Diurnal and seasonal ground temperature variations at Wairakei

Abstract

Weekly readings of ground temperatures to a depth of 6.6 metres were made at Wairakei, New Zealand, between 1958 and 1962. These readings were supplemented by multi-point Brown potentiometer recordings to a depth of 3 m for more than a year. Air temperatures were also available for the five-year period. Results show that the mean annual air temperature was 12°C; the range of the diurnal variation (from max. to min.) was less than 1 deg C at depths greater than 30 cm; the range of the seasonal variation was less than 1 deg c at depths greater than 6 m; the mean annual temperature at a depth of 1 m was 3 deg c greater than the mean annual air temperature; the mean annual thermal dilfusivity and conductivity were 3.5 × 10^-3 cm²/sec and 16 × 10^-4 cal/sec cm deg c respectively; the thermal conductivity of the pumice soil varied with moisture content, being 40 × 10^-4 cal/sec cm deg c when the water content was 73½% by volume (voids completely filled with water) and 4 × 10^-4 cal/sec cm deg c when the water content was 5 %; and the mean annual geothermal heat flux at the surface was 40 × 10^-6 cal/cm² sec, which is about 40 times the world average.