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Scientific Investigations Report 2015–5010

Prepared in cooperation with the State of Hawai‘i Commission on Water Resource Management and the City and County of Honolulu Board of Water Supply

Spatially Distributed Groundwater Recharge for 2010 Land Cover Estimated Using a Water-Budget Model for the Island of O‘ahu, Hawai‘i (Version 2.0)

By John A. Engott, Adam G. Johnson, Maoya Bassiouni, Scot K. Izuka, and Kolja Rotzoll

Thumbnail of and link to report PDF (31 MB)Abstract

Owing mainly to projected population growth, demand for freshwater on the Island of Oʻahu is expected to increase by about 26 percent between 2010 and 2030, according to the City and County of Honolulu. Estimates of groundwater recharge are needed to evaluate the availability of fresh groundwater. For this study, a water-budget model with a daily computation interval was developed and used to estimate the spatial distribution of recharge on Oʻahu for average climate conditions (1978–2007 rainfall and 2010 land cover) and for drought conditions (1998–2002 rainfall and 2010 land cover). For average climate conditions, mean annual recharge for Oʻahu is about 660 million gallons per day, or about 36 percent of precipitation (rainfall and fog interception). Recharge for average climate conditions is about 34 percent of total water inflow, which consists of precipitation, irrigation, septic leachate, water-main leakage, and seepage from reservoirs and cesspools. Recharge is high along the crest of the Koʻolau Range, reaching as much as about 180 inches per year in the north-central part of the range. Recharge is much lower outside of the mountainous areas of the island, commonly less than 5 inches per year in unirrigated areas. The island-wide estimate of groundwater recharge for average climate conditions from this study is within 1 percent of the recharge estimate used in the 2008 State of Hawaiʻi Water Resource Protection Plan, which divides the Island of Oʻahu into 23 aquifer systems for groundwater management purposes. To facilitate direct comparisons with this study, these 23 aquifer systems were consolidated into 21 aquifer systems. Recharge estimates from this study are higher for 12 of the aquifer-system areas and lower for 9. Differences in mean rainfall distribution and the inclusion of irrigation in this study are the primary reasons for discrepancies in recharge estimates between this study and the 2008 Hawaiʻi Water Resources Protection Plan. For drought conditions, mean annual recharge for Oʻahu is about 417 million gallons per day, which is about 37 percent less than recharge for average climate conditions. For individual aquifer-system areas, recharge for drought conditions is about 25 to 70 percent less than recharge for average climate conditions.

First posted February 25, 2015

Revised December 7, 2017

This publication is online only.

For additional information, contact:
Director, Pacific Islands Water Science Center
U.S. Geological Survey
Inouye Regional Center
1845 Wasp Blvd., B176
Honolulu, HI 

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Suggested citation:

Engott, J.A., Johnson, A.G., Bassiouni, Maoya, Izuka, S.K., and Rotzoll, Kolja, 2017, Spatially distributed groundwater recharge for 2010 land cover estimated using a water-budget model for the Island of O‘ahu, Hawai‘i (ver. 2.0, December 2017): U.S. Geological Survey Scientific Investigations Report 2015–5010, 49 p., https://doi.org/10.3133/sir20155010.

ISSN 2328-0328 (online)



Contents

Abstract

Introduction

Description of O‘ahu

Water-Budget Model

Model Input

Model Exclusions and Limitations

Water-Budget and Groundwater-Recharge Estimates

Suggestions for Future Study and Additional Data Collection

Summary and Conclusions

References

Figures (12)

Tables (11)


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