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Spatial Distribution of Groundwater Recharge on the Island of Oahu, Hawaii
Project Chief: John A. Engott
An updated, island-wide estimate of groundwater-recharge distribution is needed for the island of Oahu, Hawaii. A new water-budget model for Oahu also would take into account the substantial changes in urban and agricultural land use that have occurred during the last several decades, as well as project the effects of future land-use and climate changes. Estimates of past, present, and future groundwater recharge are needed to refine an existing USGS three-dimensional numerical groundwater model of the Pearl Harbor aquifer. The Pearl Harbor aquifer is the most important aquifer on the island of Oahu and currently supplies about 100 Mgal/d of fresh groundwater mainly for public supply. Decisions related to future infrastructure development and alternate sources of fresh water, including desalinization, will depend on the long-term sustainability of the groundwater resources in the Pearl Harbor aquifer.
The objective of this study is to estimate the spatial distribution of groundwater recharge on the island of Oahu. Results from this study can be used to update sustainable-yield estimates and will be used to refine the USGS numerical groundwater model of the Pearl Harbor aquifer.
Relevance and Benefits
The results from this study are necessary to properly manage groundwater in the State of Hawaii. The study is consistent with the mission of the USGS Science Strategy (U.S. Geological Survey, 2007) to provide citizens, communities, natural-resource managers, and policymakers with a clearer knowledge of the status of their water resources. By providing estimates of current groundwater recharge and analyses of the effects of land-use and climate change on recharge to aquifers that provide public water supply and support fragile ecosystems, this study broadly supports three of the six science directions in the USGS Science Strategy (U.S. Geological Survey, 2007), including (1) a water census of the United States, (2) understanding ecosystems and predicting ecosystem change, and (3) climate variability and change.
To meet the objective of this study, a GIS-based water-budget model will be developed for this study to estimate the spatial distribution recharge. For this study, a daily time step will be used to avoid possible biases associated with monthly or annual time steps. Daily recharge estimates for current average climatic conditions and land use will be aggregated for each month of the year. Past recharge will be estimated for 10-year intervals, beginning in about 1900, using land use and rainfall representative of each time period. Several other scenarios will also be examined, including possible future land use, drought, and effects of climate change.
Gathered information on precipitation, fog, irrigation, runoff, pan evaporation, soils, and historical land cover and prepared datasets that will be used as input to the water-budget model.
Fiscal Year 2013
Modify the water-budget model code for Oahu to determine spatial distribution recharge. Run model simulations for various scenarios including a long-term mean, drought, future land use, effects of climate change, and a 100-year historical time series in the Pearl Harbor area. Complete report documenting the study approach and findings.