A novel heat flux study of a geothermally active lake — Lake Rotomahana, New Zealand

• First application of novel thermal blankets to measure conductive heat flux in a lake (85).
• Two separate areas of high heat flux are found in the western part of Lake Rotomahana (85).
• The mean heat flux of Lake Rotomahana is estimated to be ~ 6 W/m2 (64).
• Total conductive thermal output of Lake Rotomahana is estimated to be 47 MW (74).
• Lake calorimetry estimates 112–132 MW of total thermal output of Lake Rotomahana (80).

A new technique for measuring conductive heat flux in a lake was adapted from the marine environment to allow for multiple measurements to be made in areas where bottom sediment cover is sparse, or even absent. This thermal blanket technique, pioneered in the deep ocean for use in volcanic mid-ocean rift environments, was recently used in the geothermally active Lake Rotomahana, New Zealand. Heat flow from the lake floor propagates into the 0.5 m diameter blanket and establishes a thermal gradient across the known blanket thickness and thereby provides an estimate of the conductive heat flux of the underlying terrain. This approach allows conductive heat flux to be measured over a spatially dense set of stations in a relatively short period of time. We used 10 blankets and deployed them for 1 day each to complete 110 stations over an 11-day program in the 6 × 3 km lake. Results show that Lake Rotomahana has a total conductive heat flux of about 47 MW averaging 6 W/m2 over the geothermally active lake. The western half of the lake has two main areas of high heat flux; 1) a high heat flux area averaging 21.3 W/m2 along the western shoreline, which is likely the location of the pre-existing geothermal system that fed the famous Pink Terraces, mostly destroyed during the 1886 eruption 2) a region southwest of Patiti Island with a heat flux averaging 13.1 W/m2 that appears to be related to the explosive rift that formed the lake in the 1886 Tarawera eruption. A small rise in bottom water temperature over the survey period of 0.01 °C/day suggests the total thermal output of the lake is ~ 112–132 MW and when compared to the conductive heat output suggests that 18–42% of the total thermal energy is by conductive heat transfer.