Recharge of water in Wairakei Geothermal Field determined from repeat gravity measurements

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Abstract
Repeat gravity measurements at Wairakei Geothermal Field, New Zealand, indicate for the period 1958–1961 about 44 × 109 kg of water replaced 145 × 109 kg withdrawn, for 1961–1967 about 126 × 109 kg replaced 360 × 109 kg withdrawn, and for 1967–1974 about 364 × 109 kg replaced 400 × 109 kg withdrawn. The amount of recharge appears to be inversely related to the rate of change of corrected gravity values at benchmark A97 in the main production borefield. Using this relationship it is estimated that during the initial period of exploitation (1958–1962) the annual recharge dropped to about 10%, but subsequently rose, and since 1966 has been about 90%. In any period, the mass replaced has not exceeded that withdrawn, but since 1966 the net mass loss has been small, and at the present rate of withdrawal it is unlikely that the field will be depleted of water for a long time.