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Photo of Maunalua Bay, Oahu. (Photo by Scot Izuka)

HAWAII VOLCANIC-ROCK AQUIFER STUDY

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HAWAII VOLCANIC-ROCK AQUIFER STUDY


STUDY PLANS

The study plan includes defining the hydrogeologic framework, quantifying components of the groundwater budget, and developing conceptual models of groundwater flow for Kauai, Oahu, Maui and the Big Island. These islands have more than 99 percent of the population and constitute 92 percent of the land area in Hawaii. The study plan also includes construction of three separate whole-island numerical groundwater models: Oahu, Maui, and Kauai. These islands have 86 percent of the population and constitute the bulk of the groundwater demand in Hawaii. A numerical model of the Big Island will not be constructed because compared to the other islands, much less is known about the Big Island. The models together with input from the groundwater budget will be used to assess changes in groundwater availability in Hawaii.

Block diagram showing example of a conceptual model of the Waikele area on Oahu (modified from Anthony and others, 2004).

Example of a conceptual model of the Waikele area on southern Oahu (modified from Anthony and others, 2004).

Hydrogeologic Frameworks

The hydrogeologic framework of an island is a description of the properties, geometry, and structures of the rocks through which groundwater moves and is stored. The basics of the hydrogeologic framework for Hawaii were developed by extensive descriptive studies in the 20th century, but subsequent numerical groundwater modeling studies demanded more quantitative descriptions of the hydrogeologic framework. Detailed frameworks currently exist only for parts of some islands where modeling studies have been focused. Hydrologic data exist for some of the other areas, but there has been no effort to synthesize the existing separate hydrogeologic frameworks into a comprehensive hydrogeologic framework for the entire Hawaiian Islands region. The objective of this task is to extend understanding of the hydrogeologic framework of Kauai, Oahu, Maui and the Big Island insofar as current data allow.


Groundwater Budgets

The groundwater budget is an accounting of inflows and outflows from the groundwater system. Groundwater budgets for Kauai, Oahu, Maui, and the Big Island will be quantified for current (2000-2010) and predevelopment conditions. Future conditions will also be investigated for Oahu. Inflow of freshwater to the groundwater system of an island comes from groundwater recharge (including water from rainfall and irrigation). Groundwater recharge will be determined using a soil water mass balance that incorporates precipitation, soil, land cover, potential evapotranspiration, irrigation, and other data. Outflows from the groundwater budget include well pumping, groundwater discharge to streams, and groundwater discharge to the ocean. Well pumping will be quantified from water use records. Groundwater discharge to streams and the ocean will be determined using numerical groundwater models. Where an imbalance between inflows and outflows exists, the amount of groundwater in storage changes; change in groundwater storage will also be determined from groundwater modeling.


Conceptual Models

Conceptual models describe groundwater flow and occurrence in the islands and are developed on the basis of the hydrogeologic-framework and groundwater-budget information. Conceptual models were developed along with thier frameworks early in the 20th century, but the emergence of numerical models and their ability to test conceptual models led to greater quantitative consistency in the conceptual models. Also like the frameworks, conceptual models have been tested for parts of some islands where numerical modeling studies have been done, but conceptual models for many areas are nonexistent or untested. No region-wide synthesis of conceptual models has been developed for Hawaii, and little is known about whether certain conceptual models may be transferable to areas that have not been studied. In this study, whole-island conceptual models for Kauai, Oahu, Maui and the Big Island will be developed using the framework and groundwater budget information acquired in this study as well as updates of existing conceptual models.


Construction of Numerical Groundwater Models

Numerical groundwater modeling is needed to quantify certain aspects of groundwater resources, such as streamflow depletion, saltwater intrusion, and change in storage that may result from both anthropogenic and natural factors. Simulated model stresses will include changes in recharge and groundwater withdrawals as indicated in the information obtained as part of the groundwater-budget component of this study.


Assessment of Groundwater Availability

The numerical models will be used to investigate historical anthropogenic effects on groundwater resources for Kauai, Oahu and Maui. The model for Oahu will also be used to investigate effects of future conditions of climate, urbanization, land use, and groundwater withdrawals. Comparison among predevelopment, current, and future scenarios will be the basis for assessing changes in groundwater availability in Hawaii. The current hydrologic monitoring network will be evaluated on the basis of data inadequacies that become apparent during this study. The evaluation will identify where and what types of data are needed to more accurately assess changes in groundwater resources in the future.


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