Ground-water recharge is estimated as the residual component of a monthly water budget calculated using soil characteristics and long-term average rainfall, streamflow, irrigation, and pan-evaporation data. The water-budget components of rainfall, direct runoff, evapotranspiration, and ground-water recharge are defined seasonally, through the use of monthly data, and spatially by land-use and geohydrologic areas, through the use of a geographic information system model.
The long-term average ground-water recharge for the Iao area was estimated for four scenarios using natural land-use, and using 1926-79, 1980-85, and 1986-95 land-use and irrigation data. The recharge rate for natural conditions is 34 million gallons per day, which is 34 percent of rainfall. The average annual ground-water recharge rate for 1926-79 conditions is 51 million gallons per day, which is 41 percent of the sum of rainfall and irrigation. The recharge rates for 1980-85 and 1986-95 conditions are 40 and 36 million gallons per day, which are 37 and 35 percent of rainfall plus irrigation, respectively.