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.
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