Groundwater levels in Jeju Island vary based on rainfall, with levels rising in summer and falling in winter. The speed of rise and fall differs between the eastern and western regions, indicating varying permeability. The average recharge volume of groundwater between the base level and surface is 9.83 billion cubic meters, with a maximum of 26.67 billion cubic meters after a typhoon. The porous ratio of Jeju Island is estimated at 16.8% on average, but drops to 4.6% after heavy rainfall due to time needed for groundwater to fill porous areas. This method helps estimate recharged groundwater mass based on observed levels.