Ground-air temperature tracking from a geothermal climate-change observatory in South India

Abstract

A geothermal climate-change observatory at Choutuppal (17.29 oN, 78.92°E) in south India was put into operation in July 2009 to study the inter-relationships between subsurface, ground-surface and surface-air temperature. The observatory is the first of its kind in a low latitude region with a significant monsoon season. Subsurface temperature measurements were obtained at different times of the year in two boreholes at the observatory drilled into granite to 21 m and 210 m depths respectively; a continuous record of shallow ground temperatures was acquired at six different depths to 1.2 m next to the deeper boreholes. Meteorological parameters recorded at the same site include surface air temperature (SAT), relative humidity, precipitation, solar radiation, wind speed and direction. Analysis of ground-air temperature data for the first five years (2009–2014) reveals the following. (i) Air temperature changes diffuse into the granitic subsurface with associated attenuation of high frequency components and phase lag characteristic of conductive heat transfer. (ii) Ground temperatures measured in the 1.2 m and 21 m deep holes track the diurnal and seasonal changes in SAT respectively, (iii) The ground is warmer than air on average; the difference is not constant but varies from −2.1 to +7.6 °C with a mean of 3.4 ± 1.8 (SD) oC, (iv) Ground-air temperature difference is influenced by incoming solar radiation and monsoon precipitation. On average, the ground warms by 2.5 °C for 100 Wm−2 increase in average incident solar radiation. The study highlights the importance of long-term monitoring to test ground-air temperature tracking critical to using borehole temperatures to infer climate change.