Understanding heat transport processes in shallow subground resulting from external and internal impacts
von Maria de Fatima Santos Pinheiro
Datum der mündl. Prüfung:2023-07-24
Erschienen:2024-01-18
Betreuer:Prof. Dr. Martin Sauter
Gutachter:Prof. Dr. Andreas Tilgner
Gutachter:Prof. Dr. Günter Buntebarth
Gutachter:Dr. Alexandru Tatomir
Gutachter:Dr. Iulia Ghergut
Gutachter:Dr. Thomas Jahr
Dateien
Name:SantosPinheiro_2023_Thesis_SUB.pdf
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Description:SantosPinheiro_2023_Thesis_SUB
Zusammenfassung
Englisch
Heat conduction in the subsurface environment is the result of internal and external impacts and is governed by different forces in the short and long term. Externally, this phenomenon is related to daily temperature variations, terrestrial, oceanic, and atmospheric tides, and vegetation activity. However, beyond depths of up to 30 meters, thermal conduction is not applied. The transpiration of the vegetation plays a key role in this dynamic by being the connection point between the soil and the atmosphere. The main objectives of this thesis are (1) to identify the variation of groundwater microtemperature at daily and seasonal level, (2) to compare the variation of groundwater microtemperature with different meteorological parameters, (3) with terrestrial tides, (4) with vegetation activities, and (5) to analytically compare the groundwater microtemperature variation with land use changes in the studied area. Daily and annual variations in groundwater microtemperature were compared to variations in meteorological parameters and electrical potential of plants. A decrease of 2 mK was observed in daily subsurface temperature when the surface temperature exceeded 9 ºC. This diurnal temperature variation occurs during the phenological growing season of the vegetation. The same pattern was also observed during summer. However, 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. With an increase in surface temperature an increase in tree electrical potential can be observed. This increase in electrical potential is concomitant with a change in groundwater temperature of approximately 2 mK. A frequency analysis of all data showed a daily frequency of high magnitude in all parameters. The lag time between changes in electric potential and subsurface microtemperature changes amounts to 17 hours, 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. Atmospheric tides can be correlated with the changes in north and south electric potentials. Annually, a linear decrease of 0.0407 K/year was estimated, and 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, showing that the vegetation causes additional convective heat transport triggered by the annual surface temperature.
Keywords: Groundwater temperature; Land use change; Reforestation; Growing season; Climate proxy; Geothermal