Heat Discharge of Steaming Ground at Karapiti (Wairakei), New Zealand

Abstract

The Karapiti fumarole field is arguably one of the largest coherent steaming ground areas associated with liquid-dominated geothermal systems. It covers 0.35 km2 and transfers, during summer, 245 +/- 20 MW of heat to the surface. Two modes of discharge occur:
1) localized discharge of steam by fumaroles and steam vents (c. 107 MW), and
2) wide-spread diffuse discharge (steaming ground in the strict sense) involving both convective and conductive heat transfer (c. 69 MW each).
Assessment of heat discharged by fumaroles and steam vents involved measurement of the steam flux of selected fumaroles and analysis of the extent of all visible steam clouds.
Assessment of the diffuse discharge required measurement of the heat flux at over 100 sites using a ground calorimeter and recording of soil temperatures down to depths where boiling prevails. The total heat flux is dominantly controlled by the boiling point depth. The most intense heat transfer occurs within a 0.07 km2 area where boiling occurs at depths less than 0.2 m; the area is also characterized by anomalous surface temperatures detected using aerial infra-red imagery.