Ground-water recharge is estimated as the residual component of a monthly water budget calculated using long-term average rainfall, streamflow, irrigation, pan-evaporation data, and soil characteristics. The water-budget components are defined seasonally, through the use of monthly data, and spatially by topographic and geologic areas, through the use of a geographic information system model.
The long-term average ground-water recharge for the Lahaina District was estimated for three scenarios using 1923-78 land-use and irrigation data, 1986-93 land-use and irrigation data, and natural conditions. The average annual ground-water recharge rate for 1923-78 conditions is 190 million gallons per day, which is 45 percent of the sum of rainfall and irrigation. The recharge rate for 1986-93 conditions is 163 million gallons per day, which is 42 percent of rainfall plus irrigation. The recharge rate for natural conditions is 145 million gallons per day, which is 44 percent of rainfall.