Geothermal vegetation and ground temperature data collection Parimahana Scenic Reserve, Kawerau

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Mertens, A. 2007 Geothermal vegetation and ground temperature data collection Parimahana Scenic Reserve, Kawerau. Lower Hutt, N.Z.: GNS Science. GNS Science report 2007/01 45 p.

Abstract: Parimahana is a reserve containing thermal features and associated vegetation. It is located in low hill country on the western side of the Tarawera River, west of the Norsk Skog pulp and paper mill at Kawerau. The reserve comprises secondary forest and pockets of thermal vegetation. Two permanent vegetation plots and 13 photo-points were established in thermal vegetation in 1997. This report compares data collected in 1997 with data collected in 2006 at the same locations. The data in this report is also intended as base line data to investigate if the thermal vegetation and soil temperatures are affected by future geothermal fluid extraction or injection, associated with direct geothermal energy use at the mill and increased power generation. Ground temperatures at both plots were measured using a Fluke digital temperature probe at 5 cm, 10 cm and 15 cm depths below ground level (bgl) in 2006. Comparisons between the data collected in June 1997 and June 2006 at 10 cm bgl showed signs of cooling in some areas and a little heating in other areas in both plots. The significance of these apparent differences in shallow temperatures with time is difficult to assess because of the transient effects of daily and seasonal air temperature changes. These penetrate into the ground, with an exponential amplitude decay and a time lag. Repeat temperature data collected at one plot in September 2006 showed an average apparent increase over 3 months that exceeded the apparent decrease over 9 years. There has been a significant change in vegetation composition, plant size and abundance during the 10 years between 1997 and 2006. This is particularly evident in areas of prostrate kanuka and bare ground, and could be due to several factors, including: a) decrease in soil temperatures, b) weed eradication, and c) natural high growth rates. The photo survey showed that bare ground in 1997 is now mostly vegetated by bryophytes. It is noticeable that prostrate kanuka has increased in height and width. A weed eradication programme by DoC (in 2004) removed all invasive weeds. This would have favour of the growth of native plants in this habitat. Future re-measurements of the plots require accurate transect location and plant identification. The ground temperature measurements should take into account daily and seasonal climatic variations (rainfall and pressure). Repeat measurements should be done in the same months of each year. For more reliable comparisons between surveys temperature measurements should be made at 1 m depth. The vegetation data collection should be repeated every 2 years to account for natural growth rates of plants in this habitat. (auth)