Ground temperature profiles and thermal rock properties at Wairakei, New Zealand

Here we present ground temperature data from Wairakei, Taupo, New Zealand. The surface zone in this area extends to 0.5 m, while the shallow zone extends to at least 7.39 m. It is expected that the deep zone starts close to this depth and will exhibit a constant temperature of approximately 14.3 °C, 1–2 °C higher than the mean ambient temperature at this site. Shallow ground temperatures are predominantly influenced by the ambient air temperature and to a lesser extent the amount of rainfall. Modelling of the thermal waves, taking conductive and convective heat transfer into account, enables estimates of thermal diffusivity, volumetric heat capacity and thermal conductivity, whose mean values based on data from August 2010–April 2011 were 3.48 × 10−7 m2 s−1, 2.35 × 106 J m−3 K−1 and 0.89 W m−1 K−1 respectively. These estimates are within the same ranges as previous estimates of the thermal properties at Wairakei, although the mean values are slightly higher. This finding is consistent with the increased amount of recharge that occurred during the experimental period. Increased understanding of ground temperature and thermal rock properties is valuable for the development of low enthalpy geothermal resources as well as having applications in hydrology, agronomy and climatology.

► Ground temperatures and modelled thermal rock properties at Wairakei, New Zealand.
► Ground temperatures influenced by the ambient air temperature and rainfall.
► Modelling highlights need for a convective term to account for recharge.
► Values thermal rock properties within published ranges, increased recharge evident.
► Important for the development of low enthalpy geothermal resources.