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.

Several comparative studies of the earth's surface provide evidence that vegetation and other bio-physical processes at the earth's surface can directly affect the atmospheric boundary layer, leading to changes in temperature and precipitation patterns. In this study, we demonstrate how vegetation cover can be responsible for the subsurface temperature variation as well as how this temperature variation can be related to past events. A linear decrease of 0.0407 K/year was estimated, and a decrease of 2 mK was observed in subsurface temperature when the surface temperature exceeded 9 oC. This diurnal temperature variation occurs during the phenological growing season of the vegetation. The transient temperature shows an annual cycle at a depth of 40 m. Model calculation applying a linear decrease in surface temperature of 2 K as a boundary condition was simulated. Comparing the results with the trend it is realistic to assume that when an apparent thermal diffusivity of 1.8*10-6 m²/s is applied an event starting between 10 and 20 years ago is responsible for the detected decrease in temperature. However, with this thermal diffusivity the conductive annual temperature variation reaches an amplitude of 1.1 mK instead of the measured 5.4 mK at 40 m. In conclusion, beside the vegetation causing additional convective heat transport triggered by the annual surface temperature, the influence of reduced solar incoming heat radiation reaching the ground caused by the increased shadowing effect of vegetation cover might be responsible for a continuous decrease in local temperature of 2 K being active approximately 20 years after plantation.

Observations of summer microthermal temperature variations suggest, next to hydrological factors, a significant influence of plant activity on groundwater flow in fractured claystone materials. Variations in groundwater microtemperature were compared to variations in meteorological parameters and electrical potential of plants. With an increase in surface temperature, relative air humidity decreases and an increase in tree electrical potential, measured as the difference between the northern and the southern stem exposure (N–S), can be observed. This increase in electrical potential is concomitant with a change in groundwater temperature of approximately 2 mK. This relationship does not always occur. At high temperatures (+30°C) the decrease amounts to just 1 mK. This fact is related to the change in transpiration of plants, decreased or even suspended at high surface temperatures. A frequency analysis of all data showed a daily frequency of high magnitude in all parameters. Possibly changes in the macro weather situation events were observed in the results of atmospheric pressure, southern electric potential and groundwater temperature. The lag time between changes in electric potential and subsurface microtemperature changes amounts to 17 hours, possibly a result of the electrical potential difference between the northern and the southern exposure of the stem (N–S), and 5 hours, the result of the change in electrical potential difference between the southern and the northern stem exposure side (S–N). A comparison between potential changes and the computed change in gravity resulting from earth tidal effects showed that the correlation between the subsurface temperature variation with up to 2 mK and the change in surface temperature variation does not match directly. Other study shows that the impact of earth tides on subsurface microtemperature variation amounts to ca. 1mK. The effect of groundwater abstraction by mature vegetation is determined at the same range. Atmospheric tides can be correlated with the changes in north and south electric potentials.