About the penetration of the diurnal and annual temperature variation into the subsurface
In order to evaluate the effect of the penetration of diurnal and annual wave temperature into the subsurface, the temperature has been monitored at an hourly recording frequency at depths of 40, 60 and 78 m between summer 2016 and summer 2018, at the geothermal experimental test site “Neutra” of the Georg-August-University of Göttingen, Germany. It has been asserted that the mean temperature gradient between 40 and 78 m continuously increases, because the temperature decreases at 40 m. The decrease can be explained by an increase in vegetation cover (trees, shrubs, etc.) in the perimeter of the test area, increasing the absorption of solar energy by the leaves. During the phenological growth season the diurnal temperature variation at the surface can be recorded in phase with opposite sign, even at a depth of 40 m, and the drop of the temperature at 40 m, when surface temperature reaches a value of nearly 9 °C, can be observed during small events of eco-dormancy during winter. The annual surface temperature variation of ±10 K induce the same effect with an amplitude of ±2mK at 40 m. It is stated that the dormant state of the vegetation cells is the reason of the annual variation of the residual temperature. At greater depths groundwater flows prevail and influence the temperature according to the structural properties of the encountered lithologies and the precipitation. The vegetation can transfer the daily and seasonal temperature variation to larger depths than expected based on the theory of heat conduction. This timely variation of the temperature gradient demonstrates that the determination of the terrestrial heat flow density is subject to several impacts induced from the surface as well as from the Earth’s interior. As a conclusion, temperature gradients determined at shallow depths may be influenced by changes in surface coverage.