Geothermal Climate Change Observatory in south India 2: Set-up and first results

A Geothermal Climate Change Observatory has been established in south India to quantify the inter-relationships between surface air temperatures, surface ground temperatures and subsurface temperatures on annual to decadal timescales and therefore contribute to better interpretation of the geothermal record of climate change. The observatory is located in the Choutuppal campus of National Geophysical Research Institute (17.29°N, 78.92°E), about 60 km to the east of Hyderabad. Surface meteorological parameters including air temperature, relative humidity, rainfall, solar radiation, wind speed and wind direction are being recorded continuously at an automatic weather station. At the same site, ground temperatures are being recorded continuously at depths of 0.015, 0.09, 0.19, 0.49, 0.99 and 1.19 m using a 1.2 m ground temperature probe buried into the granite regolith to track the diurnal changes in surface temperature. To track the seasonal and decadal changes, repeat temperature measurements are being carried out periodically in two boreholes CH-11 and CH-10 drilled to depths of 21 m and 210 m respectively and located within 3 m of the weather station. Surface meteorological data during the first year of operation indicate a general correlation between the time series for air temperature and incident solar radiation over the length of the record. Comparisons of the surface air temperature and ground temperatures measured at depths of a few cm to 21 m below the surface reveal both attenuation of high frequency temperature variations and time lag in the signals with increasing depth. These observations support a conductive heat transfer in the subsurface. Diurnal variations of surface air temperature decay to insignificant levels at a depth of ∼1 m, whereas the seasonal variations persist to depths of at least 10 m at the site. Ongoing measurements over the next few years would bring out a wealth of information on the nature and extent of tracking between the surface and subsurface temperature variations and lead to better understanding of the energy balance near the ground–air interface.