The water supply for the population of the island of Oahu, Hawaii is obtained primarily from ground-water sources, recharged by the infiltration of rainfall and irrigation water. The importance and limited nature of these sources caused the island of Oahu to be selected, along with 27 other aquifer systems in the United States, as part of the U.S. Geological Survey's Regional Aquifer System Analysis (RASA) program. The primary purpose of this program is to investigate the effects of that ground-water development has had or could have on the ground-water levels and the distribution of ground-water flow on a regional scale.
An important factor in the study of an aquifer system is the quantity of water recharging and flowing through the system. Ground-water recharge can be estimated from water-balance calculations, which subtract runoff and evapotranspiration from rainfall and irrigation. The quantification and distribution of recharge are necessary for ground-water flow simulation, an important element of the RASA project.