Gravity Changes at Wairakei Geothermal Field, New Zealand

Measurements of the value of gravity at 50 bench marks at Wairakei Geothermal Field, New Zealand, show that differences of up to 0.5 mgal have occurred between 1961 and 1967 and up to 0.1 mgal between 1967and 1968. These differences, corrected for known changes in elevation, are interpreted as reflecting the net mass of water lost from the aquifer. The net loss between 1961 and 1967 is determined to be about 2.9 × 10¹⁴g and hence only about 20 percent of the water drawn off was replaced, but between 1967 and 1968 there was little or no net loss.