Detailed water budgets calculated for southern and southeastern Oahu are used with a geographic information system to develop simplified methods for estimating areal water budgets for predevelopment and mid-1980's land use. The methods were applied to estimate water budgets for the Waianae area of western Oahu, and for north-central, southern, and southeastern Oahu. A water budget was calculated for windward Oahu by developing a separate geographic information system model of the area. The water budgets for these areas were combined into a single water budget for the entire island. The geographic information system model was used to calculate mid-1980's ground-water recharge to small areas of specific interest and the distribution of recharge by geologic formation.
The most significant changes in the water budget and ground-water recharge have occurred in north-central and southern Oahu as a result of large-scale agricultural development and urbanization by the mid-1980's. Runoff increased by 23 million gallons per day in southern Oahu where extensive urban areas have been developed. Evapotranspiration increased by 8 million gallons per day in southern Oahu and 28 million gallons per day in north-central Oahu as result of the 146 million gallons per day of agricultural irrigation. Ground-water recharge increased in both areas: by about 56 million gallons per day in southern Oahu and by about 32 million gallons per day in north-central Oahu. Predevelopment ground-water recharge to Oahu was an estimated 792 million gallons per day. Changes in land-use practices in the mid-1980's resulted in an estimated island-wide recharge of 880 million gallons per day.