental Studies, Australian National University, Canberra, Australian Capital Territory 0200, Australia,;(2) ECOWISE Environmental, ACTEW Corporation, 16 Lithgow Street, Fyshwick, Australia Capital Territory 2609, Australia,;(3) Groundwater Research Group, University of Toronto, 1265 Military Trail, Scarborough, Ontario M1C 1A4, Canada, |
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Abstract: | Fresh-water lenses are the major sources of water supply in many atoll islands in the Pacific and Indian Oceans, particularly
in dry seasons. Several two- and three-dimensional models are currently available for the simulation of atoll-island aquifers;
however, 2D models cannot include 3D spatial variability of material properties, they must simplify the boundary conditions,
and they cannot correctly simulate pumping wells. In an attempt to overcome these difficulties, a 3D model, SALTFLOW, was
adopted for the simulation of Home Island in the Indian Ocean. This exercise required a discretisation on the order of a few
metres and time steps of a few hours requiring significantly high CPU times. High CPU demand proved to be a difficult challenge
but cannot be considered a serious practical limitation with today's advanced computers. The exhaustive data demands of the
model (e.g., 3D distributions of hydraulic conductivity, porosity, dispersivities, and spatial and temporal variations of
recharge and extraction rates) proved to be more problematical. Although the Home Island data set is unusually comprehensive
by any standards, nonetheless the quality and quantity of the available data proved inadequate to meet the calibration needs
of a highly karstic aquifer system. The Home Island modeling demonstrates the practical limitations of 3D models. It raises
the concern that our ability to develop computer codes capable of simulating complex systems now exceeds our ability to supply
the input data necessary for reliable calibration. Finally, the paper demonstrates the importance of the transient calibration
in reliable simulation of various management options and emphasises that transient calibration should be considered as an
integral part of any similar 2D or 3D modeling.
Electronic Publication |
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Keywords: | coral atolls fresh-water lenses numerical modeling island hydrogeology |
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