| Pacific islands are long-settled by mankind, dating back several hundreds to thousands of years ago since discovery by islanders traveling by boat. Amongst these islands are atoll islands, which are small coral islands that lie at a low elevation and are usually part of a ring-like coral reef formation. Past and present islanders collect water from rainwater catchments and groundwater wells, with rainwater used primarily for drinking water and groundwater used as a supplement for wash water. Unfortunately, this region can experience severe drought, over-wash events during strong tropical storms, and typhoons, all of which threaten the freshwater supply for these islands. Due to rising concerns over climate change, there is interest in studying the water security of these especially vulnerable land forms. This thesis evaluates the reliability of the water supplies on four atoll islands in Yap, Micronesia by modeling the reliability of the two main sources. To first analyze rainwater catchment performance, Ifalik Island is evaluated using data collected on the island in 2015 by a collaborative research field team and a water balance model. Second, the results are used to develop design curves as a tool for rainwater catchment design and improvement. The fresh groundwater source is also modeled for each of the four islands to test the effects of varying climate conditions on the shallow, freshwater lens.;Rainwater catchment systems on Ifalik Island are evaluated for their performance using a mass balance model that quantifies water storage through time. Performance is quantified primarily by reliability, which is a term to represent the percentage of days a rainwater catchment supplies sufficient water to the users. Based on the data from the Ifalik field survey, the average household rainwater catchment system on Ifalik uses a 16.5 square meter guttered roof with a 2,000 liter storage tank and serves seven individuals at 12 liters per capita per day. As a result of a rainwater catchment system sensitivity analyses based on the average rainwater catchment conditions, the most important factors in performance are effective roof area size, water demand, and gutter-downspout efficiency. Further analyses using the mass balance model found that the performance of each individual catchment is sufficient to provide water to the community during conditions similar to the severe drought year of 1997-1998, as well as projected rainfall conditions for the next 30 years. Therefore, analyses suggest Ifalik Island has sufficient rainwater catchment performance to provide water for the community. However, to introduce a conservative measure for water security, it is recommended that the catchment area be extended to the full size of the roof area. This would thereby increase the storage supply for the community by 25 cubic meters on the day of lowest supply under the 1997-1998 severe drought conditions.;Design curves developed from the mass balance model are created to serve as a tool for rainwater catchment expansion and design. This tool allows water users and managers to determine the size of rainwater catchment components based on a selected reliability goal of 80%, 90%, 95%, or 99%. Curves developed using the severe drought conditions provide the most conservative design requirements.;A groundwater modeling approach is used to simulate saltwater movement between the thin, freshwater lens located under the surface of the islands and the surrounding seawater. SEAWAT, a computer program that models three-dimensional variable-density groundwater flow and salt transport, is used to estimate the volume and thickness of the freshwater lens for the four islands in this study under steady, average rainfall and projected future climate conditions through year 2040.;Projected, precipitation data generated through climate models developed by NASA are used to analyze groundwater lens dynamics from 2010-2040. For these scenarios, the islands show a relatively stable lens shape under the low-level climate change scenario, however the high-level climate change scenario appears to decrease the groundwater lens stability. Findings show the groundwater lens may fluctuate over 2 meters in thickness within just 2-3 years and decadal episodes of periodic increasing and decreasing lens size indicate an oscillatory climate forcing. Falalop, Ifalik, and Satawal did not deplete the freshwater lens at any point in time during either simulation. Due to the strong groundwater responses seen from the high-level climate scenario, incorporating such a GCM into a groundwater evaluation is recommended.;Results from this thesis can be used by coral island water managers to examine the community water supply during droughts and future climate conditions. This thesis also provides tools and approaches for improving rainwater catchment system performance by identifying influential parameters, design rainfall conditions, and reliability curves for expansion. (Abstract shortened by ProQuest.). |
