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Coral atolls are areas of high biological productivity even though they are usually located in regions of the tropical ocean characterized by low primary production and extremely low levels of vital dissolved nutrient materials. Recent studies have indicated the possible importance of in situ dinitrogen fixation on shallow reef flats in supplementing low oceanic nitrate levels and thus contributing to the maintenance of high reef productivity. Variations in the structure of atolls may have a direct bearing on the accumulation of fixed nitrogen and other nutrient materials, and consequently on lagoonal and reefal primary productivity. This paper investigates the effect of differing atoll configurations by comparing neighbouring, but structurally dissimilar, mid-ocean atolls. The findings are discussed in terms of ecosystem function and possible influences on the structural evolution of atolls. 相似文献
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We present a simple modelling method to estimate the volume of available groundwater in the freshwater lens of atoll islands under steady-state conditions. Model inputs include annual rainfall depth, island width for cross-sections along the length of the island, aquifer hydraulic conductivity, and depth to the contact between the upper sand aquifer and the lower limestone aquifer. The methodology is tested for nine islands of varying size in the Maldives and Micronesia. Sensitivity analysis indicates that lens volume on large islands typically is governed by the depth to the discontinuity, whereas lens volume for smaller islands is governed by rainfall rate and hydraulic conductivity. Volume curves, which relate lens volume to lens thickness, are developed for each of the nine islands and for three generic island shapes to allow rapid estimation of lens volume given field-estimated lens thickness. The methods presented in this study can be used for any small atoll island. 相似文献
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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 相似文献
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Quantifying threats to groundwater resources in the Republic of Maldives Part I: Future rainfall patterns and sea‐level rise 
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下载免费PDF全文 Reserves of fresh groundwater on atoll islands are extremely fragile due to climatic and anthropogenic stresses. Of major concern is the quantity of water to be available in the coming decades under the influence of variable rainfall patterns, rising sea level, environmental conditions, and expected population growth that depends on groundwater resources. In this study, a 3‐dimensional numerical modelling approach using the SEAWAT modelling code is used to estimate freshwater lens volume fluctuation for 4 representative islands in the Republic of Maldives in response to long‐term changes in rainfall, sea‐level rise (SLR), and anthropogenic stresses such as groundwater pumping and short‐term impacts from tsunami‐induced marine overwash events. This work is divided into 2 papers. This first paper presents numerical model set‐up and calibration, and the effect of future rainfall patterns and SLR on fresh groundwater reserves. The second paper focuses on marine overwash events. The results of simulated future freshwater lens volume presented in the first study contribute to efficient groundwater resources planning and management for the Maldives in the upcoming decades. Freshwater lenses in small atoll islands (area < 0.6 km2) are shown to have a strong variability trends in the upcoming decades with expected reduction in lens volume between 11% and 36% due to SLR. In contrast, freshwater lenses in larger atoll islands (area > 1.0 km2) are shown to have less variability to changing patterns with expected reduction in lens volume between 8% and 26% due to SLR. Study results can provide water resource managers with valuable findings for consideration in water security measures. 相似文献
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Quantifying transient post‐overwash aquifer recovery for atoll islands in the Western Pacific 下载免费PDF全文
下载免费PDF全文 Ryan T. Bailey 《水文研究》2015,29(20):4470-4482
Marine overwash events for atoll islands in the Pacific and Indian Oceans, which cause salinization of fresh groundwater because of infiltrating seawater, pose a significant challenge for island community sustainability in regard to water supply. Understanding transient fresh groundwater development during a post‐overwash period for a range of island sizes, geologic characteristics, and rainfall patterns is essential for water management. This paper presents a methodology for quantifying this development for an atoll nation, with methods applied to the 32 atolls of the Federated States of Micronesia (FSM) in the western Pacific. Using the numerical groundwater modelling code SUTRA, overwash events and post‐overwash freshwater–seawater dynamics are simulated for the range of island widths (200 to 1100 m), geologic characteristics (hydraulic conductivity corresponding to leeward and windward islands), and rainfall patterns (western, central, and eastern regions) present in the FSM, thereby providing results for each atoll island. Results show that 10–17, 8–12, and 6–12 months are required to achieve 60% freshwater lens recovery for leeward islands in the western, central, and eastern FSM, respectively, with variation due to rainfall rate and island width. In contrast, 4–9 months is required for 60% recovery for windward islands. However, the natural thinness of the lend on windward islands typically precludes extensive use of groundwater under average rainfall conditions. Overwash characteristics (depth, duration, and seasonal timing) did not significantly affect recovery times. For the region of lowest rainfall (western FSM), 6–10 months is required to achieve potable groundwater at the typical depth of hand‐dug wells. Results provide water resource managers and atoll island communities with important information regarding timing of potential fresh groundwater use following an overwash event. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
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Fresh groundwater reserves on small coral islands are under continual threat of salinization and contamination because of droughts, storm‐surge overwash events, over‐extraction, island community urbanization, and sea level rise. Whereas storm‐surge overwash events can cause sudden groundwater salinization, long‐term changes in rainfall patterns and sea level elevation have the potential of rendering these islands uninhabitable in the coming decades. This study demonstrates the use of a tested freshwater lens thickness simulator to estimate the groundwater resources of a set of atoll islands in the coming decades. The method uses ranges of projected rates of annual rainfall and sea level rise (SLR) to provide a range of probable lens thickness for each island. Projected rainfall is provided by General Circulation Models that accurately replicate the historical rainfall patterns in the geographic region of the islands. Methodology is applied to 68 atoll islands in the Federated States of Micronesia. These islands have widths that range between 150 and 1000 m, and experience annual rainfall rates of between 2.8 and 4.8 m. Results indicate that under average conditions of SLR, beach slope, and rainfall, almost half of the island will experience a 20% decrease in lens thickness by the year 2050. For worst‐case scenarios (high SLR, low rainfall), average decrease in lens thickness is 55%, with almost half of the islands experiencing a decrease of greater than 75% and half of the islands having a lens thickness less than 1.0 m. Small islands (widths less than 400 m) are particularly vulnerable because of shoreline recession. Groundwater on islands in the western region is less vulnerable to SLR because of a projected increase in rainfall during the coming decades. Results indicate the vulnerability of small islands to changing climatic conditions, and can be used for water resources management and community planning. Methodology can be applied to any group of islands as a first approximation of the effect of future climate conditions on groundwater resources. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
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Quantifying threats to groundwater resources in the Republic of Maldives Part II: Recovery from tsunami marine overwash events 下载免费PDF全文
下载免费PDF全文 Marine overwash events are among the most serious short‐term threats to groundwater supply of small coral islands. During such events, seawater can inundate small islands partially or completely, causing salinization of the aquifer. A comprehensive knowledge of freshwater lens recovery is essential for water planners on these islands. In this study, a numerical modelling approach is used to quantify recovery of the freshwater lens on 4 islands of the Maldives after a tsunami‐induced overwash event similar to that experienced from the Indian Ocean earthquake in December 2004. The islands vary in size (0.2 to 10.1 km2) and span the climatic regions of the Maldives. A tested 3‐dimensional SEAWAT groundwater model for each island is used to simulate the recovery process. Recharge rates from historical rainfall data and from global climate models are imposed on each island during the post‐overwash recovery period. The effect of groundwater pumping on lens recovery also is examined. Results show abrupt decrease in fresh groundwater volumes for each island, followed by recovery that is significantly influenced by island size and recharge patterns. Overall, salinization is more widespread on small islands (<1 km2), but recovery is more rapid than for large islands. Between 50% and 90% of lens recovery occurs after 2 years for small islands (<1 km2) whereas only 35% and 55% for large islands. Imposing pumping rates required to sustain the local population lengthened the recovery time between 5% and 15%, with smaller islands having the higher percentage. However, the governing factor on recovery time is the spatial extent of land surface inundation by the overwash event, with wave height and duration of the event having a negligible impact. A strong relationship exists between required recovery time and island surface area, thereby providing a method to determine recovery time for other atoll islands not investigated in this study with similar geologic structure. Our results can be used to aid in managing water resources during the post‐overwash period. 相似文献
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Groundwater resources of the Republic of the Maldives are threatened by a variety of factors including variable future rainfall patterns, continued population growth and associated pumping demands, rising sea level, and contamination from the land surface. This study assesses changes in groundwater availability due to variable rainfall patterns and sea level rise (SLR) in the coming decades, a key component of water resources management for the country. Using a suite of two‐dimensional density‐dependent groundwater flow models, time‐dependent thickness of the freshwater lens is simulated for a range of island sizes (200 to 1,100 m) during the time period of 2011 to 2050, with recharge to the freshwater lens calculated using rainfall patterns provided by general circulation models for the three distinct geographic regions of the Maldives. The effect of SLR on the freshwater lens is quantified using estimates of shoreline recession and associated decreases in island width. If rainfall is solely considered, groundwater availability is projected to increase, as lens thickness during the 2031–2050 time periods is slightly greater (1–5%) than during the 2011–2030 time period. However, including the impact of SLR indicates an overall decrease in lens thickness, with drastic decreases (60% to 100%) projected for small islands (200 m) and moderate decreases (12% to 14%) expected for 400 m islands, which accommodate one third of the national population. Similar methodologies can be used for other atoll island nations, such as the Republic of Marshall Islands, Federated States of Micronesia, and the Republic of Kiribati. For the Maldives, results from this study can be used in conjunction with population growth estimates to determine the feasibility of including groundwater in water resources planning and management for the country. 相似文献
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