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Spatial Distribution of Groundwater Recharge on the Island of Hawaii
Project Chief: John A. Engott
No recent, island-wide estimate of groundwater recharge distribution exists for the Island of Hawaii, Hawaii (Big Island). Recharge is an important hydrologic variable that affects a multitude of water-resource management issues. Identified as a "critical need" by the State of Hawaii Commission on Water Resource Management, accurate estimation of the spatial distribution and seasonal variability of groundwater recharge on the Big Island would help: (1) assess the availability of groundwater within island aquifer systems, (2) improve groundwater flow simulations, and (3) estimate coastal groundwater discharge.
The objective of this study is to estimate the spatial distribution of groundwater recharge on the Island of Hawaii. The scope of the study will include the modification of an existing daily soil-moisture budget model and the preparation of available climate, land use, land cover, soil, and streamflow data necessary to execute the soil-moisture budget model. At several locations around the Big Island with little or no soil cover, samples of rain and ground water will be collected and analyzed for chloride and stable isotopes. These data will be used for a chloride mass balance.
Relevance and Benefits
The results from this study are necessary for the proper management of the ground and surface waters in the State of Hawaii. This study will provide information needed for water-use permitting and land-use planning in areas that are rapidly being developed. This study will produce the best available estimation of the spatial distribution of groundwater recharge on the Big Island and aid responsible parties and agencies in making informed water-resources decisions and policies.
A GIS-based soil-moisture budget model will be developed for this study to estimate the spatial distribution of long-term average recharge. A daily time step will be used to avoid possible biases associated with monthly and annual time steps. Recharge will be estimated for current climate and land-use conditions. Several drought and possible future land-use scenarios may also be simulated. Average daily recharge will then be aggregated into monthly and annual average recharge distributions for the entire island. The accuracy of recharge estimates from a soil-moisture budget is limited by factors including the accuracy of the soil-moisture budget model and the input data. Sensitivity of recharge estimates to model input will be quantified. The soil-moisture budget model may be less accurate in areas with little or no soil. To address this problem, a second recharge-estimation method—the chloride mass balance—will be used in one or more dry, thin-soil areas. Recharge estimated from the chloride mass balance will be compared to that estimated from the soil-moisture budget model. Results from this study will be published in the USGS Scientific Investigations Report series and made available on the internet.
Engott, J.A., 2011, A water-budget model and assessment of groundwater recharge for the Island of Hawai'i: U.S. Geological Survey Scientific Investigations Report 2011-5078, 53 p.